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    INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY



    ENVIRONMENTAL HEALTH CRITERIA 8





    SULFUR OXIDES AND SUSPENDED PARTICULATE MATTER







    This report contains the collective views of an international group of
    experts and does not necessarily represent the decisions or the stated
    policy of either the World Health Organization or the United Nations
    Environment Programme

    Published under the joint sponsorship of the United Nations
    Environment Programme and the World Health Organization

    World Health Organization Geneva, 1979

    ISBN 92 4 154068 0

    (c) World Health Organization 1979

        Publications of the World Health Organization enjoy copyright
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    welcomes such applications.

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    concerning the legal status of any country, territory, city or area or
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    CONTENTS

    ENVIRONMENTAL HEALTH CRITERIA FOR SULFUR OXIDES AND SUSPENDED
    PARTICULATE MATTER

    1. SUMMARY AND RECOMMENDATIONS FOR FURTHER RESEARCH AND ACTION
         1.1. Summary
               1.1.1. Chemistry and analytical methods
               1.1.2. Sources of sulfur oxides and particulate matter
               1.1.3. Dispersion and environmental transformations
               1.1.4. Environmental concentrations and exposures
               1.1.5. Absorption, distribution, and elimination
               1.1.6. Effects on experimental animals
               1.1.7. Effects on man
                       1.1.7.1   Controlled exposures
                       1.1.7.2   Industrial exposure
                       1.1.7.3   Community exposure
               1.1.8. Evaluation of health risks
         1.2. Recommendations for further research and action

    2. CHEMISTRY AND ANALYTICAL METHODS
         2.1. Chemical and physical properties
               2.1.1. Sulfur oxides
               2.1.2. Suspended particulate matter
         2.2. Methods of sampling and analysis
               2.2.1. Sulfur dioxide
               2.2.2. Suspended sulfates and surfuric acid
               2.2.3. Suspended particulate matter
               2.2.4. Dustfall (deposited matter)

    3. SOURCES OF SULFUR OXIDES AND PARTICULATE MATTER
         3.1. Natural occurrence
         3.2. Man-made sources
         3.3. Characteristics of sources

    4. DISPERSION AND ENVIRONMENTAL TRANSFORMATIONS
         4.1. Dispersion
         4.2. Transformation and degradation

    5. ENVIRONMENTAL CONCENTRATIONS AND EXPOSURES
         5.1. Concentrations in outdoor air
         5.2. Concentrations in indoor air
         5.3. Concentrations in work places
         5.4. Assessment of exposures

    6. ABSORPTION, DISTRIBUTION, AND ELIMINATION
         6.1. Absorption and deposition in the respiratory tract
               6.1.1. Sulfur dioxide
               6.1.2. Airborne particles

         6.2. Clearance from the respiratory tract and distribution
               6.2.1. Sulfur dioxide
               6.2.2. Particulate matter

    7. EFFECTS ON EXPERIMENTAL ANIMALS
         7.1. Short-term exposure studies
               7.1.1. Exposure to sulfur dioxide singly or in combination
                       with other agents
               7.1.2. Exposure to sulfuric acid aerosols or suspended
                       sulfates
         7.2. Long-term exposure studies

               7.2.1. Exposure to sulfur dioxide
               7.2.2. Exposure to sulfuric acid aerosols
               7.2.3. Exposure to a mixture of sulfur dioxide and
                       surfuric acid aerosols or this mixture combined
                       with other agents
               7.2.4. Combined exposure to sulfur dioxide and particulate
                       matter or other gaseous pollutants

    8. EFFECTS ON MAN
         8.1. Controlled exposures
               8.1.1. Effects on respiratory organs
                       8.1.1.1   Exposure to sulfur dioxide
                       8.1.1.2   Exposure to sulfuric acid aerosols
                       8.1.1.3   Exposure to mixtures of sulfur dioxide
                                 and other compounds
               8.1.2. Effects on sensory or reflex functions
         8.2. Industrial exposure
               8.2.1. Exposure to sulfur dioxide singly or in combination
                       with particulate matter
               8.2.2. Exposure to surfuric acid mist:
         8.3. Community exposure
               8.3.1. Mortality -- effects of short-term exposures
               8.3.2. Mortality -- effects of long-term exposures
               8.3.3. Morbidity -- effects of short-term exposures
               8.3.4. Morbidity in adults -- effects of long-term
                       exposures
               8.3.5. Morbidity in children
               8.3.6. CHESS studies
               8.3.7. Lung cancer and air pollution
               8.3.8. Annoyance
         8.4. Exposure-effect relationships

    9. EVALUATION OF HEALTH RISKS FROM EXPOSURE TO SULFUR OXIDES,
         SMOKE, AND SUSPENDED PARTICULATE MATTER
         9.1. Exposure levels
         9.2. Experimental animal studies
         9.3. Controlled studies in man
         9.4. Effects of industrial exposures
         9.5. Effects of community exposures
         9.6. Guidelines for the protection of public health

    REFERENCES
    

    NOTE TO READERS OF THE CRITERIA DOCUMENTS

        While every effort has been made to present information in the
    criteria documents as accurately as possible without unduly delaying
    their publication, mistakes might have occurred and are likely to
    occur in the future. In the interest of all users of the environmental
    health criteria documents, readers are kindly requested to communicate
    any errors found to the Division of Environmental Health, World Health
    Organization, Geneva, Switzerland, in order that they may be included
    in corrigenda which will appear in subsequent volumes.

        In addition, experts in any particular field dealt with in the
    criteria documents are kindly requested to make available to the WHO
    Secretariat any important published information that may have
    inadvertently been omitted and which may change the evaluation of
    health risks from exposure to the environmental agent under
    examination, so that the information may be considered in the event of
    updating and re-evaluation of the conclusions contained in the
    criteria documents.

    WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR SULFUR OXIDES
    AND SUSPENDED PARTICULATE MATTER

    Participants

     Membersa

    Professor K. Biersteker, Medical Research Division, Municipal Health
        Department, Rotterdam, Netherlands  (Vice-Chairman).

    Professor K. A. Bustueva, Department of Community Hygiene, Central
        Institute for Advanced Medical Training, Moscow, USSR

    Dr P. Camner, Department of Environmental Hygiene, The Karolinska
        Institute, Stockholm, Sweden

    Professor L. Friberg, Department of Environmental Hygiene, The
        Karolinska Institute, Stockholm, Sweden  (Chairman)

    Mrs M. Fugas Laboratory for Environmental Hygiene, Institute for
        Medical Research and Occupational Health, Zagreb, Yugoslavia

    Dr R. J. M. Horton, Health Effects Research Laboratory, US
        Environmental Protection Agency, Research Triangle Park, NC, USA

    Professor S. Maziarka, National Institute of Hygiene, Warsaw, Poland

    Dr B. Prinz, State Institute for Protection of Air Quality and Land
        Usage, Essen, Federal Republic of Germany

    Dr H. P. Ribeiro, Laboratory of Pulmonary Function, Santa Casa de
        Misericordia de Sao Paulo, Sao Paulo, Brazil

    Dr T. Suzuki, Institute of Public Health, Tokyo, Japan

    Mr G. Verduyn, Institut d'Hygiene et d'Epidemiologie, Brussels,
        Belgium

    Mr R. E. Waller, Medical Research Council, Air Pollution Unit, St
        Bartholomew's Hospital Medical College, London, England
         (Rapporteur)

    Mr D. A. Williams, Surveillance Division, Air Pollution Control
        Directorate, Environment Canada, Ottawa, Ontario, Canada

    Professor M. H. Wahdan, High Institute of Public Health, University of
        Alexandria, Alexandria, Egypt

              

    a  Unable to attend:

     Representatives of other Organizations

    Mr J. Janczak, Environment and Housing Division, United Nations
        Economic Commission for Europe, Geneva, Switzerland

    Mr D. Larr, Division of Geophysics, Global Pollution and Health,
        United Nations Environmental Programme, Nairobi, Kenya

    Dr D. Djordevic, Occupational Safety and Health Branch, International
        Labour Organisation, Geneva, Switzerland

    Mr G. W. Kronebach, Technical Supporting Services Branch, World
        Meteorological Organization, Geneva, Switzerland

    Dr A. Berlin, Health Protection Directorate, Commission of the
        European Communities, Luxembourg

    Mr J. A. Bromley, Environmental Directorate, Organization for Economic
        Co-operation and Development, Paris, France

     Secretariat

    Professor B. G. Ferris, Jr, Department of Physiology, Harvard
        University School of Public Health, Boston, MA, USA  (Temporary
         Adviser)

    Dr Y. Hasegawa, Medical Officer, Control of Environmental Pollution
        and Hazards, World Health Organization, Geneva, Switzerland
         (Secretary)

    Dr H. W. de Koning, Scientist, Control of Environmental Pollution and
        Hazards, World Health Organization, Geneva, Switzerland

    Dr B. Marschall, Medical Officer, Occupational Health, World Health
        Organization, Geneva, Switzerland

    Dr R. Masironi, Scientist, Cardiovascular Diseases, World Health
        Organization, Geneva, Switzerland

    Dr S. I. Muravieva, Institute of Industrial Hygiene and Occupational
        Diseases, Academy of Medical Sciences of the USSR, Moscow, USSR
         (Temporary Adviser)

    Dr V. B. Vouk, Chief, Control of Environmental Pollution and Hazards,
        World Health Organization, Geneva, Switzerland

    ENVIRONMENTAL HEALTH CRITERIA FOR SULFUR OXIDES AND SUSPENDED
    PARTICULATE MATTER

        A WHO Task Group on Environmental Health Criteria for Sulfur
    Oxides and Suspended Particulate Matter met in Geneva from 6 to 12
    January 1976. The meeting was opened by Dr B. H. Dieterich, Director,
    Division of Environmental Health, who welcomed the participants and
    the representatives of other international organizations on behalf of
    the Director-General. Dr Dieterich briefly outlined the history and
    purpose of the WHO Environmental Health Criteria Programme and the
    progress made in its implementation, thanks to the active
    collaboration of WHO Member States and the support of the United
    Nations Environment Programme (UNEP).

        The Task Group reviewed and revised the second draft criteria
    document and made an evaluation of the health risks from exposure to
    these substances.

        The first and second drafts were prepared by Professor B. G.
    Ferris, Jr, Harvard University School of Public Health, USA. The
    comments on which the second draft was based were received from the
    national focal points collaborating in the WHO Environmental Health
    Criteria Programme in Belgium, Bulgaria, Canada, Czechoslovakia, the
    Federal Republic of Germany, Greece, Japan, New Zealand, Poland,
    Sweden, USA, USSR and from the Food and Agriculture Organization of
    the United Nations (FAO), the United Nations Educational Scientific
    and Cultural Organization (UNESCO), the United Nations Industrial
    Development Organization (UNIDO), the World Meteorological
    Organization (WMO), the International Atomic Energy Agency (IAEA), and
    the Commission of European Communities (CEC). Comments were also
    received from Professor H. Antweiler and Dr B. Prinz (Federal Republic
    of Germany), Professor K. Biersteker and Dr R. van der Lende
    (Netherlands), Professor F. Sawicki (Poland), and Professor W. W.
    Holland and Professor P. J. Lawther (United Kingdom).

        The collaboration of these national institutions, international
    organizations and individual experts is gratefully acknowledged. The
    Secretariat also wishes to thank Professor B. G. Ferris, Jr and Mr R.
    E. Waller for their invaluable assistance in the final stages of the
    preparation of the document.

        In view of the substantial amendments made to the document
    (particularly within sections 2 to 5) since the meeting of the Task
    Group, a revised version was circulated to all members in February
    1978. At the same time, copies of a newly-produced review of the
    health effects of particulate pollution (Holland et al., in press),
    that had been submitted for consideration, were distributed to the
    members. Comments were sought on the draft of the criteria document

    itself, and on any amendments or additions considered necessary in
    light of the new report. These comments, together with others received
    from the International Petroleum Industry Environmental Conservation
    Association, and the International Iron and Steel Institute, were then
    considered by a small group consisting of the Chairman of the Task
    Group meeting, the Rapporteur and some members of the Secretariat. The
    alterations suggested (mainly within section 9) were circulated again
    to the original members of the Task Group prior to publication.

        The document has been based, primarily, on original publications
    listed in the reference section. However, several recent reviews of
    health aspects of sulfur oxides and suspended particulate matter have
    also been used including those by Katz (1969), Committee on the
    Challenges of Modern Society (1971), Organization for Economic
    Cooperation and Development (1965), Rall (1974), Task Group on Lung
    Dynamics (1966), Task Group on Metal Accumulation (1973), US
    Department of Health, Education and Welfare (1969a), US Environmental
    Protection Agency (1974), World Health Organization (1976a), and World
    Meteorological Organization (1974).

        The purpose of this document is to review and evaluate available
    information on the biological effects of sulfur oxides and suspended
    particulate matter including suspended sulfates and sulfuric acid
    aerosols, and to provide a scientific basis for decisions aimed at the
    protection of human health from the adverse consequences of exposure
    to these substances in both occupational and general environments.
    Although there are various routes of exposure, such as inhalation,
    ingestion (World Health Organization, 1971, 1974) and contact with
    skin, attention in this report has been concentrated upon the effects
    of inhalation of these substances, since this is the most important
    route of exposure. The discussion has also been limited to sulfur
    dioxide, sulfur trioxide, sulfate ions, and particulate matter
    primarily resulting from the combustion of fossil fuels. The sulfate
    ion has been considered in the variety of forms in which it occurs in
    the atmosphere, e.g., sulfuric acid and various sulfate salts.

        The vast literature on these pollutants has been carefully
    evaluated and selected according to its validity and relevance for
    assessing human exposure, for understanding the mechanisms of the
    biological action of the pollutants and for establishing environmental
    health criteria, i.e., exposure-effect/response relationships in man.
    Environmental considerations have been limited to elucidating the
    pathways leading from the natural and man-made sources of these
    substances to the sites of toxic action in the human organism. The
    non-human targets (plants, animals, ecosystems) have not been
    considered unless the effects of their contamination were judged to be
    of direct relevance to human health. For similar reasons, much of the
    published information on the effects of these pollutants on
    experimental animals has not been included.

        Details concerning the WHO Environmental Health Criteria Programme
    including some terms frequently used in the document may be found in
    the general introduction to the Environmental Health Criteria
    Programme published together with the environmental health criteria
    document on mercury (Environmental Health Criteria 1, Mercury, Geneva,
    World Health Organization, 1976), now also available as a reprint.

        The following conversion factors have been used in the present
    document:a

            Sulfur dioxide               1 ppm = 2856 g/m3
            Ozone                        1 ppm = 2140 g/m3
            Carbon monoxide              1 ppm = 1250 g/m3



              

    a  When converting values expressed in ppm to g/m3, the numbers
       have been rounded up to 2 or, exceptionally, 3 significant figures,
       and concentrations higher than 10 000 g/m3 have been expressed in
       mg/m3.
    1.  SUMMARY AND RECOMMENDATIONS FOR FURTHER RESEARCH AND ACTION

    1.1  Summary

    1.1.1  Chemistry and analytical methods

        Procedures in common use for the sampling and determination of
    sulfur dioxide, sulfates, sulfuric acid, and suspended particulate
    matter have been discussed, noting their limitations and stressing the
    need to specify the method of measurement when quoting results in
    relation to studies on the effects of health.

        Several alternative methods, already in common use, can be
    recommended for the determination of sulfur dioxide using manually
    operated sampling and, providing the extent of interference from other
    pollutants is taken into account, results are reasonably comparable
    with one other. A wide range of continuous automatic instruments is
    available and, where the expense is justified, they can provide
    additional information on short-term variations in concentrationsb of
    sulfur dioxide.

        Methods for the determination of particulate sulfate do not
    present any special problems, but, at present, there does not appear
    to be any wholly satisfactory way of determining sulfuric acid
    separately from sulfates and other interrelated components.

        Much attention was given by the Task Group to the sampling and
    determination of suspended particulate matter, stressing that this was
    not a well-defined entity, and that it could only be assessed in terms
    of certain physical properties. The several methods in common use are
    based on different characteristics, and the Task Group felt that clear
    distinctions should be made between them, particularly in relation to
    those based on blackness (the "smoke" measurements commonly made in
    Europe) and those based on weight (the total suspended particulate
    matter commonly measured in USA). The need to limit the measurements
    to particles within the respiratory size range, and to consider the
    wide range in chemical composition of the samples was also stressed.

              

    b  Throughout the document, the word "concentration" refers to mass
       concentration unless otherwise stated.

    1.1.2  Sources of sulfur oxides and particulate matter

        Despite the fact that some sulfur oxides and particulate matter
    occur naturally in air in large amounts, contributions from man's
    activities are generally of prime importance in urban areas. In
    particular, the combustion of fuels for heating and power generation
    is considered responsible for most of the sulfur dioxide and
    particulate pollution to which the general population is exposed. The
    three broad categories of sources are: domestic sources associated
    with the use of coal and some other fuels for heating and cooking;
    industrial sources; and motor vehicles. Domestic and motor vehicle
    sources have a disproportionate effect on concentrations in the
    immediate vicinity, because the pollution is emitted close to ground
    level.

    1.1.3  Dispersion and environmental transformations

        The temperature of the gases, the efflux velocity, and the height
    of the chimney are important factors in securing effective dispersion
    of emissions from combustion sources. The topography of the
    surrounding area and meteorological factors determine the extent to
    which these pollutants are dispersed and diluted to tolerable levels.
    Temperature inversion can trap emissions over urban areas to produce
    concentrations up to several hundred times the normal values.

        Several processes, including photochemical reactions in the
    presence of hydrocarbons, catalytic oxidation in the presence of
    particulate matter containing iron or manganese compounds, and
    reaction with ammonia, leading to the transformation of sulfur dioxide
    to sulfates or sulfuric acid, are involved in the atmospheric
    reactions of sulfur dioxide and suspended particulate matter. The
    relative importance of each of these is not well established, but
    together they account for the gradual removal of most of the sulfur
    dioxide dispersed into the air, the remainder being deposited directly
    on soil, water, vegetation, or other surfaces.

    1.1.4  Environmental concentrations and exposures

        Sulfur dioxide and suspended particulate matter are measured
    routinely in many areas throughout the world, but care is needed to
    ensure that observations from monitoring networks set up for other
    purposes are suitable for assessing risks to health. The location of
    samplers in relation to sources, the surrounding topography, and the
    population at risk need to be considered, and also the time-resolution
    of the observations. Averaging periods of 24 h are commonly used in
    relation to short-term exposures, though, in some circumstances, still
    shorter periods are required. For long-term exposures, annual means
    based on a series of daily observations may be adequate.

        Examination of concentrations of sulfur dioxide and suspended
    particulate matter in the air of a number of cities throughout the
    world has revealed a wide range in annual mean values and an even
    wider range of peak values, reflecting the effects of climatic factors
    and liability to temperature inversions. The typical, annual,
    arithmetic mean concentrations of sulfur dioxide in urban areas range
    from 100-200 g/m3 (0.035-0.07 ppm) with the highest daily means from
    300-900 g/m3 (0.1.-0.3 ppm). For smoke, the corresponding values are
    30-200 g/m3 and 150-900 g/m3 respectively, and for suspended
    particulate matter, measured by the high volume sampler, the annual
    arithmetic means are 60-500 g/m3 with maximum daily means of about
    150-1000 g/m3. Relatively little information is available on
    sulfates but some data have been obtained in the USA.

        Indoor concentrations of these pollutants also deserve attention.
    In the absence of specific sources of sulfur dioxide or particulate
    matter, concentrations are generally lower indoors than outdoors.
    Proposals for assessing weekly-weighted average exposures of people in
    terms of the proportion of time spent in various locations were
    discussed.

        Industrial situations in which high concentrations of sulfur
    dioxide or sulfuric acid occur should be carefully assessed in each
    specific case, but it should be noted that industrial dusts are
    generally very different in character from the suspended particulate
    matter in urban air.

    1.1.5  Absorption, distribution, and elimination

        Although the major route of absorption of the relevant sulfur
    compounds and particulate matter into the body is through the
    intestinal tract, the respiratory tract is the most vulnerable area
    for airborne materials.

        Most studies on both man and animals have indicated that 40 to 90%
    or more of inhaled sulfur dioxide is absorbed in the upper respiratory
    tract. Taken into the blood stream, it appears to be widely
    distributed throughout the body, metabolized, and excreted via the
    urinary tract.

        The deposition pattern of particulate matter varies with particle
    size, shape, and density, and also with airflow conditions. Deposited
    particles are largely phagocytized and transported to the mucociliary
    escalator, into the interstitium, or to the lymphatic system. The
    biological half-times range from days to years depending on their
    chemical composition.

        Soluble particles may dissolve in the mucous or aqueous lining of
    the lungs. In the first case, they will be eliminated via the
    mucociliary route. In the second, they may diffuse into the lymph or
    blood.

    1.1.6  Effects on experimental animals

        Selected studies on animals that involve both short-term (24-h or
    less) and long-term (more than 24-h) exposures have been reviewed in
    this document; certain interactions between the effects of sulfur
    oxides, particulate matter, and other air contaminants have also been
    reported. The lowest, adverse-effect concentrations vary considerably
    from study to study. The discrepancies may be due to differences in
    the sensitivity of the test animals used or in exposure conditions
    including the duration of exposure and the pattern of exposure
    (single, continuous, repeated, or intermittent). Furthermore, exposure
    may have been to a single pollutant or to a mixture of various agents,
    or different effects may have been analysed.

        In general, however, it has been noted that sulfuric acid aerosols
    and some sulfate salts such as zinc ammonium sulfate are more
    irritating to respiratory organs than sulfur dioxide, and that some
    aerosols, particularly those in the submicron size range, enhance the
    effect of sulfur dioxide when they are present simultaneously.

        Caution must be exercised in light of the fact that differences in
    metabolism and life span make extrapolation of results of animal
    experiments to man difficult. However, some of these studies have
    indicated possible mechanisms of biological action on the respiratory
    system -- e.g., interference with mechanisms for the clearance of
    bacteria and inert particles from the lung.

    1.1.7  Effects on man

    1.1.7.1  Controlled exposures

        Inhalation studies on human volunteers have been performed under
    controlled, short-term exposure conditions with sulfur dioxide or
    sulfuric acid mist singly or in combination, or with mixtures of these
    and other compounds. Some of these studies have proved useful for
    developing exposure-effect relationships.

        When exposed to sulfur dioxide alone, slight effects on
    respiratory function were demonstrated at a concentration of
    2.1 mg/m3 (0.75 ppm) but not at 1.1 mg/m3 (0.37 ppm), while sulfuric
    acid mist affected respiratory function at levels as low as
    0.35 mg/m3. Synergistic effects on pulmonary function were reported
    from joint exposure to sulfur dioxide and hydrogen peroxide as well as
    sulfur dioxide and ozone.

        The effects of sulfuric acid mist and sulfur dioxide on sensory
    receptors and cerebral cortical function have been studied extensively
    in the USSR. In these reflex actions, threshold levels for sulfuric
    acid were always much lower than those for sulfur dioxide. Synergistic
    effects of these compounds have also been noted.

    1.1.7.2  Industrial exposure

        Effects of exposure to sulfur dioxide, particulate matter, or
    sulfuric acid mist have been studied in workers in refrigerator
    manufacturing plants, steel mills, paper and pulp mills, and in the
    battery industries.

        Although the exposure levels were very high (daily mean
    concentrations of sulfur dioxide of up to 70 mg/m3 or 25 ppm) in some
    studies, no significant differences in effects were found when
    compared with the controls. In another study, effects on respiratory
    function were not detected with joint exposure to sulfur dioxide and
    suspended particulate matter at mean concentrations over 3 years of
    1.8 to 2.1 mg/m3 (0.6-0.7 ppm) and 600 to 1800 g/m3, respectively.

        Exposure to sulfuric acid mist produced effects (nose and throat
    irritation) at a concentration of 2.0 mg/m3, while exposure to a
    concentration of 1.4 mg/m3 did not affect pulmonary function.
    However, the effects of this pollutant are also closely dependent on
    particle size.

    1.1.7.3  Community exposure

        The large amount of literature on the effects of community
    exposure has been reviewed and detailed consideration has been given
    to those studies that appeared to have adequate data and design, in
    particular, due control for cigarette smoking and satisfactory
    measurements of exposure levels. Certain of these studies were
    selected to develop exposure-effect relationships. In the evaluation
    of the studies it became apparent that it was not possible to compare
    two fundamentally different methods of measuring exposure to
    particulate matter -- one measuring black smoke and the other
    measuring total suspended particulates, usually by the high-volume
    sampling method.

        Studies have been performed in terms of both short and long-term
    exposures and in relation to changes in the incidences of mortality
    and morbidity. In morbidity studies concerned with short-term exposure
    to a combination of sulfur dioxide and total particulates, the lowest
    concentrations (24-h mean) at which adverse effects were noted were

    200 g/m3 (0.07 ppm) and 150 g/m3 (high volume sampler),
    respectively. In long-term, joint exposure studies, effects were noted
    at annual mean concentrations of 60-140 g/m3 (0.02-0.05 ppm) for
    sulfur dioxide and 100-200 g/m3 for total suspended particulates
    (light-scattering method). However, there were reservations about the
    validity of some of these studies.

        Increases in mortality were reported in relation to episodes of
    high pollution with 24-h mean concentrations of the order of
    500 g/m3 (0.18 ppm) for sulfur dioxide and 500 g/m3 for smoke.

        The question as to whether carcinogenic components of suspended
    particulate matter, such as benzo(a)pyrene may have some influence on
    the incidence of lung cancer was not discussed by the Task Group.a

    1.1.8  Evaluation of health risks

        From a critical evaluation of the studies on the health effects of
    community exposures, the Task Group developed two summary tables; one
    for the expected effects on the health of selected populations of
    short-term exposures to sulfur dioxide and smoke; the other for the
    effects of long-term exposures to these substances.

        As an estimate of the lowest adverse-effect levels for short-term
    exposures, the Group selected the 24-h mean concentrations of
    500 g/m3 (0.18 ppm) for sulfur dioxide and 500 g/m3 for smoke, as
    levels at which excess mortality might be expected among elderly
    people or patients with pulmonary diseases, and a sulfur dioxide
    concentration of 250 g/m3 (0.09 ppm) and a smoke concentration of
    250 g/m3 as levels at which the conditions of patients with
    respiratory disease might become worse.



              

    a  Since the Task Group meeting, an International Symposium on Air
       Pollution and Cancer has been held at the Karolinska Institute,
       Stockholm, with the collaboration of the World Health Organization.
       One of the conclusions quoted in the report (Task Group on Air
       Pollution and Cancer, 1978) is as follows: "Combustion products of
       fossil fuels in ambient air, probably acting together with cigarette
       smoke, have been responsible for cases of lung cancer in large urban
       areas, the numbers produced being of the order of 5-10 cases per
       100 000 males per year. The actual rate will vary from place to place
       and from time to time, depending on local conditions over the
       previous few decades."

        For long-term exposures, annual mean concentrations of 100 g/m3
    (0.035 ppm) for sulfur dioxide and 100 g/m3 for smoke were selected
    as the lowest concentrations at which adverse health effects such as
    increases in respiratory symptoms, or respiratory disease incidence in
    the general population might be expected.

        Based on these evaluations, guidelines for the protection of the
    health of the public were developed in terms of 24-h values
    (100-150 g/m3 for sulfur dioxide, and for smoke) and in terms of
    annual means (40-60 g/m3 for sulfur dioxide, and for smoke). In view
    of the limited amount of data available in relation to total suspended
    particulates, firm guidelines could not be recommended, but it was
    suggested that interim guidelines might be of the order of
    60-90 g/m3 for annual arithmetic means and 150-230 g/m3 for 24-h
    values. Guidelines were not developed for sulfuric acid or sulfates,
    also because of lack of data.

    1.2  Recommendations for Further Research and Action

         (a) As most of the knowledge of effects discussed in this
    document relates to combinations of sulfur oxides with smoke, and as
    these pollutants are not wholly representative of the current exposure
    situation in a number of communities, there is a need to carry out
    epidemiological studies where possible effects can be related to
    particulate pollutants of other types, including sulfuric acid and
    sulfates, and to other gaseous components of the mixture.

         (b) As epidemiological studies are still being carried out that
    do not take other variables, particularly smoking, into account when
    considering effects, and in which the exposure to pollution is not
    adequately assessed, it is recommended that the World Health
    Organization should provide guidance and advice on the minimum
    requirements for such studies.

         (c) As a consequence of the conclusions reached on the expected
    effects on health of sulfur oxides, smoke, and total suspended
    particulates, and on the related guidelines for the protection of
    public health, it is recommended that existing monitoring practices
    should be reviewed and, if necessary, appropriately modified. To
    assist regulatory agencies in this respect, it is recommended that the
    World Health Organization should undertake consultations with
    environmental scientists, inhalation toxicologists, and
    epidemiologists to consider both the epidemiological and control
    aspects of the problem.

         (d) As there is little information from occupational exposures
    that can be used for exposure-effect evaluations, it is recommended
    that such studies should be carried out particularly in relation to
    sulfur dioxide and related pollutants. These studies should include
    measurements of pollution over complete working shifts, taking into
    account variations with space and time over shorter periods, and
    exposures outside the working environment. The importance of following
    up people who may have left work because of effects on their health
    must also be stressed.

         (e) The Group did not carry out a thorough evaluation of any
    possible association between lung cancer and air pollution. It is
    recommended that a separate evaluation should be carried out.

         (f) As deposition and clearance of particles from the
    respiratory system is of fundamental importance for evaluating risks,
    and for the design of measuring instruments for use in monitoring
    systems, it is recommended that a thorough review of the relevance of
    existing information on the mixture of pollutants present in the
    ambient air be carried out, taking into account particle size
    distribution and chemical composition.

         (g) Some laboratory experiments on the effects of sulfur oxides,
    smoke, and total suspended particulates do not need to be repeated,
    but the Task Group considered that it was necessary for more
    experimental work to be carried out on the mechanism of the biological
    action of these pollutants and of their interactions with other
    agents.

         (h) The Task Group found that information concerning the nature
    and effects of pollution had considerably increased since the WHO
    meeting on air quality criteria and guides in 1972, and that this had
    resulted in a somewhat different approach to the preparation of
    criteria for sulfur oxides and suspended particulate matter and to the
    recommendations for future action. It was considered, therefore, that
    the criteria should be reviewed at least every five years to take into
    account any new data on effects that may become available and the
    implications of any further changes in the character of pollution.

         (i) Since observations on sulfur oxides, smoke, and total
    suspended particulates are considered mainly as indices of the complex
    mixture of pollutants in the ambient air, the Task Group recommended
    that, in addition to the continuation of efforts to reduce these
    pollutants, efforts should be made to control other pollutants.

    2.  CHEMISTRY AND ANALYTICAL METHODS

    2.1  Chemical and Physical Properties

    2.1.1  Sulfur oxides

        Sulfur dioxide is a colourless gas that can be detected by taste
    by most people at concentrations in the range of 1000 to 3000 g/m3
    (0.35-1.05 ppm). At higher concentrations (above about 10 000 g/m3;
    3.5 ppm), it has a pungent, irritating odour. It dissolves readily in
    water to form sulfurous acid (H2SO3), and in pure solutions this is
    slowly oxidized to sulfuric acid by the oxygen from the air. In the
    presence of catalysing impurities such as manganese or iron salts, it
    is more rapidly converted (Freiberg, 1975, Johnstone & Coughanowr,
    1958). Sulfur dioxide can also react either catalytically or
    photochemically in the gas phase with other air pollutants to form
    sulfur trioxide, sulfuric acid, and sulfates (see section 4).

        Sulfur trioxide (SO3) is a highly reactive gas, and, in the
    presence of moisture in the air, it is rapidly hydrated to sulfuric
    acid. In the air, therefore, it is sulfuric acid in the form of an
    aerosol that is found rather than sulfur trioxide, and, in general, it
    is associated with other pollutants in droplets or solid particles
    extending over a wide range of sizes (Waller, 1963). It can be emitted
    into the atmosphere directly, or may result from the various reactions
    mentioned earlier. Sulfuric acid may also be formed from the oxidation
    of hydrogen sulfide in the air. The acid is strongly hygroscopic, and
    droplets containing it readily take up further moisture from the air
    until they are in equilibrium with their surroundings. If there is any
    ammonia present, it will rapidly react with sulfuric acid to form
    ammonium sulfate, which will continue to exist as an aerosol (in
    droplet or crystalline form, depending on the relative humidity). The
    sulfuric acid may react further with compounds in the air to produce
    other sulfates. Some sulfate reaches the air directly, from combustion
    sources or industrial emissions, and, in the proximity of oceans,
    magnesium sulfate is present in the aerosol generated from ocean
    spray.

        A wide range of sulfur compounds is represented in the complex
    mixture of urban air pollutants, but, from a practical point of view,
    only the gas sulfur dioxide, and sulfuric acid and sulfates as
    components of the suspended particulate matter need be considered.

    2.1.2  Suspended particulate matter

        The term suspended particulate matter covers a wide range of
    finely divided solids or liquids that may be dispersed into the air
    from combustion processes, industrial activities, or natural sources,
    as discussed further in section 3. The composition of this material is
    dependent upon the types of sources contributing to it, and the broad
    definition is in terms of the settling velocity of the particles. For
    ideal spherical particles, the velocity can be predicted from Stokes'
    Law (Fuchs, 1964, see Table 1).

        In the size range under about 10 m, the settling velocity is
    negligible compared with the movement produced by wind and air
    turbulence, and such particles are liable to remain in suspension for
    periods of the order of hours or days, until they are removed by
    impaction or diffusion on to surfaces or are scavenged by rain. It is
    these particles, with diameters ranging from well below 0.1 m, up to
    about 5 to 10 m, that are referred to as suspended particulates, but
    there is clearly no sharp dividing line between them and the larger
    particles of deposited matter (or "dustfall") that are liable to fall
    out rapidly, close to their source.

        The suspended particulates are important in relation to health not
    only because they persist in the atmosphere longer than larger
    particles, but also because they are small enough to be inhaled and to
    penetrate deeply into the respiratory tract, as discussed in section
    6. They are also responsible for reduction in visibility, and take
    part in reactions with other air pollutants.

        Many of the particles in the air have complex shapes, as
    illustrated in the electron micrograph (Fig. 1). Among the particles
    shown are a number of "smoke aggregates", typical of the incomplete
    combustion of hydrocarbon fuels, consisting of small spherical
    particles of carbon or higher hydrocarbons having diameters of the
    order of 0.05 m, clustered Together in loose structures with overall
    diameters up to several micrometres. From the point of view of their
    behaviour during sampling or inhalation, such particles are classified
    in terms of their equivalent aerodynamic diameters, i.e., the
    diameters of unit density spheres having the same settling velocities.
    Some truly spherical material may be present, mainly as aqueous
    droplets containing dissolved salts, sulfuric acid, or occluded solid
    particles. These cannot be examined directly under the electron
    microscope, since the aqueous component evaporates completely, but
    some residues can be seen in Fig. 1, and the rings of small droplets
    indicate the presence of sulfuric acid (Waller et al., 1963). Many
    other types of particles, including small flakes and fibres can be

        Table 1.  Settling velocities of spherical particles of unit density in still air
                                                                                             

                                          Diameter          Settling
                                            m           velocity, ms-1
                                       ^                           
                                       |
    Suspended particulate matter       |      0.1           8 x 10-7
                                       |
                                       |                    4 x 10-5
                                       |
                                    |  |     10             3 x 10-3
                                    |  |
                                    |  |    100               0.25
                                    |
       Deposited matter             v      1000               3.9

                                                                                             

    
    FIGURE 1

    seen in Fig. 1. and a wide range of sizes, shapes, and densities is
    commonly seen in all samples of suspended particulates in urban areas.
    For routine monitoring purposes, it is clearly out of the question to
    characterize the material completely in terms of size distribution and
    composition, but it is important to recognize that the suspended
    particulates generally comprise a heterogenous mixture, that can
    differ greatly in its characteristics from one location to another,
    and even from one occasion to another at any one site.

        Estimates of size distribution can be obtained from electron
    micrographs by considering the particles compressed into equivalent
    spheres. The results are commonly plotted as cumulative frequency
    curves, and Fig. 2 shows results for a sample consisting largely of
    smoke aggregates. In this the mass median diameter (MMD) is
    approximately 1 m, i.e., half the mass of material collected is
    contained in particles having effective (aerodynamic) diameters under
    one micrometer.a

        The results shown in Fig. 2 indicate a log-normal distribution of
    particle size in that specific sample, but Willeke & Whitby (1975)
    have shown that distributions are often multimodal. These authors also
    stressed the importance of examining the distribution in terms of
    numbers of particles, and surface area, in addition to volume (or
    mass), for each curve may reveal features not shown by the others. An
    example of results obtained with their Minnesota Aerosol Analyzing
    System is shown in Fig. 3. This shows a mode in the volume
    distribution in the range 0.1 to 1 m that is related to particles
    formed by coagulation or condensation from smaller units, and a
    further mode of the order of 10 m that corresponds with
    mechanically-produced particles, some of which are large enough to
    settle rapidly, and are not, therefore, strictly part of the suspended
    particulates.

        The impression gained of size distributions will, however, always
    depend on the characteristics of the instruments used. The most
    extensive series of results that has been reported was based on a
    modified Andersen cascade impactor (Lee & Goranson, 1972). This allows
    samples to be collected in five, roughly size-graded fractions, with a
    back-up filter as a sixth stage to collect the finest particles. Mass
    median diameters have generally been found to be below 1 m in samples
    collected in urban areas of the USA, but this method cannot describe
    the size distribution as completely as that of Willeke & Whitby
    (1975).

              

    a  For further details concerning the definition of poly-dispersed
       aerosols containing particles of irregular shape see, for example,
       Fuchs (1964) or Task Group on Lung Dynamics (1966).

    FIGURE 2

    FIGURE 3

        Although it is possible to investigate the composition of
    individual particles to a certain extent, data on chemical composition
    are usually derived from larger samples as collected for the
    determination of the total mass of suspended particulates. Among the
    principal components are carbon, tarry material (hydrocarbons, soluble
    in organic solvents such as benzene), water soluble material (such as
    ammonium sulfate), and insoluble ash (containing small amounts of
    iron, lead, and a wide variety of other elements).

        The proportions of these components vary widely, depending on the
    types of sources in the locality. For example, the special feature of
    the suspended particulate matter in the United Kingdom prior to the
    implementation of the Clean Air Act was their high tar content, and
    this was particularly evident in high pollution episodes (Table 2).

        Table 2.  Examples of analyses of suspended particulates sampled in London prior
              to smoke control (high volume samples)a
                                                                                   

                           Typical summer    Typical winter     High pollution
                               sample            sample             episode
                             (July 1955)     (February 1955)    (January 1956)
                                                                                   

    Total suspended
    particulates (g/m3)         97               485               5111

    Components as %
    of total:

    Organic (tar)                 7.5              19.1               45.7

    Sulfate                      11.3               9.0                5.8
    Chloride                      0.8               0.2                1.0
    Nitrate                       0.8               0.5                0.7

    Iron                          1.3               1.4                0.1
    Lead                          0.7               0.4                0.1
    Zinc                          0.5               0.2                0.8
                                                                                   

    a From:  The Medical Research Council Air Pollution Unit, now. Clinical
             Section of Medical Research Council Toxicology Unit
             (unpublished data)

    
        Results from the extensive series of analyses of high volume
    samples of suspended particulate matter at sites in the USA indicate
    organic contents of the order of 10% of the total particulates (US
    Environmental Protection Agency, 1974a). Although little is known of
    the influence of the composition of the suspended particulate matter
    on effects on health, detailed analyses can be of value in identifying
    sources and are relevant in the study of reactions between pollutants.
    Thus iron and manganese, although only trace constituents, may be of
    importance in catalysing the oxidation of sulfur dioxide to sulfuric
    acid or sulfates. The lead content is usually related to pollution
    from motor vehicles, and traces of vanadium that are present come
    mainly from the combustion of residual oils. There may also be a
    variety of substances from noncombustion sources, such as road dust,
    material from the degradation of tyres, windblown soil, pollen, and
    emissions from industrial processes such as cement manufacturing or
    steel making.

        The most important distinction to be made in relation to suspended
    particulate matter at the present time, however, is neither the
    precise size distribution, nor the detailed chemical composition, but
    the very broad characterization that results from the different
    methods of assessment that are in common use for routine monitoring
    purposes. These are discussed further in section 2.2.3. In subsequent
    sections, the term "smoke" has been used for observations of suspended
    particulate matter based on its soiling properties, and "total
    suspended particulates" for those based directly on weight. Since the
    former is mainly influenced by incomplete combustion products from the
    burning of fossil fuels, and is little affected by white or colourless
    materials such as ammonium sulfate, it is clear that the two terms are
    not interchangeable.

    2.2  Methods of Sampling and Analysis

        In general, the outdoor air has been sampled for sulfur oxides and
    suspended particulate matter in relation to community exposures,
    whereas indoor environments have been examined in connection with
    occupational exposures.

        The most commonly used methods have been described in detail in a
    recently published manual (World Health Organization, 1976a) and their
    application in monitoring networks has been discussed in a further
    publication (World Health Organization, 1977). However, the bases of
    these and some other methods, that have been used in reporting
    concentrations of sulfur oxides and suspended particulates in the air,
    are discussed below to provide a better understanding of the
    measurements cited in epidemiological studies. It is important to
    ensure that any measurements that are made are supervised by someone
    competent in the field of air pollution monitoring, and the methods
    used must be reported together with the results.


        Table 3.  Methods of analysis for sulfur dioxidea
                                                                                                                      

    Method                             Principle                                     Comment
                                                                                                                      

    Pararosaniline      Absorption of sulfur dioxide in solution of         Uses simple apparatus, and suitable
       methodb          potassium tetrachloromercurate (TCM);               for sampling periods ranging from
                        complex formed reacts with pararosaniline           30 min to 24 h; samples should be
                        and formaldehyde to produce a red-purple            analysed soon after collection;
                        colour, determined colorimetrically (West &         specific for sulfur dioxide, and
                        Gaeke, 1956).                                       possible interference from oxides of
                                                                            nitrogen and some metals can be
                                                                            eliminated (Pate et al., 1965;
                                                                            Scaringelli et al., 1967) Widely used
                                                                            in USA.
                                                                                                                      

    Acidimetric         Simple apparatus, often combined with smoke         Absorption of sulfur dioxide in
       methodb          filter (see the Organization for Economic           dilute hydrogen peroxide solution;
                        Cooperation and Development filter soiling          the sulfuric acid formed is titrated
                        method in Table 5); suitable for sampling           against standard alkali (Organization
                        periods of 24-h, or less in some circumstances      for Economic Cooperation and
                        (e.g. high pollution episodes, or occupational      Development, 1965).
                        environments).
                                                                                                                      

    Conductivity        Sulfur dioxide is sampled in deionized water        Simple apparatus, suitable for
       measurements     containing hydrogen peroxide where it is            sampling periods of the order of
                        oxidized to sulfuric acid, as in the acidimetric    24-h, usually combined with a filter
                        method; increase in conductivity measured           to remove particulate matter; less
                        with a conductivity bridge (Adams et al., 1971).    reliable than acidimetric method,
                                                                            and not widely used in manual
                                                                            form, but the principle often used
                                                                            in automatic instruments (Derrett
                                                                                                                      

    Table 3.  (cont'd).
                                                                                                                      

    Method                             Principle                                     Comment
                                                                                                                      

    Conductivity                                                            & Brown, 1978), applicable also to
       measurements                                                         simple portable instruments for
       cont'd.                                                              spot checks in urban or industrial
                                                                            environments (Nash, 1964), and to
                                                                            personal samplers for assessing
                                                                            occupational exposures (Sherwood,
                                                                            1969).
                                                                                                                      

    Detector tube       Air is drawn through tubes containing silica        Portable, and no power supply
       measurements     gel impregnated with indicator sensitive to         required. Widely used for spot
                        sulfur dioxide; concentration assessed from the     checks in occupational environments,
                        length of the stain (Ash & Lynch, 1972).            or in other situations where
                                                                            the concentrations may be high
                                                                            (from about 3000 g/m3 upwards).
                                                                                                                      

    Iodine              Sulfur dioxide absorbed in a solution of iodine     Applicable to occupational
       method           contained in a wash bottle with a fritted bubbler,  environments, but not now widely used;
                        and solution titrated with thiosulfate (Elkins,     the method has been modified for
                        1959).                                              colorimetric assessment, providing
                                                                            a basis for portable instruments for
                                                                            survey use (Cummings & Redfearn,
                                                                            1957).
                                                                                                                      

    Table 3.  (cont'd).
                                                                                                                      

    Method                             Principle                                     Comment
                                                                                                                      

    Automatic           Based on conductivity, colorimetry, coulometry      Particularly valuable for following
       instruments      flame photometry, or gas chromatography             short-term variations in
                        (Hollowell et al., 1973).                           concentration, but difficult to
                                                                            assess 24-h average concentrations,
                                                                            unless linked with data processing
                                                                            equipment; instruments expensive, and
                                                                            must be under the control of
                                                                            experienced operators.
                                                                                                                      

    Sulfation           Sulfur compounds in the air react with an           Simple and requires no power
       rate             exposed cylinder or plate covered with a paste      supply; sampling period long (30
                        containing lead peroxide; sulfate formed is         days); results expressed in
                        determined by precipitation with barium             SO3/100cm2/day, indicating rate
                        chloride (British Standards Institution, 1969a).    of reaction of sulfur compounds
                                                                            with surfaces; not specific for sulfur
                                                                            dioxide, does not indicate
                                                                            concentrations in the air, and although
                                                                            often quoted in epidemiological
                                                                            studies, of little value for these.
                                                                                                                      

    a  From: Pate et al., (1965)
    b  Methods that are described fully in the manual of the World Health Organization (1976a).

    

    2.2.1  Sulfur dioxide

        If sulfur dioxide were the only contaminant of the air and
    providing the samples were of adequate size, each of the methods
    mentioned in Table 3 would give comparable results, indicating the
    true amount of sulfur dioxide. In normal urban environments, however,
    other pollutants are always present and although the sampling
    procedure can be arranged to minimize interference from particulate
    matter by filtering the air first, errors can still arise due to the
    presence of various gases and vapours. The choice of method depends on
    many factors, including the averaging time required: 24-h sampling
    periods are commonly used, and many of the methods are suitable for
    this. For shorter periods the choice is more limited, and for detailed
    information on short-term variations in concentration, instrumental
    methods are required.

        In occupational environments, the mixture of air pollutants may be
    simple and more clearly defined than that in urban air. Sulfur dioxide
    may be emitted from a specific process rather than from a variety of
    combustion sources, and the air may then be relatively free from
    interfering gases. Concentrations may also be much higher than those
    encountered in urban air, allowing short sampling periods to be used,
    and, since concentrations are liable to change rapidly, this may even
    be essential. Also, concentrations may vary greatly over short
    distances, depending on the proximity of the source of pollution; this
    makes the assessment of exposure based on measurements at fixed sites
    difficult. There may be a preference in these circumstances for
    methods suitable for use with portable instruments.

    2.2.2  Suspended sulfates and sulfuric acid

        Most of the methods mentioned in Table 4 assess the total
    water-soluble sulfates collected on filters as part of the suspended
    particulates. In general, any sulfuric acid present is included with
    this, and some of the material present as acid in the air may be
    converted to neutral sulfate on the filter during sampling. There is
    no completely satisfactory method for the determination of sulfuric
    acid in the presence of other pollutants, but some procedures for
    examining the acidic properties of suspended particulates have been
    referred to. There is an urgent need for more research in this field.
    No methods, other than those mentioned in Table 4, are, as yet,
    sufficiently well established for widespread application in
    epidemiological studies, but much research work is in progress.



        Table 4.  Methods of analysis for suspended sulfates and sulfuric acid
                                                                                                            

    Method                             Principle                                     Comment
                                                                                                            

    Turbidimetric       Sample collected on sulfate-free glass              Samples normally collected
    method              fibre or other efficient filter: sulfate            over 24-h periods by high
                        extracted and precipitated with barium              volume sampler (see Table 5).
                        chloride, measuring the turbidity of the            No distinction made between
                        suspension spectrophotometrically (US               sulfates and sulfuric acid.
                        Environmental Protection Agency,
                        1974b).

    Methylthymol        Samples collected as in the turbidimetric           This modification allows the
    blue method         method above and extract reacted with               procedure to be automated,
                        barium chloride, but barium remaining               comments as in the turbidimetric
                        in solution then reacts with methylthymol           method apply.
                        blue; sulfate determined colorimetrically
                        by measurement of uncomplexed methylthymol
                        blue (US Environmental Protection Agency,
                        1974b).

                                                                                                            
    


        The most recent trends are towards the application of more
    sensitive techniques, such as X-ray fluorescence (Dzubay & Stevens,
    1973), or the thermal conversion of sulfates, measuring the resulting
    sulfur dioxide by flame photometry (Husar et al., 1975), or by the
    pararosaniline method (Maddalone et al., 1975).

        Approaches to the difficult problem of determining sulfuric acid
    have been made by back-titrating a sodium tetraborate extract of
    suspended particulates collected on a small filter paper (Commins,
    1963), and by observing the acidic properties of individual particles
    collected on indicator-treated slides in a cascade impactor (Waller,
    1963). A procedure for the separate determination of sulfuric acid and
    ammonium sulfate by nephelometry of a humidified sample of air has
    also been described (Charlson et al., 1973) and work is in progress on
    the prevention of the reaction of sulfuric acid on filter papers
    (Thomas et al., 1976). However, at present, there is not enough field
    experience with methods for sulfuric acid to warrant their general use
    in connexion with epidemiological studies.

    2.2.3  Suspended particulate matter

        In general, it is not practicable to discriminate on the basis of
    either particle size or chemical composition when assessing
    particulate matter for routine monitoring purposes. The
    characteristics of the sample are determined by the types of sources
    in the vicinity, the weather conditions, and the sampling procedure
    adopted. The difficulties that result and the limitations of
    measurements have been discussed by Ellison (1965)and are illustrated
    in the discussion of the merits and shortcomings of the various
    methods described below and in Table 5.

        When considering measurements of suspended particulate matter, it
    is essential to specify the method used and to recognize that, even
    then, results obtained in one set of circumstances will not
    necessarily be applicable to others. The main difficulty has arisen in
    attempts to apply findings based on smoke measurements that relate
    only to the dark coloured material characteristic of the incomplete
    combustion of coal or other hydrocarbon fuels, to situations involving
    total suspended particulates assessed more directly in terms of
    weight. Because the former have been used in much of the early
    epidemiological work and the latter are now used for monitoring
    purposes in many countries, some kind of conversion from one type of
    measurement to the other would be desirable, but, for the reasons
    already stated, there can be no generally applicable conversion
    factor. Comparative evaluation of the two methods has been undertaken
    at a number of sites (Ball & Hume, 1977; Commins & Waller, 1967; Lee
    et al., 1972), but the results have only served to emphasize that they
    measure different qualities of the particulate matter and that they
    should not be compared with one another.



        Table 5.  Methods of analysis for suspended particulate matter
                                                                                                                           

    Method                                   Principle                                    Comment
                                                                                                                           

    Smoke measurement:        Air is drawn through a white filter paper,         Widely used in Europe and recommended
    Organization for          usually over periods of 24 h, and the darkness     by the Organization for Economic
    Economic Cooperation      of the stain obtained measured by reflectometer;   Cooperation and Development (1965);
    and Development filter    values converted to equivalent international       low intake velocity ensures sample
    soiling method            smoke units, expressed conventionally, in          restricted to respirable size range; often
                              g/m3; simple apparatus, suitable for              combined with sulfur dioxide measurement
                              continuous operation.                              by acidimetric method (see Table
                                                                                 2-3); results influenced primarily by
                                                                                 black material do not necessarily
                                                                                 represent true weights; only a limited
                                                                                 range of chemical analyses possible on
                                                                                 these small samples.
                                                                                                                           

    Smoke measurement:        Similar to the Organization for Economic           Flow rate a little greater than in the
    American Society          Cooperation and Development filter soiling         Organization for Economic Cooperation
    for Testing and           method, but samples collected on a filter          and Development filter soiling method
    Materials filter          paper tape moved on automatically to provide       but sample still effectively within
    soiling method            a series of stains over intervals of 2-6 h         respirable size range used in USA;
                              (American Society for Testing and Materials,       interrelationships between Coh units and
                              1964); results usually assessed by transmittance,  RUDS investigated by Saucier & Sansone
                              and expressed in coefficient of haze (COH)         (1972); suitable for continuous
                              units (Hemeon et al., 1953; reflectance            operation.
                              has sometimes been used, expressing results
                              in reflectance units of dirt shade (RUDS)
                              (Gruber & Alpaugh, 1954).
                                                                                                                           

    Table 5.  (cont'd).
                                                                                                                           

    Method                                   Principle                                    Comment
                                                                                                                           

    Determination of          Air drawn through a glass fibre filter sheet,      Widely used in USA. Liable to collect
    total suspended           usually with a turbine blower, and the amount      particles well beyond the respiratory size
    particulates,             of material collected determined by weighing       range and this may bias results,
    gravimetric high          under controlled temperature and humidity          particularly in dry, dusty locations;
    volume                    conditions; the most widely used instrument        not very suitable for continuous
                              is the high volume sampler (US Department          operation, samples commonly collected
                              of Health, Education and Welfare, 1962), but       over 24-h periods every sixth day;
                              instruments based on rotary pumps with             samples large enough for a wide range
                              membrane rather than glass fibre filters have      of chemical analyses.
                              been used (Verein Deutscher Ingenieure,
                              1974).
                                                                                                                           

    Indirect determination    Series of samples collected on filter paper        Instrument relatively expensive; used for
    of mass concentration:    strip over selected periods (usually 30 min),      monitoring purposes in Federal Republic
     ray sampler             and mass of material determined by                 of Germany, but not to any large extent
                              attenuation of  radiation from a built-in         elsewhere; valuable for studying
                              source (Husar, 1974).                              short-term variations in total
                                                                                 suspended particulates.
                                                                                                                           

    Light scattering          Direct determination of suspended particulate      Used to some extent in Japan for monitoring
                              matter as aerosols by light scattering, either     suspended particulate matter, but
                              counting and sizing individual particles (Liu et   calibration required and results not
                              al., 1974) or integrating light scattered from     necessarily comparable with those from
                              given volume of air (Horvath & Charlson,           direct weighings; otherwise main
                              1969).                                             application in industrial environments,
                                                                                 some instruments allow particles to be
                                                                                 counted and classified within a large
                                                                                 number of size ranges.
                                                                                                                           

    Table 5.  (cont'd).
                                                                                                                           

    Method                                   Principle                                    Comment
                                                                                                                           

    Size selective            Particles separated into several roughly size-     Allows concentrations to be assessed
    sampling:                 graded fractions by impaction, the amount of       within specified size ranges; some series
    modified cascade          material in each being determined by direct        of results available from USA but not
    impactor                  weighing (Carson & Paulus, 1974).                  yet widely adopted; applicable also to
                                                                                 the sampling of dusts in industrial
                                                                                 environments.
                                                                                                                           

    Electrostatic             Particles charged by passing through metal         Not suitable for outdoor measurements,
    precipitators             tube with large potential gradient between         but useful in occupational environments;
                              wall and needle along centre; deposited on         advantage over direct weighing of filters
                              wall and determined by direct weighing             is that the collector is unaffected by
                              (Lauterbach et al., 1954).                         changes in humidity.
                                                                                                                           

    Personal samplers         Air drawn through small glass-fibre filters        Applicable primarily to industrial
                              using battery operated pump, so that               environments to assess exposures in
                              instrument can be worn by individuals              series of working shifts; elutriator can be
                              (Sherwood & Greenhalgh, 1960); particulates        added to exclude large particles.
                              assessed by weighing, or analysed for specific
                              constituents.
                                                                                                                           

    

        From their study in central London, Commins & Waller (1967) showed
    that additional material was collected by the high volume sampler that
    had little effect on smoke measurements and that for their particular
    series, the total suspended particulate results were approximately
    100 g/m3 higher than the corresponding smoke figures. Other authors
    have calculated regression equations for their series and, although
    there are variations in these relationships with time and place, the
    general picture is of a large proportional difference between total
    suspended particulate and smoke figures at low values, but relatively
    little difference at high values (of the order of 500 g of smoke/m3
    or more).

        Thus, it is recommended that "smoke" and "total suspended
    particulate" measured by the various methods described should be
    regarded as separate entities; this principle has been adopted in
    later sections relating to the effects on health.

        In occupational environments, suspended particulate matter from
    combustion sources may be of some concern, but more commonly it is the
    dusts and aerosols associated with particular occupations or processes
    that are of interest. In such cases, the composition of the material
    may be relatively uniform and well established, and specific methods
    of assessment can be devised. There has, for example, been a great
    deal of research and development on methods for determining dust
    concentrations in coal mines (Jacobsen, 1972). With industrial dusts,
    the particle size distribution must always be considered carefully,
    for it is liable to extend beyond the respirable range, and
    elutriators or cyclones may be needed in conjunction with gravimetric
    samplers. A valuable discussion of methods for collecting size-graded
    samples has been included in a recent review (International Atomic
    Energy Agency, 1978).

    2.2.4  Dustfall (deposited matter)

        In some of the older epidemiological studies, measurements of
    dustfall were quoted as an index of particulate pollution. This is
    inappropriate as the results are influenced primarily by large
    particles (diameters from about 10 m upwards: see section 2.1.2) that
    do not penetrate the respiratory system and, generally, are not
    relevant to health problems, apart from possible annoyance reactions.
    For reference purposes, however, a brief description of one of the
    more commonly used instruments is included (Table 6).

        Table 6.  Method of analysis for dust fall
                                                                                             

    Method               Principle                         Comment
                                                                                             

    Deposit   A receiver containing a                 Results expressed in terms
    gauge     nonfreezing solution is left in         of deposit per unit area and time,
              the open and the quantity of            not convertible in any way to
              material collected (usually             concentrations of suspended matter
              over 1-month periods) is determined     in the air; strongly influenced by
              by weighing, water-soluble and          sources nearby, hence results only
              insoluble components being considered   relevant to immediate vicinity
              separately (British Standards
              Institution, 1969b)
                                                                                             

    
    3.  SOURCES OF SULFUR OXIDES AND PARTICULATE MATTER

    3.1  Natural Occurrence

        Compounds of sulfur are found in small quantities in ambient air,
    even in remote areas far from sources of pollution. In the gas phase,
    they are present as hydrogen sulfide or sulfur dioxide, and in
    particulate form they may be present as sulfate. Sulfur dioxide and
    hydrogen sulfide are emitted by volcanoes and the latter is also
    produced by anaerobic bacteria in soil, marshes, and tidal flats (Grey
    & Jensen, 1972). Some of the particulate sulfate may also be emitted
    directly by volcanoes or sea spray, but most of it is the end-product
    of the oxidation of hydrogen sulfide or sulfur dioxide.

        In general, suspended particulate matter can result from volcanic
    activity, from dust storms, or from strong winds blowing over dry soil
    and may include pollen from trees and other plants. Forest fires also
    produce large amounts of particulate matter.

        Some of these natural contributions to the particulate matter in
    the air consist of particles too large to remain in suspension for
    long periods, and their composition and properties may be quite
    different from those of the emissions from man's activities.

    3.2  Man-made sources

        Most emissions of sulfur into the air are in the form of sulfur
    dioxide resulting from the combustion of fossil fuel for heating and
    energy production. Various industrial activities such as petroleum
    processing, smelter operations, wood-pulping, etc., also produce
    significant emissions of sulfur dioxide and other sulfur compounds.

        It has been estimated (Robinson & Robbins, 1968) that on a
    worldwide scale about 146  106 tonnes of sulfur dioxide are emitted
    annually, 70% of which result from coal burning, 16% from the
    combustion of petroleum products, and the remainder from petroleum
    refining and nonferrous smelting. These estimates are based mainly on
    1965 world figures for coal production, petroleum refining, and
    smelter operations, each combined with an estimate of a sulfur dioxide
    "emission factor" per unit of production. A similar basis was used for
    the Committee on the Challenges of Modern Society (1971) assessment of
    emissions in the northern and southern hemispheres, reproduced in
    Table 7.

    Table 7.  Hemispheric sulfur dioxide emmissions due to man's
              activities (106 tonnes per year)a
                                                                    

    Source                Total      Northern       Southern
                                     Hemisphere     Hemisphere
                                                                    

    Coal                  102        98   (96%)      4   (4%)
    Petroleum,
      combustion
      and refining         28.5      27.1 (95%)      1.4 (5%)

    Smelting, copper       12.9       8.6 (67%)      4.3 (33%)
      lead                  1.5       1.2 (80%)      0.3 (20%)
      zinc                  1.3       1.2 (90%)      0.1 (10%)
                                                                    

    Total                 146       136   (93%)     10   (7%)
                                                                    

    a From:  Committee on the Challenges of Modern Society (1971).


        On a global scale, the emissions of sulfur compounds into the
    atmosphere by man-made activities are about equal to those from
    natural sources. On the other hand, the emissions from man's
    activities are the main contributors to pollution in large cities and
    their surrounding areas. Assuming that world energy demand increases
    at its historic rate, the total emissions of sulfur dioxide will
    increase unless appropriate control measures are applied, or there
    is a shift from the use of fossil fuels to the use of nonpolluting
    energy sources. However, with the stabilization of the population in
    some countries, including the United Kingdom and USA, and increasing
    concern about the use of limited fuel reserves, there are prospects
    that the rate of increase in emissions may be reduced in some parts
    of the world.

        Combustion and industrial processes are also prime sources of
    particulate emissions. As with sulfur dioxide, the burning of fuel
    (especially coal) for heating and for the generation of power has been
    one of the major contributors to the suspended particulate matter in
    urban air. Vehicular traffic also generates dust from the road and
    from the wear of tyres as well as particulate lead compounds from the
    exhausts of petrol-engined vehicles, and black smoke from those of
    diesel vehicles. The incineration of domestic and industrial refuse
    may disperse particulate matter and other pollutants into the air
    unless carefully controlled. Table 8 shows estimates of the global
    emission of all particulate matter (Robinson & Robbins, 1968).

    Table 8.  Global emission of all particulate matter (106 tonnes
              per year)a
                                                                    

    Man-made

        Particles                                             92
        Gas-particle conversion:    sulfur dioxide           147
                                    oxides of nitrogen        30
        Photochemical compounds from hydrocarbons             27
                                                                 
                                                             296

    Natural

        Soil dust                                            200
        Gas-particle conversion:    hydrogen-sulfide         204
                                    oxides of nitrogen       432
                                    ammonia                  269
        Photochemical compounds from terpenes, etc           200
        Volcanic                                               4
        Forest fires                                           3
        Sea salt                                            1000
                                                                 
                                                            2312
                                                                    

    a From:  Robinson & Robbins (1968).


    3.3  Characteristics of Sources

        In urban areas, most of the sulfur dioxide and suspended
    particulate matter in the air come from the combustion of fuels, but
    many factors, including the type of fuel, the combustion efficiency,
    and the flue velocity, influence the quantity and quality of
    emissions. The incomplete combustion of soft coal in domestic fires,
    for example, produces much smoke, consisting of finely divided
    particles of carbon and tarry material, whereas the efficient burning
    of pulverised coal in a power station leads to little or no smoke, but
    the production of coarser ash particles, which must be removed at
    source to avoid their being carried up the flues at a high velocity
    and dispersed into the air. The relationship between types of sources
    and emissions is summarized in Table 9.

        Table 9.  Pollutants from combustion sources
                                                                                                

    Type of source                 Fuel                 Sulfur dioxide      Particulate matter
                                                                          Smoke        Ash etc.
                                                                                                

    Domestic heating         Wood, peat etc.                   -            +             +
      or cooking             Soft coal                        ++           ++             +
                             Hard coal, coke                  ++            -             +
                             Oil (light distillates)           +            -             -
                             Gas                               -            -             -
    Industrial heating       Coal, coke                       ++            -            ++
      and power generation   Oil (heavy residuals)            ++            -             -
    Motor vehicles           Petrol                            -            -             +
                             Diesel                            +            +             -
                                                                                                

    Notes:  The term smoke is used for incomplete combustion products (notably carbon and tar),
            and ash for inorganic components from complete combustion (including lead compounds
            in the case of petrol). The signs give only a rough indication of emissions, in the
            absence of direct control at source:
            - = little or none, + = moderate quantities, ++ = large quantities

    
        In cold and temperate parts of the world, the burning of coal for
    domestic heating purposes has been a major contributor to both the
    sulfur dioxide and suspended particulate contents of urban air. This
    is particularly true of the situation in the United Kingdom prior to
    the implementation of the Clean Air Act (Committee on Air Pollution,
    1954). Such sources are liable to have a disproportionate effect on
    concentrations in the immediate vicinity, because of the low levels of
    the chimneys and the low emission velocity. Even in warmer climates,
    domestic sources may be of importance, particularly if coal is used
    for cooking purposes.

        In densely populated areas where domestic sources dominate, the
    many chimneys can be considered for some purposes as a diffuse area
    source, and, within such an area, concentrations of pollutants in air
    remain relatively stable over short distances and short periods of
    time. In contrast, large industrial sources should be treated as point
    sources, and, at any given location around them, the concentration of
    air pollution is liable to vary greatly, even from minute to minute.

        The emission into the air of sulfur dioxide and particulate matter
    from motor vehicles is relatively small in comparison with those from
    domestic and industrial chimneys but it is close to the ground and
    within the breathing zone. In these circumstances, concentrations vary
    greatly over short distances as well as over short time intervals,
    depending on the proximity of the traffic. At points very close to
    mixed traffic, smoke from diesel engines may make a substantial
    contribution to the concentration of suspended particulate matter in
    air (Waller et al., 1965).

        Source strength may vary with time of day, day of week, and season
    of the year. Accompanying meteorological conditions are also important
    in determining the ultimate air concentrations of pollutants arising
    from sources. Where heating is required during the winter season,
    emissions of sulfur dioxide and particulate matter are usually much
    higher than they are in the summer. In a number of cities, however,
    where a considerable amount of fuel is used for running cooling
    systems, emissions of these pollutants during the summer time are not
    always lower than those in winter. Some industrial sources of
    pollution may emit little at weekends, and emissions for most sources
    are at a minimum during the night.

        Although the control of emissions is outside the scope of the
    present discussion, the general point can be made that while the
    control of particulate emissions is a practicable proposition in many
    circumstances, the control of sulfur dioxide at source is relatively
    difficult and costly, and the more effective means of reducing
    emissions is to change to fuels with a lower sulfur content.

    4.  DISPERSION AND ENVIRONMENTAL TRANSFORMATIONS

        Sulfur compounds dispersed into the air eventually return to the
    land or oceans either unchanged, or converted into sulfates. The
    sulfur cycle so set up is shown diagrammatically in Fig. 4 (Kellogg et
    al., 1972). Particulate matter also returns, its residence time in the
    air varying widely according to its physical and chemical
    characteristics. As far as direct effects on health are concerned, it
    is the local concentration of pollutants in the air at a given time
    that is important, as discussed in section 5, but an outline of
    dispersion and transformation phenomena is given here.

    4.1  Dispersion

        The maintenance of a tolerable environment in modern towns depends
    very much on the ability of wind and turbulence to disperse the
    pollutants rapidly as they are emitted. When these processes fail, the
    results can be disastrous, as they were in London in 1952 (Ministry of
    Health, United Kingdom, 1954). There are some localities where natural
    ventilation is so poor that the emission of pollutants must, at all
    times, be carefully controlled. This is especially true of the Los
    Angeles area, where emissions of sulfur dioxide and particulate matter
    have been successfully curtailed, leaving, however, the major problems
    associated with the emission of oxides of nitrogen, hydrocarbons, and
    carbon monoxide from motor vehicles (Goldsmith, 1969).

        Factors affecting the dispersion of sulfur dioxide and particulate
    matter from combustion sources, include:

         (a) Temperature and efflux velocity of the gases. Emissions from
    small sources, such as domestic fires or incinerators have relatively
    little buoyancy, since the temperature at the point of emission is not
    much greater than that of the surrounding air. Such sources are liable
    therefore to have their greatest impact in the immediate vicinity
    (Williams, 1960). Emissions from large-scale industrial installations,
    on the other hand, may be at higher temperatures, or may be assisted
    by forced-draught to rise more rapidly. Thus, any major impact in the
    immediate vicinity may be avoided but weaker effects may be produced
    over a wider area (Bosanquet, 1957).

         (b) Stack height. Dilution and dispersion over a wide area is
    also aided by the use of tall stacks. Much is known of the
    relationship between source strengths, stack heights, and ground-level
    concentrations of pollutants, through the application of mathematical
    modelling techniques, coupled with observations around selected
    sources (Briggs, 1965, Pasquill, 1971, Turner, 1968). It is also
    possible to devise models to predict concentrations of sulfur dioxide
    in urban areas on the basis of emissions from multiple sources

    (Fortak, 1970). Conversely, techniques have also been developed for
    estimating the pollution inventory of an area from measurements of
    sulfur dioxide concentration, fitted to a dispersion model (East,
    1972). The height of emission of sulfur dioxide and particulate matter
    from domestic sources is primarily a function of the height of the
    building itself. Thus the effect on ground level concentrations in the
    vicinity is liable to be greater in areas with closely-packed, single
    or two-storey houses than in those with high-rise apartments. Tall
    stacks are widely used for electricity-generating stations and other
    major industrial sources, but the pollutants may then be carried great
    distances, often over national boundaries, to be deposited eventually
    far from their source (Royal Ministry of Foreign Affairs, Sweden,
    1971; Zeeduk & Velds, 1973).

    FIGURE 4

         (c) Topography and the proximity of other buildings. The
    presence of hills or tall buildings, and many other features of the
    landscape, have important effects on the dispersion of plumes from
    individual stacks, or of the pollution from an area source as a whole.
    Many industrial cities have developed in river valleys, initially to
    take advantage of water transport, but, in general, the dispersion of
    pollutants in such a situation is poorer than it would be from a more
    exposed location.

         (d) Meteorology. Meteorological factors are of fundamental
    importance in determining the whole spatial and temporal distribution
    of pollution, and the subject has been well reviewed in a recent
    publication (Munn, 1976). Apart from the general influence of the
    local climate, the great variability of the weather in any one
    locality is liable to lead to considerable changes in the
    concentrations of sulfur dioxide. In particular, temperature
    inversions can trap these pollutants to produce concentrations up to
    several hundred times the usual values (Waller & Commins, 1966).

    4.2  Transformation and Degradation

        In recent years, there has been a rapid escalation of interest in
    the ultimate fate of sulfur dioxide and particulate matter emitted
    into the air. This is concerned partly with the nature of reaction
    products and their possible effects on health, and also with the
    ecological effects of these products when deposited (Brosset, 1973)
    and their possible role, as aerosols, in modifying the climate on a
    global scale (Hobbs et al., 1974).

        Some of the sulfur dioxide emitted into the air is removed
    unchanged by various surfaces, including soil (Abeles et al., 1971),
    water (Liss, 1971; Spedding, 1972), grass (Garland et al., 1973) and
    vegetation in general (Hill, 1971). It has been estimated that, in the
    United Kingdom, about 25% of the sulfur dioxide is removed by these
    direct ("dry" deposition) processes (Garland et al., 1974). The
    remainder is transformed into sulfuric acid or sulfates by a variety
    of processes, in the presence of moisture, and is then mainly washed
    out in rain. Although this self-cleansing process limits the build-up
    of sulfur compounds in the air, so minimizing the effects on health,
    the "acid rain" produced is considered to be a serious general
    environmental problem in some areas (Likens & Bormann, 1974).

        A schematic representation of a natural sulfur cycle is shown in
    Fig. 5 (Kellogg et al., 1972). Additions to this cycle due to man's
    activity are possible at each stage, although 95% of such
    contributions are added as sulfur dioxide. Also represented here are
    the possible reactions with sunlight. There have been many laboratory
    investigations of this process: the reaction is slow (Allen et al.,
    1972; Cox & Penkett, 1970), but it is enhanced in the presence of
    hydrocarbons and other pollutants associated with motor vehicle
    emissions (Cox & Penkett, 1971; Wilson & Levy, 1970). In general,
    however, other processes are of even greater importance in the
    transformation and removal of sulfur dioxide, including reactions in
    water droplets with ammonia (McKay, 1971), and catalytic oxidation in
    the presence of manganese or iron (Barrie & Georgii, 1976; Chun &
    Quon, 1973). These various reactions involving sulfur dioxide,

    FIGURE 5

    particulate matter, and other pollutants have been discussed in a
    number of reviews (Bufalini, 1971; Calvert, 1973). There may be
    limitations to the catalytic processes because of the restricted
    availability of reactive metallic oxide, catalytic particles, and
    neutralizing compounds in the air. Some of the photochemical reactions
    are severely rate-limited, but in others, sulfur dioxide can be
    oxidized at an appreciable rate and it has been estimated that
    conversion rates as high as 18% per hour might be possible (Rall,
    1974). The end-products are similar in all these reactions, i.e., the
    formation of aerosols, initially in the submicron size-range,
    consisting of a mixture of sulfates and sulfuric acid. There is no
    uniform relationship between the proportions of sulfur dioxide,
    sulfates, or sulfuric acid in the total sulfur pollution. Emissions
    are primarily in the form of sulfur dioxide; thus, in cities close to
    sources, the major proportion is in this form, but it has been
    reported that, in the USA, the proportion present as sulfate is higher
    in western than in eastern urban areas (Altshuller, 1973). At nonurban
    sites, concentrations of sulfates may be similar to those of sulfur
    dioxide.

        The overall conversion of sulfur dioxide to sulfate is an
    extremely complex process with many interrelated variables that are
    poorly characterized. These include the absorption rate of sulfur
    dioxide, the sizes of the particles or droplets involved, their
    chemical composition, the rate of diffusion of reactants within the
    aerosol, and the relative humidity. The last of these variables is a
    major factor, as the catalytic reactions occur with water droplets
    containing absorbed sulfur dioxide and other pollutants. Furthermore,
    as the pH decreases, the rate of oxidation of sulfur dioxide also
    decreases (Junge & Ryan, 1958). Thus, the formation of sulfuric acid
    tends to be self-limiting unless the fall in pH is offset by
    additional water vapour. On the other hand, the presence of alkaline
    compounds, such as ammonia, in the droplet can enhance the reaction
    rate due to its buffering capacity. Extrapolated levels for the rate
    of oxidation of sulfur dioxide by catalytic processes in urban air
    range upwards from 2% per hour (Rall, 1974). Overall, the half-life of
    sulfur dioxide in ambient air is estimated to be three to five hours.

        The physical and chemical forms of suspended particulate matter in
    general may be changed in the air. Some components, such as
    hydrocarbons, absorbed initially onto particulate matter, may
    evaporate or be oxidized. Even some of the complex hydrocarbons in the
    tarry matter from coal burning may be volatile enough to be lost
    gradually, and there is evidence that they can be lost from filters
    during sampling (Commins & Lawther, 1958). The sizes of the particles
    may vary according to the relative humidity, particularly if sulfuric
    acid, sulfates, or other salts are present, and this can lead to
    precipitation even before the onset of rain (Waller, 1963). The
    question of overwhelming importance is the role of particulate matter
    in the conversion of sulfur dioxide to sulfuric acid and sulfates.

    Traces of metallic compounds, some of which serve as catalysts in
    these reactions, are present in particulate matter from the combustion
    of coal, and also in the relatively small amount of particulate matter
    that may come from the combustion of oil. It has long been considered
    that the acute effects on health seen in episodes of high pollution
    are crucially dependent on the mixture of sulfur dioxide and
    particulate matter present, together with the relative humidity: the
    worst effects have been seen with each of these factors in a high
    range (Ministry of Health, United Kingdom, 1954).

    5.  ENVIRONMENTAL CONCENTRATIONS AND EXPOSURES

        Sulfur dioxide and suspended particulate matter are the most
    widely monitored air pollutants. National sampling networks exist in
    many of the industrialized countries of the world, and summaries of
    the observations are commonly published in annual reports. Results
    from selected sampling sites are also collated by a number of
    international organizations (Commission of the European Communities,
    1976; Pan American Health Organization, 1976; World Health
    Organization, 1976b).

    5.1  Concentrations in Outdoor Air

        Most sampling networks for sulfur dioxide, smoke, and suspended
    particulate matter have been set up for control purposes, to examine
    the distribution of these pollutants in various areas and to follow
    the long-term trends. Such measurements are normally made on outdoor
    air. Where adequate networks exist, there is obviously an advantage in
    trying to use them to assess exposure, but this may be far from ideal.
    The limitations of the data obtained from the usual monitoring sites
    may, to some extent, account for inconsistencies in results in studies
    reviewed in section 8. Requirements for monitoring networks have been
    discussed further in a recent report (World Health Organization,
    1977).

        Concentrations of sulfur dioxide and suspended particulate matter
    vary greatly from one area to another depending on the nature and
    intensity of local sources, and on other factors such as topography,
    general weather conditions, and liability to temperature inversions.
    Even within a single city there may be large differences in
    concentrations. Where sufficient monitoring stations exist, it may be
    possible to construct isopleths, showing "contours" of equal
    concentration. An example, for the city of Antwerp, is shown in Fig. 6
    (Derouane et al., 1972). From this, it is clear that the distribution
    of sulfur dioxide is not necessarily the same as that of smoke. There
    is a general tendency for the concentrations of these pollutants to be
    highest in the largest cities of the world, and within them for the
    highest concentrations to be in the central areas. The implementation
    of control measures has, however, changed the situation in recent
    years, for, in some instances, these have been applied most vigorously
    in the central areas of large cities (Masters, 1974).

    FIGURE 6

        Table 10.  Concentrations of sulfur dioxide, smoke, and suspended particulate
               matter (1974)a
                                                                                   

         Site                             Concentration (g/m 3)
                                annual arithmetic mean     maximum daily mean
                                                                                   

    Sulfur dioxide

         Brussels                        107                      347
         Frankfurt                       119                      455
         London                          150                      503
         Madrid                          161                      763
         Prague                          126                      482
         Rome                            108                      600
         Zagreb                          173                      893

    Smoke, by reflectance

         Brussels                         37                        -
         London                           26                      149
         Madrid                          190                      908
         Rome                             60                      160

    Suspended particulate by high volume sampler

         Calcutta                        519                     1090
         St Louis                         87                      189
         Vancouver                        64                      134
         Zagreb                          167                      806
                                                                                   

    a From:  World Health Organization (1976b). Sites selected for inclusion
             here are all classified as city centre commercial sites. The
             selection is also limited to sites using 24-h averaging periods.
             Further information is available in the WHO report of frequency
             distributions, standard deviations, and monthly and annual
             geometric means.

    
    Examples of concentrations of sulfur dioxide and smoke or suspended
    particulate matter, drawn from the WHO air quality monitoring
    programme (World Health Organization, 1976b) are shown in Table 10.
    For present purposes, results included in this table are limited to

    those from one type of site (city centre commercial sites), where the
    sampling methods and averaging periods are also comparable with one
    another. Even so, much caution must be exercised in drawing
    comparisons between cities, for the sites can only be representative
    of their immediate surroundings.

        The annual mean concentrations of sulfur dioxide are fairly
    uniform at the particular sites quoted in Table 10, ranging from about
    100 to 200 g/m3 (0.035-0.070 ppm). For particulate matter, however,
    the variation between cities appears to be much greater and it seems
    likely that some of the results are unduly influenced by sources close
    to the samplers, or by high background levels of dust from
    noncombustion sources.

        There are few reports about ambient levels of sulfates. A study
    from the USA (Altshuller, 1973) reported annual, arithmetic mean
    concentrations at urban sites in the range of 2.4 to 48.7 g/m3, with
    an average ratio of sulfur dioxide to sulfate of 4.7. The relationship
    between these two pollutants was not, however, entirely consistent and
    the ratio tended to be higher at western sites than at eastern sites.
    In the east, there was a general background level of sulfate of about
    5 g/m3, even at nonurban sites, and this was attributed to the long
    distance transport of sulfur dioxide, with conversion to sulfate
    during transport. There also appeared to be a "saturation" level of
    sulfate at about 17 g/m3 at eastern urban sites, within the sulfur
    dioxide range of 100-200 g/m3 (0.035-0.070 ppm).

        There is even less published information concerning concentrations
    in air of sulfuric acid, and such observations must always be related
    to the method of measurement. One series of measurements of net
    particulate acid, as determined by titration of samples collected on
    filter papers, has indicated a mean concentration in London of
    approximately 4 g/m3  representing a few percent of the
    corresponding concentration of sulfur dioxide (Commins & Waller,
    unpublished data). However, the concentration of this pollutant is
    liable to increase rapidly during temperature inversions, particularly
    if the relative humidity is high (Commins, 1967).

        For each of the pollutants considered, there are generally large
    variations in concentration with time at any one place. The extent to
    which this can be followed depends on the time resolution of the
    sampling instruments. Usually, integrated samples are collected over
    24-h periods to yield daily mean values, and from these monthly,
    seasonal, and annual means are calculated. Shorter sampling periods
    may however be used, and where continuous automatic instruments are
    used, virtually instantaneous values can be obtained. Relationships
    between values averaged over different periods have been extensively
    studied in the USA (Larsen, 1971). An indication of the day-to-day
    variation in smoke and sulfur dioxide concentrations in a large city
    (London) is given in Fig. 7.

    FIGURE 7

        Although annual means, coupled with daily maxima, give a general
    impression of pollution levels in any given locality, long series of
    results are often summarized as frequency distributions. These have
    been shown to be log-normal for a wide range of averaging times,
    pollutants, and localities (Pollack, 1975). This suggests that the
    geometric mean, which, in such a case, is equivalent to the median, is
    perhaps the most appropriate central value to use. Historically,
    however, the arithmetic mean has been more widely used. For the usual
    log-normal distribution, the geometric mean is a little lower than the
    arithmetic mean. Percentiles of the frequency distribution are
    tabulated in some monitoring networks (US Environmental Protection
    Agency, 1974a), and the complete distributions can conveniently be
    plotted on log-probability paper, as in the example in Fig. 8 drawn
    from data for a recent 5-year period in London (Commins & Waller,
    unpublished data).

        Relationships between peak and mean values for sulfur dioxide have
    also been considered on an empirical basis. For a number of cities in
    Europe, the highest daily mean concentrations during the year have
    been found to be of the order of four times the annual means
    (Commission of the European Communities, 1976). Transient peaks in
    continuous records have been examined in the USA in relation to
    averaging times: the ratio of peak to mean values has been found to be
    2.3 for hourly averaging periods, increasing for successively longer
    periods (Montgomery & Coleman, 1975). Some highly sophisticated
    networks exist for the measurement of sulfur dioxide on a continuous
    basis at many points. The collection and interpretation of such data
    then presents a formidable task, and in some of these networks on-line
    computers are used for data acquisition (Lauer & Benson, 1975).

        The examination of trends in the concentrations of sulfur dioxide
    and particulate matter is important when the effects of long-term
    exposures are investigated. In urban areas of most developed
    countries, there has been a tendency for levels to decline in recent
    years as a result of control efforts, although elsewhere, and
    particularly where concentrations of these pollutants had previously
    been low, increases have occurred as emissions from industrial and
    other sources have increased. Since variations in weather patterns
    from year-to-year can affect even the annual mean concentrations, long
    series are required to examine trends adequately. The declining trend
    in sulfur dioxide concentrations seen in a number of large cities in
    Europe during the 1960s (Commission of the European Communities, 1976)
    may, to a large extent, reflect declining emissions or improved
    dispersion from chimneys, but some authors have considered that
    changing weather conditions have been a contributory factor (Van Dop &
    Kruizinga, 1976). In the United Kingdom, there has been an overall
    decline in sulfur dioxide concentrations without a corresponding
    decline in total emissions (Fig. 9). This is attributable to the

    FIGURE 8

    FIGURE 9

    gradual elimination of sources, such as domestic fires, that had a
    substantial effect on local concentrations, and their replacement by a
    smaller number of large sources dispersing the sulfur dioxide more
    widely. In the case of smoke, concentrations in the United Kingdom
    have declined in parallel with the emissions (Fig. 10). Domestic fires
    always had a dominant effect, and these have been subject to control
    in an increasing number of urban areas.

    5.2  Concentrations in Indoor Air

        As yet, there is relatively little information available
    concerning the concentrations of sulfur dioxide and particulate matter
    in indoor environments (excluding those specifically related to
    occupational exposures). It is quite possible to make such
    measurements indoors by most of the methods mentioned in section 2,
    subject to additional care about interfering substances, and
    limitations of noise for equipment such as the high volume sampler.
    The results are, however, of limited value for general monitoring
    purposes, because of the additional variability introduced by the
    circumstances within each building. As far as human exposure is
    concerned, much time is spent indoors, particularly by the oldest and
    youngest members of the community, and information on indoor
    concentrations is required for epidemiological studies.

        Whether there are substantial differences in indoor and outdoor
    concentrations of sulfur dioxide and particulate matter will depend on
    the degree of ventilation, the capacity of surfaces within to absorb
    or otherwise collect these pollutants, and the presence of sources
    either of the pollutants themselves, or of others that may interact
    with them.

        In warm climates not subject to frequent rain or other adverse
    weather conditions, buildings may be left open enough to ensure that
    indoor concentrations of pollutants are virtually the same as those
    outdoors. Even so, there can be local problems associated with the use
    of fuels in equipment with poor flues or without flues. Extremely high
    concentrations of pollutants, including smoke, have been reported
    inside primitive dwellings in tropical regions, where cooking is
    carried out over open fires (Sofoluwe, 1968). The open coal fires that
    were so widely used in the United Kingdom prior to the Clean Air Act
    of 1956 had two possible, and opposing, effects on indoor
    concentrations. The very large ventilation rate that they induced
    helped to maintain indoor concentrations of smoke and sulfur dioxide
    close to those outdoors, but, in unfavourable wind conditions,
    downdraughts could force these pollutants from the fire itself into
    the room, producing concentrations far in excess of those outdoors.
    Examples of indoor concentrations of suspended particulate matter or
    (more rarely) of sulfur dioxide exceeding those out of doors have also
    been found in studies in the Netherlands (Biersteker et al., 1956) and
    in the USA (Yocom et al., 1971).

    FIGURE 10

        In general, however, in the absence of specific sources of sulfur
    dioxide or fine particulate matter indoors, concentrations are
    generally less than those outdoors. Sulfur dioxide as a gas, can
    diffuse readily onto walls and other surfaces. There is evidence that
    it reacts with ammonia from the indoor air on painted surfaces,
    particularly in the presence of moisture (Holbrow, 1958), but it is
    most effectively absorbed on clothing, curtains, carpets, and other
    soft furnishings, so that in domestic surroundings where these abound
    concentrations of sulfur dioxide are only of the order of 20% of those
    outdoors (Weatherley, 1966), while in offices and other buildings
    containing less absorbing material, concentrations may be 40-50% of
    those outdoors (Andersen, 1972; Derouane, 1972). The presence of
    ammonia in occupied rooms is important in relation to measurements of
    sulfur dioxide. Concentrations of this "natural" pollutant may be much
    higher than outdoors, particularly where there are young babies or old
    people with incontinence problems. The ammonia will only react
    effectively with sulfur dioxide in the presence of moisture, but it
    will, in any case, interfere with the determination of the sulfur
    dioxide by acidimetric or conductometric methods.

        Smoke, that is to say the finely divided black material from
    incomplete combustion, can penetrate fairly readily into buildings,
    and since the mobility of the particles is less than that of sulfur
    dioxide molecules, they are less rapidly removed onto surfaces.
    Concentrations of smoke indoors, assessed by soiling methods, have
    been found to be in the range of 50-90% of those outdoors (Derouane,
    1972; Yocom et al., 1971), again assuming there is no specific source
    of this material inside. Cigarette smoke can make a very substantial
    contribution to the finely divided particulate matter indoors (Elliott
    & Rowe, 1976; Hoegg, 1972). It is liable to affect direct gravimetric
    concentrations, but it has relatively little effect on concentrations
    measured by blackness. The composition of cigarette smoke is quite
    different from that of the general urban particulate matter, and it
    has been stressed by De Graaf & Biersteker (1972) that there may be
    many differences in the type and composition of suspended particulate
    matter indoors and outdoors. This point is particularly important in
    relation to samples obtained with the high volume sampler. The larger
    particles (greater than 10 m diameter) that are liable to be
    collected by this method would penetrate into buildings less readily
    than the fine particles, and, with reduced air movement inside, they
    may fall out fairly rapidly under gravity. This may account for the
    smaller proportion of total suspended particles (as compared with
    smoke) found indoors in some studies (Yocom et al., 1971). However, on
    the other hand, there is a risk that textile and other dusts dispersed
    in the home may be sampled and assessed as part of the general
    suspended particulate matter.

        Sulfuric acid is unlikely to remain for any appreciable time in
    occupied rooms, as it is readily neutralized by ammonia. However,
    finely divided sulfate particles are likely to behave in the same way
    as smoke, with a modest reduction indoors compared with outdoors.
    Relative humidity is normally lower indoors than outdoors and this
    will help to keep sulfate particles in suspension, but where the
    humidity is especially high, as it may be in kitchens, particles
    containing sulfates or other salts will grow rapidly and be deposited.

        Studies on indoor concentrations of sulfur dioxide and particulate
    matter are limited primarily by the need to consider the circumstances
    of each location individually; nevertheless, there is a growing body
    of literature on the subject that has been assembled and reviewed in
    recent reports from the USA (Benson et al., 1972; Henderson et al,
    1973).

    5.3  Concentrations in Work Places

        Concentrations of sulfur dioxide, much higher than those commonly
    found in urban air, may be present in some industrial environments,
    arising from processes in which the gas is handled or evolved, as well
    as from combustion sources. Paper mills, sulfuric acid plants, steel
    works, nonferrous metal foundries, and oil refineries are among the
    places where such concentrations may be found. However, emissions are
    usually highly localized and intermittent, presenting major problems
    in assessing concentrations to which workers may be exposed. Results
    are not normally published, but in a number of countries there is a
    requirement to ensure that a specified limit is not exceeded. For
    example, in the USSR, the maximum permissible concentration is
    10 mg/m3 (3.8 ppm) (ILO/WHO Committee on Occupational Health, 1970)
    whereas in the USA (American Conference of Government Industrial
    Hygienists, 1977), this value, averaged over 8-h shifts, is 13 mg/m3
    (5 ppm). This figure is of the order of 100 times the average values
    in urban air and it is higher than the maximum values reported, even
    in episodes of high pollution (Waller & Commins, 1966). Mean values
    over 3 years of 1800-2100 g/m3 (0.6-0.7 ppm) have been reported
    close to blast furnaces in steelworks, with occasional 24-h mean
    values up to 17 000 g/m3 (5.95 ppm) (Lowe et al., 1970).

        In steelworks, substantial quantities of suspended particulate
    matter may be present as well as sulfur dioxide and other pollutants.
    A 3-year mean concentration of about 1000 g/m3 (respirable dust, as
    measured with a Hexhlet sampler) has been reported in the study cited
    above (Lowe et al., 1970). Even so, the situation does not necessarily
    parallel that in urban atmospheres, for the particulate matter is
    liable to include dust which differs in composition and size
    distribution from that produced in the usual range of combustion
    processes.

        Dusts encountered in industry must, in general, be considered
    separately from the suspended particulate matter in urban air. Some,
    such as the coal-dust in mines, may be present at high concentrations,
    and have substantial effects on health, but these are specific to that
    dust, and are outside the scope of the present discussion. However,
    some of the chemical compounds in dust from industrial activities are
    covered by other documents of the WHO environmental health criteria
    programme.

        In some industries, such as wood-pulping and paper-making, sulfur
    dioxide may be evolved in the process, producing high concentrations
    locally without an accompanying particulate matter problem.
    Concentrations in the range 6 to 100 mg/m3 (2 to 36 ppm) have been
    reported at a plant in Norway (Skalpe, 1964) and similar
    concentrations were found in a study in the USA (Ferris et al., 1967)
    but these were gradually reduced over a 5-year period. High
    concentrations, averaging about 71 mg/m3 (25 ppm), also existed at
    one time in charging rooms for refrigerators when sulfur dioxide was
    used as a refrigerant (Kehoe et al., 1932) and similar concentrations
    have been reported in certain areas of oil refineries (Anderson 1950).

        There is a specific problem of sulfuric acid mist in the forming
    departments of works making lead-acid accumulators, where
    concentrations up to 16.6 mg/m3 have been observed (Malcolm & Paul,
    1961; Williams, 1970). This is very much greater than the
    concentrations ever found in urban air (Commins & Waller, 1978) and
    the physical form of the aerosol is different: mass median diameters
    of the droplets have been reported to be over 10 m (Williams, 1970)
    whereas those found in urban air are of the order of 0.5 m (Waller,
    1963).

    5.4  Assessment of Exposures

        In the present context, the term exposure relates to the
    concentrations of sulfur dioxide or suspended particulate matter in
    the air breathed by individuals or populations, averaged over
    specified periods. Broadly speaking, two types of exposure are
    considered: short-term exposure, in which the relevant concentrations
    are averaged over periods of the order of a day, and long-term
    exposure, for which averages over periods of the order of a year are
    commonly used. These periods are to a certain extent arbitrary,
    imposed by the time-resolution of the pollution measurements and by
    the nature of the health indices examined, and there is no clear
    guidance on the most relevant averaging periods to use in relation
    either to short-or to long-term exposure.

        Exposures are usually estimated from measurements of concentration
    at fixed sampling sites. The number of sampling sites required to give
    an adequate representation of the exposure of people living in a given
    area will depend on the topography, the distribution of sources, and
    other factors. In urban areas, there is usually a close correlation
    between values for neighbouring stations (Prinz, 1970). Care must be
    taken, however, to ensure that these stations are not used to
    represent environmental levels in areas much larger than their
    coverage. Furthermore, the instruments for measurement may be sited on
    the roof of a building for protection and convenience, and the levels
    measured may not necessarily represent those in the ordinary breathing
    zone.

        The usual practice in epidemiological studies is to assume that
    measurements made on outdoor air in the areas where people live or
    work provide an index of exposure that allows comparison to be made
    between different groups. It has been demonstrated, however, that
    estimated weekly exposures calculated as averages of concentrations
    measured indoors and at various outdoor sites and weighted according
    to the length of time likely to be spent in each location, may differ
    substantially from those indicated by measurements at just one site
    close to the place of residence or work (Fugas, 1976). To validate
    this approach, further measurements using personal samplers would be
    required. These have been used in studies on specific components of
    suspended particulate matter such as lead (Azar et al., 1972) and they
    have wide application in assessing the exposures of individuals to
    pollutants, including sulfur dioxide, in industry, but they are not
    generally practicable for large-scale epidemiological studies on the
    general population.

        A further problem in assessing exposures is the great variation in
    concentrations with time for, as far as sulfur dioxide is concerned, a
    brief exposure to a very high concentration, with no appreciable
    exposure during the rest of the day, would create a different
    situation from a steady exposure to a low level throughout the day. At
    present, there is no satisfactory way of measuring such situations, at
    least for people moving about during the course of the day. Providing
    studies are confined to groups of people living or working in
    reasonably uniform circumstances, measurements made over extended
    periods at a few fixed sites may give results that are valid for
    comparative purposes. Difficulties are liable to arise however if
    dissimilar groups are compared. In particular, people such as the very
    old and the very young may be partially protected from sulfur dioxide
    in the outside air if they spend long periods indoors, whereas an
    active worker may be exposed not only to a higher mean concentration

    in the course of the day, but also to a greater range. A further point
    is that increased activity will also lead to a greater ventilation
    rate, increasing the overall intake, and, in the case of sulfur
    dioxide, the deep inspirations associated with exercise are liable to
    increase the penetration of the gas and to enhance its effects
    (Lawther et al., 1975). The situation is even more complex for
    sulfuric acid aerosols. Because of the hygroscopic properties of the
    droplets, they take up moisture rapidly on inspiration, becoming
    larger and more dilute, and it has been suggested, on the basis of
    experimental work, that those from urban air could be diluted to such
    an extent that their irritant properties would be lost (Carabine &
    Maddock, 1976).

        When assessing long-term exposures, annual mean concentrations at
    fixed sampling sites are commonly used as the basis. Short-term
    variations are then of little consequence, but it may still be
    necessary to consider the extent of seasonal swings in concentration,
    or the frequency of occurrence of days of exceptionally high
    pollution. Also, it is often necessary to examine trends in
    concentrations for many years back, for the relevant exposure may have
    been one that occurred earlier in life, rather than the current level.
    Since there are very few long series of measurements of sulfur dioxide
    or suspended particulate matter made by uniform methods, it is very
    difficult to assess the true exposure of people to these pollutants.

    6.  ABSORPTION, DISTRIBUTION, AND ELIMINATION

        Most of the sulfur compounds discussed in this report are absorbed
    via the digestive system. Relatively little is absorbed from the
    respiratory tract, even in areas with highly polluted ambient air.
    Even so, it is reasonable to believe that the respiratory tract is the
    most vulnerable organ for the local effects of sulfur oxides and
    particulate matter in the ambient air. Thus, this section will deal
    mainly with the absorption, deposition, and clearance of sulfur
    dioxide and particulate matter in the respiratory tract.

    6.1  Absorption and Deposition in the Respiratory Tract

    6.1.1  Sulfur dioxide

        Sulfur dioxide is highly soluble in aqueous media. Absorption
    after inhalation has been studied in rabbits (Dalhamn & Strandberg,
    1961; Strandberg, 1964), dogs (Balchum et al., 1959, 1960a, 1960b;
    Frank et al., 1969) and man (Speizer & Frank, 1966a, 1966b).

        In rabbits, about 40% of the inhaled sulfur dioxide is absorbed in
    the nose and pharynx when concentrations of about 290 g/m3 (0.1 ppm)
    are inhaled. At higher concentrations (29-290 mg/m3; 10-100 ppm), the
    fraction absorbed is much higher (about 95%),The reasons for these
    different rates of absorption are not clear. In dogs, more than 99% of
    the inhaled sulfur dioxide is absorbed by the nose at exposure levels
    of 2.9-140 mg/m3 (1-50 ppm). These observations in dogs have been
    confirmed in man by studies on human volunteers, with levels of
    exposure to sulfur dioxide ranging from 2.9 to 420 mg/m3 (1 to
    140 ppm) and exposure times of a few minutes at the higher levels and
    30-40 minutes at the lower levels. Absorption can occur during
    mouth-breathing, but is less efficient than during nose-breathing,
    especially with increased ventilation.

    6.1.2  Airborne particles

        Particles smaller than about 10 m are deposited at different
    levels in the respiratory tract. The exact deposition pattern is
    determined by the interaction of size, shape, and density (expressed
    as aerodynamic diameter of unit density spheres) of the particles and
    by airflow conditions. Some particles in the air, such as sulfuric
    acid particles are hygroscopic. Such particles will take up water,
    expand in the respiratory tract, and be deposited in a manner other
    than would be expected from their diameters in the ambient air (Hatch
    & Gross, 1964; Stuart, 1973; Task Group on Lung Dynamics, 1966;
    Vigdorcik, 1948). Sophisticated theoretical models for the deposition
    of particles in the lung such as the ICRP model have been constructed

    (Task Group on Lung Dynamics, 1966). Deposition is caused by
    impaction, sedimentation, and diffusion. Impaction is an important
    mechanism for the deposition of the larger or heavier particles
    (5-30 m aerodynamic diameter) and where the air velocity is
    relatively high. Impaction occurs therefore at sites where the air
    stream is turbulent and is of most importance in the nose, mouth,
    pharynx, and the upper part of the tracheobronchial tree.
    Sedimentation or settling out of particles is also a function of
    particle size and density, as well as of residence time in the
    airways. It is important for the deposition of larger particles
    (1-5 m) providing they have not been deposited by impaction and is
    active in the trachea, bronchi, and bronchioles. Diffusion is of
    importance for particles smaller than a few tenths of 1 m (its effect
    increasing with decreasing particle size), with respect to the smaller
    bronchioles and especially the alveoli. In the size range of 2-5 m,
    experimental data have fitted quite well with the ICRP model, but
    deposition has varied widely (with an order of magnitude of 2 to 3)
    among human subjects (Lippmann et al., 1971). In experiments on
    animals, large reproducible differences in deposition have been seen
    among individuals within the same species (Albert et al., 1968;
    Tomenius, 1973). In the submicron range, theoretical models have not
    yet been satisfactorily verified by experiments.

    6.2  Clearance from the Respiratory Tract and Distribution

    6.2.1  Sulfur dioxide

        Sulfur dioxide is absorbed from the respiratory tract into the
    blood stream. It can then be widely distributed throughout the body
    (Balchum et al., 1960a, 1960b; Bystrova, 1957; Frank et al., 1967;
    Yokoyama et al., 1971), where it appears to be metabolized and
    excreted via the urinary tract.

    6.2.2  Particulate matter

        Knowledge concerning the elimination of insoluble particles
    deposited in the alveoli is incomplete (Hatch & Gross, 1964; Morrow,
    1973). It is clear that these particles are, to a large extent,
    phagocytized by alveolar macrophages within hours, but it is not known
    to what extent they are actively carried to the ciliated part of the
    lung or to the lymphatic system. The particles can be transported by
    the mucociliary escalator into the interstitium (where they can remain
    for a long time) or the lymphatic system. Solubility  in vivo is
    important for the clearance of "insoluble" particles deposited in the
    alveoli (Mercer, 1967; Morrow, 1973). The biological half-lives range
    from days to years depending on the chemical composition of the
    particles (Brain & Valberg, 1974; Task Group on Lung Dynamics, 1966;
    Task Group on Metal Accumulation, 1973).

        Mucociliary transport, and therefore clearance of particles, can
    be affected by various factors and can be impaired by long-term
    cigarette smoking and acute infection in the respiratory tract (Camner
    & Philipson, 1972; Camner et al., 1973a; Jarstrand et al, 1974).
    Subjects suffering from chronic obstructive lung disease may also have
    an impaired mucociliary transport (Camner et al., 1973b; Toigo et al.,
    1963). Studies in animals have shown that the inhalation of sulfur
    dioxide can interfere with the clearance of bacteria (Rylander et al.,
    1971) and inert particles (Ferin & Leach, 1973) from the lungs, but
    how much is due to the action of the sulfur dioxide on the mucociliary
    mechanism and how much to its action on the macrophages is not clear.

        The clearance of soluble particles may follow the above pathways
    depending upon their solubility. They may dissolve in the mucus in
    which case they will probably be eliminated via the mucociliary route
    and be swallowed and removed or absorbed via the intestinal tract. In
    the alveoli, the particles may diffuse into the lymph or blood and
    thus be removed from the lung.

    7.  EFFECTS ON EXPERIMENTAL ANIMALS

        It has been difficult to separate the relative effects of sulfur
    dioxide, sulfuric acid mists, sulfate salts, and particulate matter in
    the ambient air by epidemiological techniques. The effects of these
    substances individually and in various combinations have been studied
    in the laboratory. These studies have been useful in explaining some
    of the mechanisms of action but their application has been rather
    limited for the establishment of safe ambient levels, particularly for
    complex mixtures such as exist in the ambient air. In the past, the
    levels used in laboratory studies on animals have usually been far in
    excess of those seen in the ambient air and therefore have had little
    relevance for ambient air quality standards. Later studies have been
    at more relevant levels.

        The following discussion separates work on animals into short-term
    and long-term exposure studies. Short-term exposure studies are those
    that involve exposures of 24-h or less and usually last from minutes
    to at the most a few hours. Long-term exposure studies refer to
    exposures that last longer than 24-h and are usually extended over
    months. In the short-term exposure studies, only immediate or acute
    effects have been studied. In long-term exposure studies, animals may
    be studied during the period of exposure to determine whether there
    are progressive changes or not and at the end of the exposure period
    to quantify any chronic effect.

    7.1  Short-term Exposure Studies

    7.1.1  Exposure to sulfur dioxide singly or in combination with
           other agents

        Sulfur dioxide is a respiratory irritant that is very soluble in
    the aqueous surfaces of the respiratory airways. Because of this high
    solubility, most of the sulfur dioxide is absorbed in the nose and
    upper airways (section 6.1) and very little reaches the lungs
    directly. At extremely high concentrations, the absorptive capacity of
    the upper airways can be overwhelmed and death or pathological changes
    including laryngotracheal and pulmonary oedema can be induced in the
    respiratory tract of experimental animals.

        Exposure-effect curves have been developed for guineapigs (Amdur,
    1966) and dogs (Frank & Speizer, 1965). In Amdur's study, a linear
    relationship was obtained between exposure for 1 h to sulfur dioxide
    concentrations ranging from 0.46 to 2380 mg/m3 (0.16-835 ppm) and
    corresponding increases in pulmonary flow resistance. The second study
    showed that nasal flow resistance increased roughly in proportion to
    exposure to sulfur dioxide concentrations ranging from 20 to
    660 mg/m3 (7-230 ppm) for a 15-20 minute period.

        Studies with guineapigs at lower concentrations have shown that a
    sodium chloride aerosol, administered concomitantly enhances the
    effects of sulfur dioxide on the lungs in the form of
    bronchoconstriction and increased airway resistance (Amdur, 1957). It
    was postulated that the sodium chloride could act as a carrier to
    deliver the absorbed sulfur dioxide deep into the lungs, or that the
    increased humidity in the respiratory tract could react with the
    sulfur dioxide to form sulfurous acid, especially if catalysts were
    present (Amdur & Underhill, 1968). McJilton et al. (1973) have
    reaffirmed the importance of humidity in this reaction, exposing
    guineapigs for 1 h to sulfur dioxide at 3.1 mg/m3 (1.1 ppm) and a
    sodium chloride aerosol of about 1 mg/m3. They allowed the mixture to
    "age" in a reaction chamber for 8-10 minutes at various relative
    humidities. The chamber temperature was 22C and the average aerosol
    particle size was 0.1 m with a maximum size of less than 2 m. At
    relative humidities above 80%, they noted a marked increase in
    pulmonary resistance, whereas administration of the individual
    components produced little or no effect. The droplets formed at high
    humidity and in the presence of the aerosol had a pH of 3.2  0.5.
    Analysis of these particles by mass spectrometer revealed sulfur
    dioxide and bisulfite ions (HSO-3)in the solution but no sulfuric
    acid. In the high humidity, the sodium chloride aerosol apparently
    became hydrated and could then absorb sulfur dioxide. The authors
    indicated that the mixture had to "age" to ensure absorption of the
    sulfur dioxide; otherwise there would be too much competition with the
    moist surfaces in the nasopharynx which could sweep out the sulfur
    dioxide. Thus, this is another mechanism whereby the effect of sulfur
    dioxide can be enhanced. If such mixtures were allowed to "age"
    longer, it seems likely that the sulfur dioxide could be changed to
    sulfuric acid. Studies using other animal species such as the cat have
    not shown this enhancement of the effect of sulfur dioxide by sodium
    chloride (Corn et al., 1972).

        Matsumura (1970a) exposed guineapigs to ozone at 2.1, 11, or
    21 mg/m3 (1, 5, or 10 ppm), nitrogen dioxide at 40, 80, or 140 g/m3
    (20, 40, or 70 ppm), or sulfur dioxide at 60, 170, 510, or 940 mg/m3
    (20, 60, 180, or 330 ppm) for 30-50 minutes. Each group with unexposed
    controls was then exposed to an aerosolized antigen (2 m) of egg
    albumin and bovine albumin for 45 minutes, 5-7 times at intervals of a
    day or more. Two weeks after the last exposure, the animals were
    killed and blood was drawn for assay of the immunological response.
    Enhanced sensitization was noted only at the highest levels of
    exposure. During these exposures a number of animals died of
    anaphylactic reactions at the fifth or sixth inhalation. In another
    study, sulfur dioxide at a concentration of 1100 mg/m3 (400 ppm) did
    not have any effect on dyspnoeic attacks (Matsumura, 1970b).

    7.1.2  Exposure to sulfuric acid aerosols or suspended sulfates

        Sulfuric acid mist and some of the sulfate salts are more powerful
    respiratory irritants than sulfur dioxide, and this effect is also
    related to particle size (smaller particles tending to be more
    irritating) (Amdur, 1958). Treon et al. (1950) exposed rabbits, rats,
    mice, and guineapigs to sulfuric acid mist, 93-99% of which was less
    than 2 m in diameter, many droplets being about 1 m in diameter.
    Concentrations ranged from 87 to 1610 mg/m3. Despite the relatively
    small number of animals used, a clear-cut species difference was shown
    at these high concentrations. The order of increasing sensitivity was
    rabbits, rats, mice, guineapigs.

        It was reported by Amdur et al. (1952a) that the 8-h LC50 of
    sulfuric acid mist with a mass median diameter (MMD) of about 1 m was
    18 mg/m3 for 1-2 month old guineapigs. Pattle et al. (1956) showed
    that sulfuric acid mist of MMD 2.7 m was more toxic to guineapigs
    than a mist of 0.8 m, and that the toxicity of the smaller particles
    increased when the exposure occurred at 0C. However, this might be a
    response of the guineapig to low temperature rather than to the
    sulfuric acid mist. Concomitant exposures with ammonium carbonate had
    a protective action, apparently because it neutralized the sulfuric
    acid.

    Table 11.  Sulfuric acid and some sulfates in descending order of
               their irritative capacity for animals. Presented for
               equivalent amounts of sulfur and at comparable particle
               size i.e., sub-micron; short-term exposuresa
                                                                     

         Sulfuric acid                      H2SO4
         Zinc ammonium sulfate              ZnSO4(NH4)2 SO4
         Iron (III) sulfate                 Fe2 (SO4)3
         Zinc sulfate                       ZnSO4
         Ammonium sulfate                   (NH4)2 SO4

    Nonreactive

         Iron (II) sulfate                  FeSO4
         Manganese (II) sulfate             MnSO4
                                                                    

    a From:  Amdur (1969, 1970, 1971).

        Studies on guineapigs have shown that, for equivalent amounts of
    sulfur and in comparable particle size, sulfuric acid is more
    irritative than any of the sulfate compounds, some of which appear to
    be nonreactive in animals (Table 11). Whether these compounds would
    behave in the same manner in complex mixtures such as those found in
    polluted air is not known. Some of the nonreactive compounds such as
    manganese salts or oxides can catalyze the reaction of sulfur dioxide
    to sulfuric acid.

        In short-term exposure studies by Amdur (1958), concentration
    seemed to be more important than duration and death was related to
    laryngospasm and bronchospasm. Sulfuric acid mist also caused
    parenchymal lung damage that seemed to be related to the total dose
    (Amdur, 1958; Pattle et al., 1956).

    7.2  Long-term Exposure Studies

    7.2.1  Exposure to sulfur dioxide

        Rats were exposed for 96 days to sulfur dioxide at concentrations
    of 0.1, 0.5, and 1.5 mg/m3 (0.04, 0.18 & 0.53 ppm). Histological
    examination showed interstitial pneumonia, bronchitis, tracheitis, and
    peribronchitis after exposures to the two higher levels (Elfimova &
    Gusev, 1969).

        Misiakiewicz (1970) exposed rats continuously during 5 months to
    sulfur dioxide at concentrations of 0.3, 0.5, 1.0, 2.0, and
    20.0 mg/m3 (0.11, 0.18, 0.35, 0.7 and 7.0 ppm). Exposures to
    2.0 mg/m3 (0.7 ppm) and 20.0 mg/m3 (7.0 ppm) increased the activity
    of serum cholinesterase (EC 3.1.1.8) and aspartate aminotransferase
    (EC 2.6.1.1) and caused morphological changes in the upper respiratory
    tract.

        After a 120-h exposure to a sulfur dioxide concentration of
    3 mg/m3 (1.1 ppm), guineapigs showed proliferative interstitial
    pneumonia, bronchitis, and tracheitis and an increased histamine
    content in the lungs, while exposure to 167 g/m3 (0.06 ppm) of
    sulfur dioxide for one month led to interstitial changes in the
    respiratory tract (Bustueva, 1961a). Bustueva (1966) also exposed rats
    for 65 days (24 h per day) to sulfur dioxide at 4.86 mg/m3 (1.7 ppm).
    Tracheitis, desquamation of epithelial cells, an increased amount of
    purulent mucus, and interstitial pneumonia were found. It was not
    clear, however, whether this was due to the sulfur dioxide or to the
    pulmonary infection that can develop in rats.

        Beagle dogs exposed to a sulfur dioxide concentration of
    13.4 mg/m3 (4.7 ppm) for 21 h per day, for 620 days, did not develop
    any specific histopathological changes (Lewis et al., 1973).

        Guineapigs have been exposed to sulfur dioxide levels up to
    16.3 mg/m3 (5.7 ppm) for 12 months without definite effects except
    for slight cytoplasmic vacuolation in the liver (Alarie et al., 1970).
    Cynomolgus monkeys exposed continuously for 78 weeks to sulfur dioxide
    levels up to 3.7 mg/m3 (1.3 ppm) did not show any significant
    pathological changes (Alarie et al., 1972). However, rats exposed to
    sulfur dioxide at 2.9 mg/m3 (1 ppm) for 170 h showed a significant
    reduction in clearance of inert particles from the lung (Ferin &
    Leach, 1973). Syrian hamsters, made emphysematous by previous exposure
    to aerosolized papain, tolerated concentrations of sulfur dioxide up
    to 1900 mg/m3 (650 ppm). Exposures were for 4 h per day, 5 days per
    week, for a total of 19-74 exposures. Only slight changes in the
    mechanical properties of the lung were noted as well as slight
    bronchitis (Goldring et al., 1970). As Syrian hamsters are
    exceptionally resistant to the effects of sulfur dioxide, these data
    must be extrapolated with caution to other species.

    7.2.2  Exposure to sulfuric acid aerosols

        Alarie et al. (1973) exposed cynomolgus monkeys for 78 weeks to
    sulfuric acid mist at concentrations of 0.38 to 4.79 mg/m3 and
    particle sizes of 0.54 to 3.60 m. At concentrations of 2.43 and
    4.79 mg/m3 and particle sizes of 3.60 and 0.73 m, definite damage to
    the pulmonary structure was evident and there was deterioration in
    pulmonary function. At the lower concentrations, changes were slight
    or absent. The authors also exposed guineapigs for 52 weeks to
    concentrations of 0.08-0.1 mg/m3 and particle sizes of 0.84 and
    2.78 m; no detectable effects were seen.

        When beagle dogs were exposed to sulfuric acid at a concentration
    of about 0.9 mg/m3 for 21 h per day, for 620 days, there was a
    significant reduction in pulmonary function; 90%, of the sulfuric acid
    mist was less than 0.5 m in diameter (Lewis et al., 1973).
    Histopathological changes were produced in the alveolar part of the
    lung, especially in the elastic tissue, of rats exposed for 65 days
    (24 h per day) to a sulfuric acid concentration of 1 mg/m3. Ninety
    per cent of the particles were less than 2 m in diameter in this
    study (Bustueva, 1966).

        Guineapigs were exposed to sulfuric acid aerosol for 120 h at 3
    different concentrations (1.98  0.03; 4.20  0.06; and 8.27 
    0.15 mg/m3). Exposure to the highest concentration led to oedema of
    the lungs, changes in the interalveolar walls, sharp, diffused,
    interstitial changes, and increased histamine content in the lung
    tissue. Three weeks after exposure, sclerosis appeared (Bustueva,
    1957). After a 1-month exposure to 0.1 mg/m3, no changes were found
    (Bustueva, 1961a).

    7.2.3  Exposure to a mixture of sulfur dioxide and sulfuric acid
           aerosols or this mixture combined with other agents

        Rats were exposed for 65 days (24 h per day) to a combination of
    sulfur dioxide at 4.86 mg/m3 (1.7 ppm) and a sulfuric acid aerosol at
    1 mg/m3 (90% of the aerosol particles were less than 2 m in
    diameter). There was a summation of the effects (histopathological
    changes in alveolar tissue) produced by exposure to each of the
    pollutants alone (Bustueva, 1966).

        Lewis et al. (1973) exposed beagle dogs to a combination of sulfur
    dioxide at 13.4 mg/m3 (4.7 ppm) and sulfuric acid at about
    0.9 mg/m3. Ninety percent of the sulfuric acid mist was less than
    0.5 m in diameter. The duration of exposure was 21 h per day, for 620
    days. Some of the dogs had been pre-exposed to nitrogen dioxide. This
    group tended to show less response in the form of changes in pulmonary
    function than dogs not pre-exposed to nitrogen dioxide.

        Alarie et al. (1975) exposed cynomolgus monkeys and guineapigs to
    mixtures of sulfur dioxide, fly ash, and sulfuric acid mist, for 18
    months after an 8-week baseline period. Exposure concentrations varied
    from 0.29 to 143 mg/m3 (0.1 to 5.0 ppm) for sulfur dioxide and from
    0.1 to 1 mg/m3 for sulfuric acid mist; the concentration of fly ash
    was approximately 0.5 mg/m3. Particle size (MMD) varied from 0.53 to
    3.11 m in the acid mist and from 4.1 to 5.8 m in the fly ash.
    Pulmonary function tests and serum biochemical and haematological
    analyses were conducted prior to, and periodically during, the
    exposure. Lungs were examined microscopically at the end of the
    experiment. Sulfuric acid mist appeared to be responsible for the
    effects observed. These were largely histopathological changes in the
    lungs. No synergistic action was noted between the pollutants.

    7.2.4  Combined exposure to sulfur dioxide and particulate matter
           or other gaseous pollutants

        In studies by Frazer et al. (1968), white albino rats were exposed
    to sulfur dioxide at 2.9 and 8.6 mg/m3 (1 and 3 ppm) and graphite
    dust at 1 mg/m3. The mean particle size of the dust was less than
    1 m and the MMD was 3 m. The animals were exposed for 12 hours per
    day, 7 days per week, for 4 months. Both ciliary activity, and the
    number of dust-containing cells per 100 lung cells appeared to be
    unaffected after 56-109 days of exposure.

        Various aerosols that may react with sulfur dioxide have been
    studied either singly or in combination with sulfur dioxide in
    experimental animals, particularly in guineapigs. Amdur (1969, 1971)
    emphasized that the particle size of the aerosol as well as the
    concentration was extremely important in the determination of toxicity
    and that the most important size range was the submicron level. The

    changes induced in pulmonary mechanics were slow to return to
    pre-exposure levels. This indicates either that deposited aerosols may
    not be cleared promptly but remain in the lungs exerting their effect
    for a period of time, or that the repair mechanisms are slow, or that
    both conditions are present. There was also a variation in the
    toxicity of different sulfates for the same particle size and sulfur
    content (Amdur, 1969) (Table 11).

        Studies by Battigelli et al. (1969) in which rats were exposed to
    sulfur dioxide at 2.9 mg/m3 (1 ppm) and graphite dust at 1 mg/m3
    (particle size not stated) did not show any effect other than the
    accumulation of dust in the lung after exposure for 4 months, for 12
    hours per day. Amdur & Underhill (1968) pointed out that not all
    aerosols enhance the effect of sulfur dioxide and that only those
    aerosols composed of droplets in which the sulfur dioxide could
    dissolve were active.

        Mice were exposed by Zarkower (1972) to carbon particles (1.8 to
    2.2 m MMD) at a concentration of 558  154 g/m3 and sulfur dioxide
    at 5.7 mg/m3 (2 ppm). The animals were exposed to carbon alone,
    carbon with sulfur dioxide, and sulfur dioxide alone; controls were
    unexposed. Killed  Escherichia coli was used as an antigen and
    antibody production was measured. Exposures for 192 days produced
    significant immunosuppression. Shorter periods of exposure resulted in
    variable effects including stimulation of antibody production.

        A variety of metallic aerosols can catalyze the oxidation of
    sulfur dioxide including the soluble salts of ferrous iron, manganese,
    and vanadium.

        Rylander & Bergstrm (1973) exposed animals with latent upper
    respiratory disease and controls for 4 weeks to various combinations
    of sulfur dioxide at 57 mg/m3 (20 ppm), manganese dioxide at
    12-20 mg/m3 (particle size between 5.0 and 0.5 m), and carbon
    monoxide at 187-250 mg/m3 (150-200 ppm). Animals with latent upper
    respiratory disease showed more extensive histological changes and an
    increase in the number of free lung cells. The combination of sulfur
    dioxide and manganese dioxide produced the greatest changes.

        An addition of effects in rats was reported by Salamberidze (1969)
    from joint exposure to sulfur dioxide at 0.15 mg/m3 (0.05 ppm) and
    nitrogen dioxide at 0.1 mg/m3 (0.05 ppm) over a 3-month period.

        The precise mechanism by which the oxides of sulfur and
    particulate matter can affect the lungs is not known. Sulfur dioxide
    (and presumably sulfuric acid as well) can interfere with the
    clearance of bacteria (Rylander et al., 1971) and inert particles
    (Ferin & Leach, 1973) from the lungs (see also section 6). Chronic
    exposure to sulfur dioxide increased the number and area of goblet
    cells in guineapigs and lambs (Mawdesley-Thomas et al., 1971).

        The considerable variations in the results of these experiments on
    animals reflect differences in sensitivity of individual species,
    exposure levels, and methods used to assess the effects.

        It should be emphasized that extrapolation of these results from
    animals to human beings is not easy. These findings, however, do give
    some insight into possible mechanisms of action and reactions that can
    occur.

    8.  EFFECTS ON MAN

    8.1  Controlled Exposures

        A number of studies have been performed on volunteers under
    controlled conditions of exposure to sulfur dioxide or sulfuric acid
    aerosols, singly or in combination, or to mixtures of these with other
    compounds such as ozone and hydrogen peroxide. These studies, all
    conducted under short-term exposure (up to 24 h), include those on
    changes in respiratory function and effects on sensory and reflex
    organs.

    8.1.1  Effects on respiratory organs

    8.1.1.1  Exposure to sulfur dioxide

        Amdur et al. (1953) exposed 14 healthy volunteers (inhaling
    through the mouth) to sulfur dioxide at concentrations of
    2.9-23 mg/m3 (1-8 ppm) for 10 min. They noted an increased pulse
    frequency, decreased tidal volume, and an increased respiratory
    frequency which returned to normal levels after exposure. The sequence
    of exposures was randomized. Effects increased with increasing levels
    of sulfur dioxide and were detectable at the lowest concentration
    tested (2.9 mg/m3; 1 ppm). Sulfur dioxide levels were monitored by
    the conductimetric method. However, Lawther (1955) was unable to
    reproduce these effects in any consistent manner, either in urban or
    in rural dwellings. Eleven healthy volunteers were exposed to sulfur
    dioxide at levels of 2.9, 14, and 37 mg/m3 (1, 5, and 13 ppm) in
    studies by Frank et al. (1962). Respiratory mechanics were measured by
    means of a body plethysmograph and an oesophageal balloon. Exposures
    lasted 10-30 min. Only one of the 11 subjects exposed showed an
    increased pulmonary resistance at 2.9 mg/m3 (1 ppm). Increased
    pulmonary resistance was noted in all subjects and was greater at the
    higher concentration. The change occurred within 1 min of exposure and
    increased up to 10 min, after which no further increase was noted.

        In studies by Snell & Luchsinger (1969), exposure to a sulfur
    dioxide concentration of 2.9 mg/m3 (1 ppm) for 15 min produced a
    slight effect on total respiratory resistance in 9 healthy volunteers.

        Andersen et al. (1974) exposed healthy male subjects to sulfur
    dioxide at levels of 2.9, 14, and 71 mg/m3 (1, 5, and 25 ppm) for up
    to 6 h. With exposures of 1-3 h at 2.9 mg/m3 (1 ppm), there was a
    decrease in the flow of nasal mucus and a decrease in the cross
    section of the nasal passages. Thus it appears that exposure to
    concentrations of 2.9 mg/m3 (1 ppm) or more may result in impairment
    of mucociliary transport in the nose.

        Four healthy volunteers were exposed to sulfur dioxide and the
    forced vital capacity (FVC), one second forced expiratory volume
    (FEV1.0), mid-maximal flow rates (MMFR), maximal expiratory flow rate
    at 50% (MEFR50) and closing volume and capacity were measured.
    Although no changes were found in these tests at 1.1 mg/m3
    (0.37 ppm), slight changes in FVC, FEV1.0, MMFR, and MEFR50 were
    noticed at 2.1 mg/m3 (0.75 ppm) after exposure for 30 min; no effect
    on closing volume was detected (Bates & Hazucha, 1973).

        The results of these studies on human volunteers are summarized in
    Table 12.

    8.1.1.2  Exposure to sulfuric acid aerosols

        Amdur et al. (1952b) exposed 15 healthy men to sulfuric acid mist,
    through mouth breathing, at concentrations of 0.35-5 mg/m3 (particle
    size of approximately 1 m) for periods of 5-15 min. At concentrations
    below 1 mg/m3, the mist did not produce any subjective sensations
    although 5 of the 15 subjects showed a slightly increased respiratory
    rate and a decreased tidal volume at 0.35 mg/m3. All subjects noted
    irritation at a concentration of 3 mg/m3. Respiration was monitored
    by means of a pneumotachograph. The respiratory changes as well as the
    subjective sensations increased with increasing concentrations of
    sulfuric acid. Healthy male volunteers were also exposed to sulfuric
    acid mist by Sim & Pattle (1957), either by mask or in a chamber for
    periods ranging from 10 to 60 min. Twelve men were exposed to the
    mist. The temperature of the air was 18.4C with a relative humidity
    of 62%. The MMD of the aerosol was 0.99 m and the concentration was
    39.4 mg/m3. The men noted minor irritation, and lung resistance (as
    measured by the interrupter technique) rose by 35-100%. With
    re-exposure for 30 min to the mist at a temperature of 24.5C, a
    relative humidity of 91%, a concentration of 20.8 mg/m3, and a MMD of
    1.54 m, severe coughing and irritation of the throat occurred. The
    men found it almost intolerable. Lung resistance had risen by 43-150%
    when measured after 10 minutes of exposure and when coughing had
    ceased. No changes in respiration, blood pressure, or pulse rate were
    noted. Two of the men exposed to these conditions had persistent
    symptoms for some days after exposure ended.

        It is difficult to evaluate these two studies. Amdur did not
    report the temperature, though it was probably in the neighbourhood of
    24C, or the relative humidity. The studies of Sim & Pattle were at
    relatively high levels but they demonstrated the importance of the
    relative humidity or perhaps even more important, the absolute
    humidity. The results of these studies are summarized in Table 13.
    None of these studies used sulfates.


        Table 12.  Selected laboratory studies on the effects of short-term exposures to sulfur dioxide on respiratory
               function in volunteers
                                                                                                                                

       Concentration       Length of            Effects                          Subjects             Reference
                           exposure
    (mg/m3)a     (ppm)      (min)
                                                                                                                                

    2.9-23       1-8       10          Increased pulse rate, decreased      14 healthy males      Amdur et al. (1953)
                                       tidal volume, and increased
                                       respiratory rate

    2.9          1         10-30       Increased pulmonary/resistance       11 healthy males      Frank et al. (1962)

    2.9          1         15          Increased respiratory resistance     9 healthy subjects    Snell & Luchsinger (1969)
                                                                            (5 males &
                                                                            4 females)

    2.9          1         60-180      Decreased nasal mucus flow           15 healthy males      Andersen et al. (1974)
                                       and decreased cross section of
                                       nasal passages

    2.1          0.75      120         Slight effect in 30 min on           4 healthy subjects    Bates & Hazucha (1973)
                                       FVC, FEV1-0, MMFR, and
                                       MEFR50; no effect on
                                       closing volume

    1.1          0.37      120         No effect on above tests of          4 healthy subjects    Bates & Hazucha (1973)
                                       pulmonary function throughout
                                       exposure period
                                                                                                                                

    a  Original levels reported as ppm have been converted to mg/m3 and rounded off.

    Table 13.  Selected laboratory studies on the effects of short-term exposures to sulfuric acid mist on respiratory
               function in volunteers
                                                                                                                                

    Concentration   Particle Size   Length of   Relative   Temperature     Effects             Subjects      Reference
      (mg/m3)           (m)        exposure    Humidity   (C)
                                      (min)        (%)
                                                                                                                                

       0.35             1             5-15          ?      Room (?24)     5 of 15 subjects     15 healthy    Amdur et al.
                                                                          increased            subjects      (1952b)
                                                                          respiratory rate
                                                                          and decreased
                                                                          tidal volume

      39.4              0.99            60         62      18.4           Increased            12 healthy    Sim & Pattle
                                                                          pulmonary            males         (1957)
                                                                          resistance and
                                                                          minor irritation

      20.8              1.54            30         91      24.5           Marked increase      12 healthy    Sim & Pattle
                                                                          of pulmonary         males         (1957)
                                                                          resistance and
                                                                          severe irritation

                                                                                                                                

    

    8.1.1.3  Exposure to mixtures of sulfur dioxide and other compounds

        Frank et al. (1964) repeated the above exposures in combination
    with a sodium chloride aerosol. Concentrations of sulfur dioxide were:
    2.9-5.7 mg/m3 (1-2 ppm), 11-17 mg/m3 (4-6 ppm), and 40-49 mg/m3
    (14-17 ppm). The sodium chloride aerosol had a geometric mean diameter
    of 0.15 m with a geometric standard deviation of 2.3 m and an
    average concentration of 18 mg/m3. As in the earlier studies, little
    change in pulmonary flow resistance was noted at the lower levels of
    sulfur dioxide alone (2.9-5.7 mg/m3; 1-2 ppm) but a progressive
    increase was noted at higher levels. The authors did not find any
    systematic differences between the responses to sulfur dioxide alone
    or to the gas plus the aerosol. As mentioned in section 8.1.1.1, Snell
    & Luchsinger (1969) noted a slight effect of sulfur dioxide at
    2.9 mg/m3 (1 ppm) on total respiratory resistance but could not
    demonstrate an enhancing effect of either a sodium chloride aerosol or
    a distilled water aerosol. Burton et al. (1969) exposed volunteers to
    a concentration of sulfur dioxide of 6 mg/m3 (2.1 ppm) with or
    without a sodium chloride aerosol (MMD of less than 0.4 m) at a
    concentration of 2.2 mg/m3. The inhaled air was warmed and moistened.
    However, they too failed to demonstrate any effect of the sodium
    chloride aerosol on the response to sulfur dioxide.

        Thus, three groups have not been able to demonstrate any enhancing
    effect of sodium chloride aerosol on respiratory resistance in man as
    had been demonstrated in guineapigs. This does not exclude the
    possibility that other particulate matter might enhance the effect of
    sulfur dioxide on the human respiratory system. These studies should
    be repeated at levels of humidity above 80% and the mixture should be
    allowed to "age".

        The possible interaction of sulfur dioxide and hydrogen peroxide
    has been examined by Toyama & Nakamura (1964) and that of sulfur
    dioxide and ozone by Bates & Hazucha (1973). Toyama & Nakamura (1964)
    studied 24 healthy male volunteers by means of a pneumotachograph and
    the interrupter technique to measure alveolar pressure; airway
    resistance was determined from these measurements. The particle sizes
    of the hydrogen peroxide were reported to be 1.8 and 4.6 m.
    Concentrations of hydrogen peroxide were 0.8-1.4 mg/m3 for the larger
    particles and 0.01-0.1 mg/m3 for the smaller ones. Levels of sulfur
    dioxide ranged from 2.9 to 170 mg/m3 (1-60 ppm). The subjects were
    not aware whether they were breathing hydrogen peroxide, sulfur
    dioxide, or combinations. It was not stated whether the order of

    administration was randomized or not. The investigators noted a marked
    increase in airway resistance with the mixture compared with the
    individual components. They also noted that the larger particles had
    more effect than the smaller particles. This was probably due to the
    greater dose delivered, although they did not comment on this. It was
    the authors' opinion that the enhancement of the response to the
    combination was due to the conversion of sulfur dioxide to sulfuric
    acid, although no measurement for sulfuric acid was made.

        In similar studies by Bates & Hazucha (1973) healthy male
    volunteers were exposed in a chamber to sulfur dioxide, or ozone, or a
    combination of the two. Changes in FVC, FEV1.0, MMFR, MEFR at 50%
    vital capacity and peak expiratory flow rates were studied and some
    effects were noted during exposure to ozone at 540-1610 g/m3
    (0.25-0.75 ppm). Sulfur dioxide by itself had little effect over a
    similar range. Joint exposure to sulfur dioxide and ozone at
    1060 g/m3 (0.37 ppm) and 790 g/m3 (0.37 ppm), respectively,
    produced a greater effect than ozone alone. Changes were noted after
    exposure for 30 min and were marked after 2 h of exposure. Exercise
    during exposure enhanced the effect.

    8.1.2  Effects on sensory or reflex functions

        Studies in the USSR have concentrated on the effects of sulfuric
    acid aerosols and sulfur dioxide on sensory receptors, cerebral
    cortical function, and their interrelationships. When sulfuric acid
    mist produced subjective sensory stimulation such as odour or
    irritation of mucous membranes, or both, then, invariably, objective
    evidence of central nervous system depression could be demonstrated
    (Rjazanov, 1962).

        Bustueva (1961b) did not note any change in optical chronaxy in
    volunteers exposed to concentrations of sulfur dioxide of 0.5 mg/m3
    (0.18 ppm) and sulfuric acid of 0.3 mg/m3. However, when levels of
    1.5 mg/m3 for sulfur dioxide, 0.73 mg/m3 for sulfuric acid, and
    combined levels of 1.2 mg/m3 and 0.6 mg/m3, respectively, were
    exceeded, optical chronaxy increased (Table 14). Similar effects were
    seen in dark adaptation responses (Rjazanov, 1962).

        Studies have also been carried out in which the cerebral cortex
    was monitored by electroencephalography. Alpha rhythm suppression was
    used as an index of response. Threshold levels at which a response was
    noted are given in Table 14. (Bustueva et al. 1960).



        Table 14.  Threshold levels of sulfur dioxide and sulfuric acid required for effects on sensory or
               reflex functions in volunteers during short-term exposuresa
                                                                                                       

                                                             Threshold levels (mg/m3)
                                                                                                      

    Effects                       Sulfuric acid     Sulfur dioxide     Sulfuric acid + Sulfur dioxide
                                                                                                       

    Perception of odour and       0.6 to 0.85       1.6 to 2.8                   0.3 + 0.5
    irritation of mucosa

    Suppression of dark           0.63 to 0.73      0.92                         0.3 + 0.5
    adaptation

    Elevation of optical          0.73              1.5                          0.6 + 1.2
    chronaxy

    Disruption of alpha           0.63              0.9                          0.3 + 0.5
    rhythm

    Conditioning of               0.4               0.6                          0.15 + 0.5
    electrocortical reflex                                                    or 0.3 + 0.25
                                                                                                       

    a Summarized from studies in the USSR (Bustueva, 1961b; Bustueva et al., 1960; Rjazanov, 1962).

    


        The electrocortical conditioned reflex is a central nervous system
    phenomenon elicited only after a succession of repeated, conditioned
    reflex trials. After exposure to a combination of irritants (sulfur
    dioxide or sulfuric acid) and light has been repeated several times,
    desynchronization begins to appear before the light is switched on.
    This can be produced at levels generally not sensorially perceived.
    Thus, unperceived odour or stimulus appears to become the conditioning
    stimulus and generates the conditioned electrocortical reflex
    (Rjazanov, 1962).

        Elfimova & Hacaturjan (1968) exposed volunteers to sulfur dioxide,
    phenol, and carbon monoxide and noted a summation of effects as
    measured by reflex action. No interaction was noted between sulfur
    dioxide at 0.5 mg/m3 (0.18 ppm) and carbon monoxide at 3 mg/m3
    (2.4 ppm) as measured in volunteers by the sensory reflex technique
    (Mamacasvili, 1968).

    8.2  Industrial Exposure

        Workers are exposed to sulfur dioxide or sulfuric acid mist in a
    number of industries; sometimes exposure is not solely to sulfur
    dioxide or sulfuric acid. Exposed populations have been studied with
    respect to the effects of exposure on their health status or on their
    respiratory system. However, in many of these studies, only the
    currently employed workers were examined and a serious effort was not
    made to locate subjects who had left the industry and who may have
    suffered more from the disease or could have been more sensitive to
    the materials. It should also be emphasized that, in general, in the
    following reports, the exposure levels studied were from spot samples
    or for very short time intervals.

    8.2.1  Exposure to sulfur dioxide singly or in combination with
           particulate matter

        Kehoe et al. (1932) studied men working in a refrigerator company
    in the USA where sulfur dioxide was the refrigerant. Exposures
    averaged 60-90 mg/m3 (20-32 ppm) with peaks as high as 200 mg/m3
    (70 ppm). These peaks had probably been higher in the past ranging up
    to 290 mg/m3 (100 ppm) or more. The exposed group had significantly
    more respiratory symptoms and colds. They also complained more of
    fatigue and shortness of breath on exertion. Chest X-rays of the
    exposed and unexposed groups showed the same distribution of
    abnormalities. The authors concluded on their inadequate evidence that
    there was no injury to the tracheobronchial tree or alveoli.

        In studies on men working in smelters in Sweden, Sjrstrand (1947)
    found that those who had worked at the roasting and reverberatory
    furnaces and in the converter hall for 8 years or more had poorer
    respiratory function than men working in other parts of the smelter.
    Levels of exposure and the smoking histories of the subjects were not
    reported. Men working in smelters are exposed to a variety of dusts as
    well as to sulfur dioxide and it is difficult to separate the effects
    of such exposures from those due to sulfur dioxide.

        Men exposed to sulfur dioxide at daily mean concentrations up to
    70 mg/m3 (25 ppm) with occasional peaks of 290 mg/m3 (100 ppm) in
    certain areas of a petroleum refining plant in Abadan (Iran) were
    compared by Anderson (1950) with an unexposed group. No differences
    were found between the two groups. This study also did not refer to
    the smoking histories of the subjects.

        In a study in Norway, pulp mill workers were compared with paper
    mill workers using a standard questionnaire on respiration and simple
    tests of pulmonary function. The smoking histories of the subjects
    were also studied. Levels of sulfur dioxide ranged from 6-100 mg/m3
    (2-36 ppm) with peaks of 290 mg/m3 (100 ppm) when the digester was
    blown. The exposed group had more cough, sputum, and dyspnoea than the
    unexposed group but the vital capacities were similar in both groups.
    The expiratory peak flows, however, of the exposed men under 50 years
    of age were lower than those in a comparable unexposed group (Skalpe,
    1964).

        A similar study was carried out in the USA by Ferris et al. (1967)
    who reported that there was no difference between men in a pulp mill
    and men in a paper mill. Both groups had less respiratory disease than
    was reported from a survey of the general population. The authors
    noted that some of the men working in the paper mill had worked in the
    pulp mill but had left because they could not tolerate the conditions.
    Occupational levels of exposure to sulfur dioxide ranged from a trace
    to 95 mg/m3 (33 ppm). Average levels ranged from 6 to 35 mg/m3
    (2-13 ppm). Smoking habits were considered.

        Huhti et al. (1970) studied pulp and paper mill workers in Finland
    and noted that the effects of smoking were much more significant than
    the exposures at work or the effects of climate. Levels of exposure
    were not reported in this study.

        Men working in two integrated steel mills in Wales were studied by
    Lowe et al. (1968, 1970) and Warner et al. (1969). Mean concentrations
    of sulfur dioxide over 3 years ranged from 1.8 to 2.1 mg/m3 (0.6 to
    0.7 ppm) and those of suspended particulate matter in the respirable
    range, ranged from 600 to 1800 g/m3 (by elutriation technique).

    Analysis of the particulate matter showed that it was mainly composed
    of iron oxides and calcium sulfate. No effects on respiratory symptoms
    or on simple tests of pulmonary function were found after
    standardization for cigarette smoking and for age. These observations
    may reflect the limitation of studying occupational groups because of
    the effect of the selection processes, or this may be an example in
    which the suspended particulate matter present was not interacting
    with the sulfur dioxide to produce an effect.

    8.2.2  Exposure to sulfuric acid mist

        Dorsch (1913) studied men exposed to sulfuric acid mist in a plant
    manufacturing storage batteries in Germany. He reported that at a
    concentration of 0.5 mg/m3, the mist was barely noticeable; at
    2.0 mg/m3, there was nose and throat irritation, at 3-4 mg/m3, there
    was distinct discomfort, and at 6-8 mg/m3, there was marked
    discomfort. These responses are comparable to those reported in
    laboratory studies on human beings in section 8.1. No particle size
    was given but, as noted in another survey reported below, they were
    probably relatively large.

        Men in the battery industry were also examined by Malcolm & Paul
    (1961) in the United Kingdom who reported that there was significant
    erosion of the teeth of the battery room workers. This was confirmed
    by ten Bruggen Cate (1968). Apparently this was due to the direct
    impingement of relatively large droplets of sulfuric acid on the
    teeth.

        Williams (1970) studied men from the same works as Malcolm & Paul
    (1961) and reported that there was no difference in the forced vital
    capacity and the one-second forced expiratory volume between the men
    in the forming and control departments. Levels of sulfuric acid mist
    averaged 1.4 mg/m3 during working hours over two days and ranged from
    a trace to 6.1 mg/m3 in 1968. An earlier survey reported higher
    values. Particle size reported from a survey in another firming
    department doing comparable work was 14 m MMD; 4% of the particles
    being less than 4 m MMD. The level of sulfuric acid in this
    department was 2.7 mg/m3.

    8.3  Community Exposure

        Much of the information that has been gained concerning the
    effects on health of exposure to realistic concentrations of sulfur
    oxides and particulate matter has come from epidemiological studies,
    carried out on segments of population chosen by virtue of place of

    residence, age, existing state of health, or other characteristics, in
    order to present contrasts in exposure or sensitivity to these
    pollutants. Some studies have been based on the complete populations
    of urban areas, observing the total number of deaths, or the incidence
    or prevalence of illness within them in relation to differences in
    pollution between areas, or with time in any one area.

        Many epidemiological studies concerning the health effects of
    exposure to sulfur oxides and particulate matter have been reported in
    the literature. In the discussion that follows, attention has been
    directed to papers that yield information relevant to the development
    of exposure-effect and exposure-response relationships for sulfur
    oxides, smoke, and suspended particulate matter, and to some others
    that are of interest from the point of view of the method of approach.

    8.3.1  Mortality -- effects of short-term exposures

        The most clearly defined effects on mortality arising from
    exposure to sulfur oxides and particulate matter have been the sudden
    increases in the number of deaths occurring, on a day-to-day basis, in
    episodes of high pollution. The most notable of these occurred in the
    Meuse Valley in 1930 (Firket, 1931), in Donora in 1948 (Schrenk et
    al., 1949), and in London in 1952 (Ministry of Health, UK, 1954). The
    people primarily affected were those with pre-existing heart or lung
    disease or both, and the elderly. The London episode lasted for 5 days
    and it was estimated that the number of deaths during and immediately
    after this period was about 4000 more than expected under normal
    circumstances. On one day, the number of deaths was about three times
    the number expected at that time of the year. Concentrations of sulfur
    dioxide as high as 3.7 mg/m3 (1.3 ppm) were recorded in the centre of
    the urban area (48-h average). Concentrations of particulate matter
    were too great to be measured properly (British daily smoke/sulfur
    dioxide method), and the 48-h average of about 4.5 mg/m3 at a central
    site must be regarded as a conservative estimate. These were rough
    estimates for the exposures and, probably, there was considerable
    variation in individual exposures.

        Following these major episodes, attention was turned to studies on
    more moderate day-to-day variations in mortality within large cities,
    in relation to pollution. Gore & Shaddick (1958) correlated mortality
    in the County of London (the inner part of the Greater London Area)
    with pollution by smoke and sulfur dioxide for 4 foggy periods in
    1954-56, using 7-day moving averages to smooth out the data. The
    authors considered that, in two of the episodes, there was a marked
    increase in mortality from bronchitis and other lung diseases,
    particularly in the elderly. They concluded that when the 24-h mean

    concentration of smoke exceeded 2.0 mg/m3 at the same time as the
    24-h mean concentration of sulfur dioxide exceeded 1.1 mg/m3
    (0.4 ppm) (British daily smoke/sulfur dioxide method), there would be
    increased mortality. Care was taken in this study to ensure that the
    measurements were reasonably representative of the exposure of people
    anywhere within the study area: the figures quoted were the mean
    values from a group of 7 sites, all situated close to ground level in
    mainly residential areas. However, there remained the problem that the
    people at risk in a study of this type were the elderly sick, who were
    likely to remain indoors, and that outdoor measurements might not have
    provided an adequate assessment of exposure (Biersteker et al., 1965).

        The relationship between daily mortality in the more extensive
    area of Greater London and day-to-day variations in pollution (smoke
    and sulfur dioxide) and visibility was examined by Martin & Bradley
    (1960) in the winter of 1958-59. They noted that on days when the
    smoke concentration increased by more than 100 g/m3 compared with
    the previous day, or when the sulfur dioxide concentration increased
    by 70 g/m3 (0.025 ppm), there was likely to be increased mortality
    (British daily smoke/sulfur dioxide method). The increases in daily
    mortality were up to about 1.25 times expected values assessed from
    15-day moving averages. Thick fog (visibility less than 200 metres)
    was also associated with increases in mortality. The relative
    importance of the 3 factors could not be determined but, on the basis
    of other work, the authors considered that the smoke was probably the
    most important. It is not clear whether the results are best
    interpreted in terms of change in pollution from one day to the next,
    rather than in terms of absolute values, but there is support for the
    former approach from studies carried out elsewhere. When results were
    considered on an absolute basis (Lawther, 1963), it was concluded that
    increases in mortality became evident when the 24-h mean
    concentrations of smoke and sulfur dioxide exceeded 750 g/m3 and
    710 g/m3 (0.25 ppm), respectively. The measurement sites were the
    same as those used by Gore & Shaddick (1958). They could still be
    considered reasonably representative of outdoor concentrations in the
    areas where people lived, although the inclusion of outer,
    less-densely populated areas meant that the average exposures would
    tend to have been underestimated.

        Studies on day-to-day variations in mortality in London were
    continued in successive winters, and coupled with the records of
    emergency hospital admissions. In a later paper, Martin (1964) showed
    correlations between both the daily mortality and hospital admission
    data and concentrations of smoke or sulfur dioxide. There was no
    clearly defined level above which effects were seen, but there were
    fairly consistent increases in both mortality and hospital admissions

    when the concentrations of smoke and sulfur dioxide each exceeded a
    24- mean of about 500 g/m3 (0.18 ppm of sulfur dioxide, British
    daily smoke/sulfur dioxide method). In 1962, there was a major episode
    of high pollution in London, similar in terms of duration and of
    sulfur dioxide concentrations to the one in 1952, but with lower smoke
    concentrations. Again, there was a sudden increase in deaths, but the
    number of deaths was not as great as before (about 700, compared with
    4000). Whether the change in medical care could have influenced these
    results is not clear. The greater use of antibiotics in 1962 compared
    with 1952 might have reduced the number of deaths, and a greater
    awareness of the risk together with clear advice to the elderly and
    infirm to remain indoors could have had an effect. The dramatic
    reduction in smoke concentrations in London brought about by the
    implementation of the Clean Air Act, and the more gradual reduction in
    sulfur dioxide that has followed it, have meant that in more recent
    years there have been few occasions when levels of 500 g/m3 have
    been exceeded simultaneously for smoke and sulfur dioxide (Waller et
    al., 1969).

        Biersteker (1966) published a study of an episode of high
    pollution in Rotterdam in December 1962, when concentrations of smoke
    and sulfur dioxide of approximately 500 g/m3 and 1000 g/m3
    (0.35 ppm), respectively, were recorded (24-h means, OECD smoke/sulfur
    dioxide method). There were increases in admissions to local hospitals
    of people over 50 years of age with cardiovascular diseases, and there
    was also some indication of an increase in mortality. This was
    observed only once in Rotterdam and could have been due to other
    causes. Further observations during a similar episode would be needed
    to provide a convincing statistical relationship between hospital
    admissions and these levels of pollution.

        A relationship between day-to-day changes in mortality and
    pollution has also been reported from Osaka (Watanabe 1966). There
    appeared to be increases in mortality (about 20%) on days when the
    concentration of suspended particulate matter (light scattering
    method) exceeded 1 mg/m3 (4-day average) and was associated with a
    level of sulfur dioxide of 200 g/m3 (0.07 ppm). Low temperatures may
    have been partly responsible for the effects.

        Variations in daily mortality in New York in relation to sulfur
    dioxide concentrations were studied by Buechley et al. (1973). They
    examined correlations and developed regressions between a number of
    daily climatic factors and indices of pollution (sulfur dioxide,
    conductimetric method and coefficient of haze (Cohs)), and the
    mortality residuals for a given day. They noted that the day of the
    week had a special correlation with mortality (mortality rates were
    considerably higher on Mondays). Regression analysis indicated that
    heat waves and seasonal cycle were major predictors of mortality.

    Other factors were much weaker (about one third as strong) but were
    all of equal strength. Partial residual mortality values were computed
    and showed a significant correlation with the levels of air pollution
    (sulfur dioxide r = 0.14). Mortality could be predicted equally as
    well from Cohs as from sulfur dioxide levels. Mortality was 1.5% less
    than expected on 232 days when sulfur dioxide levels were below
    30 g/m3 (0.01 ppm) and 2% greater than expected on 260 days when the
    sulfur dioxide levels were above 500 g/m3 (0.18 ppm) after
    correcting for the other factors. The crossover point (i.e., that
    point below which deaths were less than expected and above which
    deaths were greater than expected) was in the vicinity of a
    concentration of sulfur dioxide of 260 g/m3 (0.09 ppm). On the other
    hand, the data from these studies could be interpreted to show a
    continuum of an effect across the levels of sulfur dioxide, which, in
    turn, should not be considered the causative agent but rather an index
    of pollution.

        Schimmel & Murawski (1975) have reported on their regression
    analysis of daily deaths and levels of pollution (smoke shade and
    sulfur dioxide) in New York City for 1963-1972. This was an extension
    of an earlier study (Schimmel & Greenberg, 1972). The authors
    controlled for season, day of week, and temperature. During this time
    there was a marked reduction in the average level of sulfur dioxide
    from 510 g/m3 (0.18 ppm) to 170 g/m3 (0.06 ppm) but virtually no
    change in smoke shade. Their observations indicated that, despite this
    reduction in sulfur dioxide, there had been no reduction in adverse
    health effects. Analysis indicated that the adverse health effects
    were associated principally (80%) with the particulate matter and only
    to a small extent (20%) with the sulfur dioxide. However, the authors
    also pointed out that, when they regressed mortality on temperature
    and sulfur dioxide alone, the effects attributable to the sulfur
    dioxide increased three-fold.

        These findings are provocative but they must be interpreted
    cautiously because of a number of limitations in the data. Air
    pollution levels were from data obtained at a single monitoring
    station and were probably not truly representative of exposures for
    the community. The standard errors reported with their data are large.
    The report should be considered to be an indicator for further studies
    in which age-specific death rates and causes of death should be
    included in the analysis as well as more relevant air pollution
    measurements. The results should not be used for developing an
    exposure-effect relationship.

        A study involving comparisons between daily mortality data in New
    York and Tokyo was carried out by Lebowitz et al. (1973). They applied
    a "stimulus-response" technique to identify associations between days
    of high pollution and days with increased mortality, and showed strong
    relationships in both cities. However, their findings do not provide
    information of direct value in the assessment of exposure-effect
    relationships.

    8.3.2  Mortality  -- effects of long-term exposures

        In countries having reliable systems for the collection and
    analysis of data on deaths, based on cause and area of residence,
    death rates for respiratory diseases have commonly been found to be
    higher in towns than in rural areas. Many factors, such as differences
    in smoking habits, occupation, or social conditions may be involved in
    these contrasts, but, in a number of countries, a general association
    between death rates from respiratory diseases and air pollution has
    been apparent for many decades.

        Analyses of these data have been of great value as a lead for
    epidemiological studies, but the absence of information concerning
    other relevant variables, such as smoking, and the relatively crude
    nature of the indices of pollution used in many of these studies make
    them unsuitable for the assessment of exposure-effect relationships.

        The studies of Daly (1954, 1959), Pemberton & Goldberg (1954), and
    Stocks (1959) were all based on mortality data from towns in England
    and Wales, and each showed a positive correlation between bronchitis
    or pneumonia death rates and some index of pollution by sulfur oxides
    or particulate matter, as assessed for periods close to those for
    which death rates were calculated. The most detailed investigation of
    this type, taking into account social factors as well as pollution,
    but still not smoking, was that conducted by Gardner et al. (1969).
    One interesting feature of their findings was a slight improvement in
    correlation when the index of pollution used was related to a period
    some 10 years earlier than that for which the death rates were
    calculated (deaths 1958-64, pollution index 1952). This illustrates
    another of the problems that has been widely recognized when trying to
    use mortality records to assess the effects of pollution i.e., that it
    may not be recent exposures that are most relevant, but those earlier
    in life; where concentrations have changed markedly over the years,
    current measurements may not provide an adequate index.

        Lave & Seskin (1970) reanalyzed some of the mortality data from
    England and Wales, and developed multiple regression equations in
    terms of pollution and socioeconomic indices. Again their findings of
    positive correlations with pollution are of general interest but
    cannot contribute to the development of dose-response relationships.
    These authors also examined analogous data for Standard Metropolitan
    Statistical Areas (SMSAs) in USA and in a later paper (Lave & Seskin,
    1972) they attempted to assess the relative effects of air pollution,
    climate, and home heating on mortality rates. Although equations were
    obtained relating death rates to measurements of suspended particulate
    matter and total sulfates (both by high volume sampler), it is
    doubtful whether these can be regarded as valid in the absence of
    adequate information on smoking.

    8.3.3  Morbidity -- effects of short-term exposures

        Prospective studies on specific occupational groups, not
    professionally exposed, can be useful in assessing the effects of air
    pollution in different communities or in areas where a change in air
    pollution is expected. In such studies, where respiratory diseases are
    followed, it is necessary to control for age distribution and
    household composition, and to employ adequate statistical methods. In
    studies on the Philadelphia area, USA, Dohan & Taylor (1960) used
    absences of seven days or more because of respiratory disease as the
    index and related this to the levels of sulfate. A later report by
    Dohan (1961) noted that this relationship was stronger during an
    epidemic of influenza. Ipsen et al., (1969) repeated this type of
    study in the same area on a slightly different occupational group
    using more detailed statistical analyses. They were not able to
    confirm the earlier observations that the sulfate levels were related
    to absences due to respiratory diseases.

        Results have been reported (Lawther et al., 1970) of a series of
    studies extending from 1954 to 1968 that were carried out mainly in
    London, but also in some other large cities in England, using a diary
    technique for the self-assessment of day-to-day changes in conditions
    among bronchitic patients. A daily illness score was calculated from
    the data contained in the diaries and this was related to the
    concentrations of smoke and sulfur dioxide (British daily smoke/sulfur
    dioxide method) and to weather variables. The pollution figures used
    for most of the London studies were the mean values from the group of
    sites associated with the mortality/morbidity studies of Martin (1964)
    and Gore & Shaddick (1958). Many of the subjects in the series were
    active enough to be out and about and at work, and the measurements
    were considered to give a reasonable assessment of the average

    exposures in the areas where they lived or worked. The method used in
    these studies has not been validated, for the subjects recorded only
    their own assessment of their condition, and this was not checked
    against regular clinical examinations or ventilatory function
    measurements, but the changes appeared to have some real meaning. In
    the earlier years of the series, when the general level of pollution
    was high, well defined peaks in the illness score were seen when
    concentrations of either smoke or sulfur dioxide exceeded 1000 g/m3.
    With the reductions in pollution that followed the gradual
    implementation of the Clean Air Act, these changes in condition became
    less frequent and of smaller magnitude, and the conclusion from the
    series as a whole, up to 1968, was that the minimum pollution
    associated with significant changes in the condition of the patients
    was a smoke level of about 250 g/m3 together with a sulfur dioxide
    concentration of about 500 g/m3 (0.18 ppm) (24-h means, British
    daily smoke/sulfur dioxide method). At these levels, there was still
    some evidence that the peaks were associated specifically with
    pollution rather than with adverse weather conditions. A later study
    that has been reported by Waller (1971), showed that, with much
    reduced average levels of pollution, there was an almost complete
    disappearance of days with smoke levels exceeding 250 g/m3 and
    sulfur dioxide levels exceeding 500 g/m3 (0.18 ppm). As in earlier
    studies, some correlation remained between changes in the condition of
    the patients and daily concentrations of smoke and sulfur dioxide but
    the changes were small at these levels. At this low range of
    pollution, discrimination between the effects of pollution and those
    of adverse weather was poor.

        Cohen et al. (1974) studied symptoms of irritation during a
    publicized and an unpublicized period of air pollution as well as
    during a control period in 3 communities in the New York metropolitan
    area during the summer of 1970. They used a telephone survey technique
    to inquire about specific symptoms such as eye irritation, throat
    irritation, chest discomfort, shortness of breath, restricted
    activity, and medical visits. No difference was noted between the 2
    episodes of pollution. Both episodes showed significantly increased
    symptoms compared with the control period. The results indicated that
    irritative symptoms increased significantly when sulfur dioxide levels
    exceeded 310 g/m3 (0.11 ppm, West-Gaeke method) and total suspended
    particulates exceeded 145 g/m3 (high volume sampler) as a 3 day
    average. Sulfate levels ranged from 6.6 to 7.6 g/m3 in one area and
    5.8 to 12.3 g/m3 in another area. In the second area, sulfate levels
    during the 2 periods of air pollution were 8.5 and 12.3 g/m3,
    respectively. Sulfate levels were not reported from the third area but
    were probably low. The authors drew attention to some of the problems

    associated with their study. The persons interviewed were generally
    wives and the symptomatology in the male population could have been
    underestimated. Also, there was an internal inconsistency possibly due
    to intercurrent infectious disease or socioeconomic differences.
    During the publicized episode, particulate pollution was considerably
    higher in the Bronx than in Queens whereas irritation symptoms were
    somewhat higher in Queens. The authors concluded that there could have
    been confounding effects of other air pollutants, intercurrent
    infection, or sociocultural factors.

        Spirometric measurements were made at approximately weekly
    intervals on 18 patients with chronic obstructive lung disease for
    various periods during 1969-71 (Emerson, 1973). The spirometric values
    (FEV1.0 and MEFR) were correlated with the levels of pollution (sulfur
    dioxide and smoke, British daily smoke/sulfur dioxide method) and
    climatic factors (temperature and humidity). Changes in FEV1.0 in
    these patients were more strongly correlated with temperature and
    humidity than with concentrations of sulfur dioxide or smoke. Levels
    of air pollution in London were: sulfur dioxide, mean 190 g/m3
    (0.07 ppm), maximum, 720 g/m3 (0.25 ppm) and smoke mean, 44 g/m3,
    maximum, 240 g/m3. One limitation of the study was that the
    pollution figures were averaged for 5-day periods, whilst the
    spirometric measurements were made on specific days.

        Studies of patients with chronic bronchitis in Chicago, USA
    (Burrows et al., 1968; Carnow et al., 1969) showed conflicting
    results. The reasons for these differences are not clear but may be
    that different criteria for the selection of patients as well as
    different methods for the determination of the exposure of the
    individuals and their responses were used.

        An unexpected finding of a possible effect of short-term exposure
    to pollution arose from a study primarily concerned with long-term
    exposures. In a resurvey of adults in Vlaardingen (Netherlands) who
    had been interviewed and had lung function measurements made in 1969,
    Van der Lende et al. (1975) found that the average lung function
    values were higher in 1972 than in 1969, even though the subjects were
    3 years older. When the authors examined the concentrations of
    pollution on the 2 occasions (each survey having been done within a
    5-day period), they found that levels were relatively high in 1969
    with daily values ranging from 15 to 140 g/m3 for smoke and from 120
    to 300 g/m3 (0.04-0.11 ppm) for sulfur dioxide compared with values
    ranging from 15 to 40 g/m3 and 45-100 g/m3 (0.02-0.04 ppm),
    respectively, in 1972. The increase in lung function was most
    pronounced on the days with the greatest difference in the levels of
    pollution. A control population in a rural area showed no comparable
    changes over the same period, and temperature differences did not
    explain the effect.

        Asthmatic subjects have also been studied. These patients
    represent a heterogeneous group and this may account for the variable
    responses that have been reported. Cohen et al. (1972) studied 20
    asthmatic subjects living in a small town (Cumberland, WV, USA) in the
    vicinity of a coal-fired power plant. They found that when the soiling
    index exceeded 1.0 Coh unit, or sulfur dioxide concentrations
    (West-Gaeke method) exceeded 200 g/m3 (0.07 ppm), or total suspended
    particulates exceeded 150 g/m3 (high volume sampling method), or the
    temperature was lower than 0C, there was a significant increase in
    the frequency of asthmatic attacks. Levels of sulfates exceeding
    20 g/m3 and nitrate levels exceeding 2 g/m3 did not result in such
    an effect. In general, the effect of temperature was stronger than
    that of the air pollutants although each of the 5 air pollutants
    measured including sulfates and nitrates showed a correlation. When
    temperature and any one of the pollutants were controlled for in the
    analysis, the effect of any of the other 4 pollutants was eliminated.
    Temperatures below 0C overwhelmed any effect of the pollutants and
    higher levels of the pollutants reduced the effect of temperature. One
    further feature of this study should be stressed. Pollution in
    Cumberland was dominated by emissions from a large single source, and
    this might have led to high transient exposures to the pollutants,
    which included oxides of nitrogen as well as sulfur oxides and
    suspended particulate matter. In these circumstances it is doubtful
    whether the 24-h values provided an adequate index of exposure of the
    population.

        Several studies on asthmatic subjects in Yokkaichi, Japan, have
    been reported by Yoshida et al. (1966) including one on a group of 13
    patients in which the number of attacks increased from 1 to 4 per week
    when the concentration of sulfur dioxide was in the range of
    140-230 g/m3 (0.05-0.08 ppm), rising to about 12 per week when the
    sulfur dioxide level reached 740 g/m3 (0.26 ppm), all expressed as
    weekly means. Suspended particulate or smoke levels were not reported.

    8.3.4  Morbidity in adults -- effects of long-term exposures

        Random samples of populations can be used for international
    comparisons where there are gradations of air pollution. The major
    difficulty here has been to ensure comparability with respect to
    occupational exposures and ethnic groups. Reid et al. (1964) reported
    such a comparison based on a study in the United Kingdom (College of
    General Practitioners' Study, 1961) and a survey in Berlin, NH, USA
    (Ferris & Anderson, 1962). The same questionnaire was used in both
    studies. In the United Kingdom, it was completed by a large number of
    practitioners who made up the survey group. In the USA, it was
    completed by 2 physicians who tried to maintain the criteria developed
    in the British survey. Results were standardized for cigarette
    smoking. The effects of air pollution were then examined by age group
    and sex. When simple bronchitis was present, i.e., phlegm production

    for 3 months out of the year for 2-3 years, standardizing for
    cigarette smoking removed any effect of air pollution for both males
    and females. A more severe form of chronic bronchitis characterized by
    phlegm production, exacerbations of colds that went to the chest, and
    shortness of breath when walking on the level at one's own pace did
    show an association with air pollution for both males and females,
    even after standardizing for cigarette smoking. Levels of air
    pollution were measured in Berlin, NH, by the lead peroxide candle,
    dustfall, and high-volume samplers. Data for the United Kingdom were
    estimated from similar measurements obtained in comparable towns and
    cities where the general practitioners collected the data. In Berlin,
    NH, the sulfation rate (lead candle) was 730 g SO3/100 cm2 per day;
    in the United Kingdom, in the large towns it was 950 and in the large
    conurbations 1650 g SO3/100 cm2 per day. The results of this study
    seem to be consistent with those of other studies but it is not known
    whether differences in socioeconomic or ethnic status could have been
    relevant factors. The population of Berlin, NH, was resurveyed in
    1967, and the results were compared with those obtained in 1961
    (Ferris et al., 1973). There had been some decrease in pollution
    levels in the interval, the sulfation rate being 470 g SO3/100 cm2
    per day in 1967, compared with the earlier figure of 730, while the
    concentration of total suspended particulates had fallen from 180 to
    132 g/m3 (high-volume samples). Small reductions in the prevalence
    of respiratory disease were noted after standardizing for age and
    cigarette smoking. Slight improvements in forced vital capacity (FVC)
    and peak expiratory flow rate (PEFR) were also noted, but there was
    little change in FEV1.0. There is some doubt about the relevance of
    the 180 g/m3 figure quoted for total suspended particulates in the
    1961 study, since it referred only to a 2-month period at a single
    site. A random population sample from Chilliwack, BC, Canada, an
    unpolluted community, was studied by Ferris & Anderson (1964) and the
    results were compared with the results of the 1961 study in Berlin,
    NH. Respiratory symptoms, after standardization for age and cigarette
    smoking, tended to be higher in Berlin than in Chilliwack. Pulmonary
    function (FEV1.0 and PEFR) was lower in Berlin than in Chilliwack,
    after standardization for age, height, sex, and smoking category.
    Pollution levels in Chilliwack (based on lead-candle measurements)
    were about one-tenth to one-sixth of those in Berlin, NH.

        A third survey in this series was carried out in 1973 (Ferris et
    al., 1976). By this time, there had been a further decline in
    pollution by particulate matter, the annual mean concentration of
    total suspended particulates (high volume sampler) being quoted as
    80 g/m3. Only limited data on sulfur dioxide were available; the
    mean of a series of 8-h samples for selected weeks was quoted as
    0.01 ppm (30 g/m3). On this occasion, the authors did not find any
    appreciable differences in the prevalence of respiratory symptoms or
    in measures of lung function, as compared with the 1973 report. The

    interpretation of these findings is difficult, for the studies were
    concerned largely with consecutive investigations of survivors of the
    original group, over a 12-year period. Although the authors took into
    account, as far as possible, the effect of selective losses of some of
    the population, and changes in smoking habits among those who
    remained, there is still some doubt as to whether the three sets of
    results are truly comparable. The authors themselves concluded that
    either the changes in air pollution levels from 1967 to 1973 (which
    included a decline in total suspended particulates from about 130 to
    80 g/m3 (annual mean) with possibly a slight increase rather than
    decrease in sulfur dioxide) were not associated with a beneficial
    effect on health, or that their methods were not sufficiently
    sensitive at the levels involved.

        One of the difficulties in interpreting these data is that
    exposure to odorous air pollutants (Berlin, NH) also occurred,
    indicating an unconventional type of air pollution. It does seem
    reasonable, however, to interpret the results of these surveys as
    showing that slight changes in respiratory symptoms and pulmonary
    function were related to levels of pollution. Sulfur oxides and
    particulate matter may have been of importance. Only sulfation data
    are available for sulfur oxides in the 1961 study.

        Extensive studies have been made of post office and telephone
    workers in the United Kingdom and USA (Holland & Reid, 1965; Holland &
    Stone, 1965; Holland et al., 1965). The authors carefully considered
    most of the relevant epidemiological variables and showed a gradation
    of symptoms across the levels of pollution, particularly in the 50 to
    59-year-old category. However, since pollution was measured in
    different ways in each part of the studies, it is difficult to deduce
    any quantitative relationships with sulfur dioxide and particulate
    matter. The differences in the prevalence of symptoms persisted when
    the authors examined the various smoking categories. They converted
    the small amount of pipe and cigar smoking to cigarette equivalents,
    which is not advisable, since other studies have indicated that pipe
    and cigar smoking are less markedly associated with respiratory
    symptoms. Lower levels of FEV1.0 and PEFR were observed in the areas
    of higher pollution. The authors indicated that a difference in height
    of 2-4 cm between the two populations studied could not account for
    the differences seen in pulmonary function. Presumably, these
    pulmonary function values had been corrected for standard temperature
    and saturated vapour pressure. If not, some of the differences between
    values in the USA and United Kingdom could be explained by the fact
    that lower temperatures in England could have resulted in lower
    measured air volumes. Similar spirometers were used in both studies.

        In another study in the United Kingdom, Lambert & Reid (1970)
    questioned about 10 000 adults by post. They estimated that their
    sample represented about 74% of those able to reply. The positive
    responses from the questionnaire (which had been recommended by the
    British Medical Research Council) were correlated with levels of
    pollution estimated from data used earlier by Douglas & Waller (1966)
    and from some data from the National Air Pollution Survey. The
    responses were controlled for social class and cigarette smoking. The
    authors reported that whereas nonsmokers showed little response to the
    levels of air pollution, cigarette smokers did respond and appeared to
    be more sensitive. They also noted a considerable rural/urban
    gradient, more pronounced in men than in women, that could not be
    explained by differences in smoking habits.

        Extensive studies were carried out in the Ruhr area of the Federal
    Republic of Germany (Reichel et al., 1970; Ulmer et al., 1970) based
    on a short questionnaire on respiratory symptoms, physical
    examinations, and measurements of airways resistance using a body
    plethysmograph. There were no clear differences in the results between
    areas with different levels of pollution, but because of selection
    factors and a low response rate, no definite conclusions can be drawn
    from these studies regarding relationships with sulfur dioxide and
    particulate matter.

        A study in Vlaardingen in the Netherlands by Van der Lende (1969)
    indicated that increased cough and phlegm production were associated
    with air pollution but decreased lung function was not. However, the
    author considered that the study might have been affected by the
    presence of allergens, such as spores and bacteria in the agricultural
    regions. Comparisons were made between an industrial area, polluted by
    sulfur dioxide and particulate matter, and an agricultural area with
    little pollution. In a later report, Van der Lende et al. (1973)
    indicated that the effect of pollution could have been masked by
    "self-selection" of the populations concerned.

        The relationship between chronic bronchitis and air pollution was
    studied in Osaka Prefecture, Japan, by Tsunetoshi et al. (1971).
    Surveys were made of all persons over 40 years of age in selected
    areas using self-administered questionnaires similar to that
    recommended by the British Medical Research Council. The authors'
    definition of chronic bronchitis was cough with sputum for 3 or more
    months, for at least 2 successive years. Pulmonary function was
    measured by the Vitalor, using the maximum value. An equation was
    developed relating prevalence of bronchitis to age, smoking, and
    sulfation rate (lead candle). The prevalence of bronchitis in the
    study areas ranged from about 4% (in males aged 40-59 years) in areas
    where the sulfation rate was close to 1 mg/100 cm2 per day to about
    10% in those around 3 mg/100 cm2 per day. No data were given
    concerning the levels of particulate matter.

        Many other studies on the prevalence of respiratory symptoms in
    relation to air pollution have been carried out in Japan such as that
    by Toyama et al. (1966). They reported prevalences ranging from 2.8 to
    3.7% (males aged 40-59 years, adjusted for age and smoking to accord
    with other Japanese studies) in areas of Kashima (a nonindustrialized
    rural town, at that time) with sulfur dioxide concentrations of less
    than 30 g/m3 (0.01 ppm, automatic conductimetric method) and
    suspended particulate concentrations (high-volume sample) of
    106-341 g/m3 (mean 197). An extensive survey in Tokyo, (Suzuki &
    Hitosugi, 1970, unpublished data)a showed a higher prevalence of
    chronic bronchitis in areas that were more polluted, ranging from 5.5%
    (males aged 40-59 years) and 1.1% (females 40-59 years) where the
    sulfur dioxide concentration was below 60 g/m3 (0.02 ppm, automatic
    conductimetric method), and the suspended particulate level was below
    100 g/m3 (light-scattering method), to 6.7% (males) and 3.9%
    (females), where sulfur dioxide was over 140 g/m3 (0.05 ppm) and
    suspended particulates more than 200 g/m3. There was also some
    increase in prevalence in an intermediate area with sulfur dioxide
    concentrations of 60-140 g/m3 (0.02-0.05 ppm) and a suspended
    particulate concentration of 100-200 g/m3. Smoking habits were
    standardized in this study. In a recent study by Tani (1975) in the
    vicinity of a pulp mill at Fuji City, and in control areas, there
    appeared to be a consistent relationship between the prevalence of
    bronchitis and sulfation rates, as measured by lead candles, with a
    prevalence of about 3% (males and females combined, aged 40-59 years)
    in areas where the sulfation rate was around 0.6 mg/100 cm2 per day
    to about 8% where it was 1.2 mg/100 cm2 per day.

        In studies on a random sample of adults in Cracow, Polandm the
    levels of pollution measured made it possible to classify the subjects
    into high and low exposure groups (Sawicki, 1972). The levels in the
    high pollution area were an annual mean smoke level of 170 g/m3 and
    an annual mean sulfur dioxide level of 125 g/m3 (0.04 ppm). In the
    low pollution area, the standard smoke annual mean was 90 g/m3 and
    the sulfur dioxide annual mean was 45 g/m3 (0.02 ppm). Persons

              

    a  Suzuki, T. & Hitosugi, M. Prevalence study of pulmonary symptoms
       in Tokyo Prefecture employees. A paper presented at a meeting of a
       Working Group on Air Pollution and Health, USA-Japan Cooperative
       Science Programme, East-West Center, Honolulu, 21-23 April, 1970. The
       pollution levels quoted in this study were obtained from reports of
       the Tokyo Metropolitan Government concerning air pollution levels for
       the years 1966-69 (Department of Environmental Pollution Control
       Tokyo, 1976).

    residing in the more polluted area had more respiratory symptoms and
    poorer pulmonary function than those residing in the area with lower
    pollution (chronic bronchitis 19% in more polluted area and 11% in
    less polluted area and asthmatic disease 11% in more polluted area and
    5% in less polluted area). Sawicki considered that there was a
    synergistic interaction with cigarette smoking. He also pointed out
    that many of the inhabitants worked outside the area in which they
    lived and, therefore, the exposure levels at their places of residence
    might not represent their true exposures.

    8.3.5  Morbidity in children

        Studies of the health of children have also provided useful
    information.

        Douglas & Waller (1966) performed a cohort study based on a
    national sample of children born in the United Kingdom in the first
    week of March 1946. The children were followed medically for 15 years
    by health visitors and school doctors. The study was restricted to the
    3131 children who remained at the same address for the first 11 years
    of the enquiry, or who moved to an area that was in a similar
    pollution group. Levels of air pollution were estimated from the
    amount of coal consumed in a given area where each child lived, and it
    was shown at the end of the study that the 4 pollution categories that
    had been defined provided a satisfactory gradation in terms of the
    measurements of smoke and sulfur dioxide that were then available. In
    each of the several periods of life considered, from birth onwards,
    the authors noted a consistent relationship between the frequency of
    lower respiratory tract infections and pollution category. There was
    no sharp change at any particular pollution level, but if the rates
    within the lowest category (mainly rural areas) were taken as a
    baseline, then increased frequencies were seen in the next category
    up, with smoke and sulfur dioxide each of the order of 140 g/m3
    (0.05 ppm sulfur dioxide) or more. Annual means were estimates based
    on the British daily smoke/sulfur dioxide method for a period after
    the end of the study and probably underestimated earlier level.
    Socioeconomic status was important in the study, but a relationship
    between air pollution and frequency of lower respiratory tract
    infections still existed within separate social classes. In a later
    follow-up of these subjects, Colley et al. (1973) showed that, at the
    age of 20 years, respiratory symptoms were related to smoking habits
    rather than to the pollution of the areas in which the subjects were
    then living. However, there was some relationship between the
    prevalence of symptoms and earlier histories of lower respiratory
    tract infections, which, in turn, were related to estimated pollution
    exposures during childhood.

        Lunn et al. (1967) studied 819 children in their first year at
    school in the industrial city of Sheffield, in the United Kingdom.
    Medical examinations were carried out on the children, the parents
    were questioned concerning the previous health of the children, and
    FEV0.75 and FVC were measured. Various socioeconomic factors were also
    carefully considered. The families were stable, and there had been
    little or no migration into or out of the specific communities.
    Measurements of pollution were made at the schools attended by the
    children (British daily smoke/sulfur dioxide method). The authors
    found increased frequencies of both upper and lower respiratory tract
    infections in the more polluted areas. When the "cleanest" area was
    taken as the baseline, most of the illness indices considered showed
    an increase in the next area up in order of pollution levels, i.e.,
    where the annual mean concentrations of smoke and sulfur dioxide were
    each about 200 g/m3 (0.07 ppm sulfur dioxide) taking figures for the
    middle year of the survey. A follow-up study was carried out on some
    of these children when they were 4 years older (i.e., aged 9) (Lunn et
    al., 1970). By then, the implementation of the Clean Air Act had
    reduced the contrast in pollution between the areas, so that the mean
    concentration of smoke in the 3 "dirty" areas combined was about
    140 g/m3 and the mean sulfur dioxide concentration was 200 g/m3
    (0.07 ppm), compared with about 50 and 100 g/m3 (0.04 ppm sulfur
    dioxide), respectively, in the "clean" area. At this time, there was
    no significant difference in the illnesses reported by the 9-year-olds
    in the different areas and the authors concluded that this was in line
    with the reduced pollution levels.

        Data were collected (Holland et al., 1969) concerning about 11 000
    children attending school in 4 areas of Kent, England, 2 of which were
    predominantly urban, and 2, rural. As part of the children's regular
    medical examinations, peak expiratory flow rates, height, weight, and
    the results of examinations of ears and tonsils were recorded. Smoking
    habits and school absences were also recorded. Questionnaires
    concerning such factors as previous respiratory diseases, father's
    occupation, and size of family were completed by the parents. Smoke
    and sulfur dioxide were measured in 3 of the areas (British daily
    smoke/sulfur dioxide method) and information on population density and
    housing was collected. Four factors emerged as important in relation
    to decreased peak expiratory flow rates. Place of residence was most
    important followed by previous history of respiratory disease; family
    size and social class were of least importance. These 4 factors seemed
    to be additive but they only accounted for 10-15% of the total
    variation. Levels of air pollution for smoke were reported to range
    from 34 to 69 g/m3 in the 3 communities. Values for sulfur dioxide
    were not reported but were stated to parallel those for smoke. There
    were clearly other factors affecting area difference, and there is
    some doubt as to whether the small contrasts in pollution could have
    had much effect.

        Colley & Reid (1970) surveyed about 11 000 children from 6 to 10
    years of age, living in urban and rural areas of England and Wales.
    The prevalence of respiratory disease was assessed in the autumn.
    Pollution was assessed in terms of sulfur dioxide concentration, from
    some direct measurements (British daily smoke/sulfur dioxide method)
    coupled with estimates based on lead candle measurements of sulfation.
    Within the English areas studied (winter mean concentrations of sulfur
    dioxide ranging from about 30 to 150 g/m3 or 0.01-0.05 ppm) there
    was a gradient in the prevalence of symptoms, but the rates were much
    higher in Wales for comparable levels of pollution. The reasons for
    this difference were not clear, although it has been suggested that it
    could be related to the fact that solid-fuel consumption was high in
    Wales.

        Respiratory function measurements were made on children aged 10-11
    years from 2 different areas of the German Democratic Republic (Berlin
    and Bitterfeld) with different levels of pollution (Grosser et al.,
    1971). The groups were matched for social class, age, and height. In
    the low pollution area, the concentration of respirable dust (method
    not mentioned) was 110 g/m3 and that of sulfur dioxide (method not
    mentioned) was 50 g/m3 (0.02 ppm); in the other area, the figures
    were 290 and 360 g/m3 (0.13 ppm), respectively (based on 24-h
    measurements, averaged for July and December). Two studies were made,
    6 months apart, in each of the 2 areas. Each of the spirometric values
    studied (FVC, FEV1.0, FEV0.7) was higher in the area with lower
    pollution.

        A variety of other techniques has been used to investigate
    possible effects of exposure to pollution. These include the work of
    Yoshii et al. (1969) who noted an association between chronic
    pharyngitis accompanied by histopathological changes at biopsy and
    level of sulfation, expressed as the annual mean, in persons attending
    their clinic in Yokkaichi, Japan, and in sixth-grade children. In the
    heavily polluted districts sulfation rate was more than 1.0 mg/100
    cm2 per day, in moderately polluted districts it ranged from 0.25 to
    1.0 mg/100 cm2 per day, and in the control area it was less than this
    level (lead candle measurements).

        Conjunctivitis, both acute and chronic, has been reported in
    Zabrze, Poland, in relation to industrial air pollution (Maziarka &
    Moroz, 1968). In Czechoslovakia, Schmidt et al. (1966) reported
    differences in the blood cells, tonsils, and cervical lymph nodes of
    children living in a highly polluted atmosphere compared with those
    living in a relatively clean atmosphere and Symon et al. (1969)
    reported retardation in growth and ossification and a reduced colour
    index in red blood cells in children living in areas with high air
    levels of sulfur dioxide (up to 2-3 mg/m3) and fly ash. None of these
    studies can be used for the exposure-effect relationship.

    8.3.6  CHESS studies

        The US Environmental Protection Agency has recognized the need for
    studies at relatively low levels of air pollution in order to
    determine whether a no-effect level can be identified to develop a
    dose-response relationship and to monitor areas where there might be a
    change in the levels of exposure. The CHESS (Community Health and
    Environmental Surveillance System) studies were developed with these
    purposes in mind. In some aspects these studies have been well
    designed and executed but in other aspects there have been
    difficulties. In many ways these are preliminary studies that need to
    be continued. The studies have included adults, children, asthmatic
    subjects, and patients with cardiovascular and pulmonary diseases.
    Both acute and chronic effects have been studied. Methods have
    involved mailed questionnaires on respiratory symptoms and illnesses
    (adults and children), mailed questionnaires on family composition,
    housing, and socioeconomic status, telephone interviews, tests of
    pulmonary function (children), and diary techniques with patients.

        Previous exposures to air pollutants were estimated from
    historical data on emissions or known production figures from the
    local industries and the application of mathematical models. More
    recent exposures were based on actual measurements. These were
    generally conducted at monitoring stations (each of which covered a
    radius of up to 2 km), and involved high volume sampling for total
    suspended particulates and suspended sulfates, and the West-Gaeke
    method for sulfur dioxide. Mathematical models were used to estimate
    the variation in exposure of the different groups. Various problems
    discussed in a summary of the results (US Environmental Protection
    Agency, 1974) include poor response rates (i.e., low participation
    rates) in some of the studies on children, high dropout rates in
    studies on patients, and, in at least one of the studies on children,
    the fact that their spirometry was probably in error.

        There was a general tendency for the authors to over-interpret the
    CHESS data. In some of the studies on children, the results were
    grouped according to high and low levels of pollution and a difference
    was found between the two groups. However, the relationship of the
    prevalence of disease to levels of pollution was not clear -- namely,
    when the individual groups were examined, some exposed to the high
    levels of pollution had a lower disease prevalence than some of the
    groups exposed to low levels of pollution.

        The design for the study of the impact on respiratory disease was
    excellent and took into account family composition, housing, and
    socioeconomic status. It is unfortunate that these studies were not
    continued for more than one year of observation before reporting. A
    number of years of observations are needed to rule out natural
    fluctuations in respiratory disease.

        These studies have been reviewed by an expert committee in the USA
    under the chairmanship of Dr D. Rall (Director, National Institute of
    Environmental Health Sciences, US Department of Health, Education and
    Welfare) (Rall, 1974). The Committee's evaluation of the studies was
    similar to that of the Task Group and the Committee concluded that the
    results of the studies did not warrant any change in the present US
    Federal Air Quality standards.

        It was the opinion of the WHO Task Group that these data could not
    be used for the estimation of an exposure-effect relationship.

    8.3.7  Lung cancer and air pollution

        The possibility that air pollution is a causal factor in cancer of
    the lung has given rise to considerable concern. The evidence in
    favour of a causal relationship is briefly:  (a) the excess
    occurrence of the disease in urban areas;  (b) the presence in the
    suspended matter in urban air of substances such as benzo(a)pyrene
    that can cause cancer under experimental conditions; and  (c) the
    general rise in lung cancer that appeared, at one time, to follow
    certain assumed trends in pollution.

        Early studies in the United Kingdom (Stocks, 1959, 1966) indicated
    that variations in lung cancer mortality in urban areas were
    associated with variations in amounts of pollution and, following a
    recommendation by a WHO Study Group in 1959 (World Health
    Organization, 1960), a pilot international study was undertaken in
    several cities where there were contrasts in lung cancer death rates.
    The results did not show any clear-cut relationship with measurements
    of particulate matter or its benzo(a)pyrene content (Waller & Commins,
    1967) and it was clear that apart from the difficulties of making
    proper allowances for differences in smoking habits, it seemed likely
    that present-day measurements of polycyclic hydrocarbons gave an
    inadequate assessment of past exposures to these compounds.

        The Royal College of Physicians of London (1970) reviewed the
    issue, and concluded that the evidence against community air pollution
    being a causal factor in lung cancer was stronger than the evidence
    for it. The urban/rural differential is greatest in countries with
    relatively low urban air pollution (Sweden, Norway, Denmark). The
    upward trend in mortality as well as other experimental and
    epidemiological evidence are best explained by the causal role of
    cigarette smoking.

        Nevertheless, in a review by Cleary (1967), evidence is presented
    to show that, in Australia, New Zealand, South Africa, and the USA,
    immigrants from the United Kingdom have a higher lung cancer death
    rate than those born in these countries: immigrants from Norway have a
    lower rate than native-born citizens of the USA, and the lung cancer
    mortality rates for all these migrants are intermediate between those
    of their countries of origin and destination, strongly suggesting an
    environmental factor in early life.

        While the existence of an urban excess of lung cancer has been
    proved, it is uncertain that air pollution is the "urban factor"
    responsible. In contrast, recent work incriminates cigarette smoking
    more strongly than ever; there is also a contribution from some
    specific occupational exposures (see footnote in section 1.1.7).

        The consideration of criteria for environmental carcinogenesis
    specifically in relation to any possible effects on lung cancer, is
    outside the scope of the present discussion, but it may be mentioned
    that any action to reduce smoke and especially that from the
    inefficient combustion of coal in domestic fires, is likely to make
    substantial reductions in the benzo(a)pyrene content of the air. Such
    a change has already been noted in London, where the concentration of
    this compound is now only about one-tenth of what it was 25 years ago
    (Lawther & Waller, 1976). A reduction of sulfur dioxide may also be
    important for any possible interactions relating to the production of
    lung cancer. Some experimental studies (Kuschner & Laskin, 1971;
    Skvorcova et al., 1973) showed an increased carcinogenic response when
    laboratory animals (rats and hamsters) were exposed to sulfur dioxide
    in addition to benzo(a)pyrene.

    8.3.8  Annoyance

        Annoyance may be defined as "a feeling of displeasure associated
    with any agent or condition believed to affect adversely an individual
    or a group". This definition was adopted by an international symposium
    on annoyance in Stockholm in 1971 (Lindvall & Radford, 1973). Very few
    studies have been performed that would make quantitative evaluations
    of this effect possible.

        The social awareness of pollution caused by particulate matter has
    been studied in a few areas. The results from different studies have
    been presented in a document on particulate matter (US Department of
    Health, Education and Welfare, 1969a); they include those from a study
    carried out in St. Louis (Schusky, 1966), where values for suspended
    particulates of around 100 g/m3 produced annoyance reactions from a
    considerable number of people.

        A similar study was carried out in Birmingham, Alabama, USA
    (Stalker & Robinson, 1967), in which levels of air pollution were
    correlated with annoyance. They found that, as dustfall reached or
    exceeded 14.1 g/m2 per month, about one-half of the population
    considered it to be a nuisance; at 10.5 g/m2 per month, about
    one-third of the population considered air pollution a nuisance.
    Stepwise multiple regression analysis showed that the variation due to
    dust fall alone explained 49% during the spring season and 68% during
    the winter season of the total association between the air pollutants
    measured and public annoyance. The association between levels of
    suspended particulate matter and public opinion, which was weaker than
    that of dustfall, was strongest during the summer season (r = 0.59).
    About one-half of the persons interviewed thought that air pollution
    was a general nuisance, when mean annual or mean summer concentrations
    of particulate matter reached 230 g/m3 and one-third, when they
    reached 150 g/m3.

        No such relationship was shown with sulfur dioxide concentrations.
    However, these levels were low. Further studies are indicated, since
    it may be that effects such as annoyance reactions will, in the
    future, be the critical effects on which criteria, as regards the
    protection of public health, will be based. Since annoyance reactions
    have a large sociocultural component, these levels may vary from place
    to place and should be determined for each locality. Surveys on
    annoyance are fraught with many problems. When proper survey
    techniques are expertly applied, however, it will be possible to
    assess reactions in a quantitative manner.

    8.4  Exposure-Effect Relationships

        Some of the studies reported in this section can be used to
    develop exposure-effect relationships. It has been necessary to
    develop 2 tables, one for the effects of short-term exposures
    (Table 15, p. 107) and another for the effects of long-term exposures
    (Table 16). The results from the studies have been based on different
    methods of measuring sulfur oxides and particulate matter. The values
    for the sulfur oxides are treated as if they were comparable. For the
    particulate matter, two categories are listed: smoke as measured by
    the Organization for Economic Cooperation and Development or British
    daily smoke/sulfur oxide methods and total suspended particulates as
    measured by the high volume sampler or light scattering. These factors
    should be considered in interpreting these tables.


        Table 15.  Exposure-effect relationships of sulfur dioxide, smoke, and total suspended
               particulates: effects of short-term exposures
                                                                                                                                

        concentration
          24-h mean
        values (g/m3)
                            
                                   Total
                                  suspended
    Sulfur dioxide    Smoke      particulates                         Effects
                                                                                                                                

       > 1000        > 1000          --          London, 1952. Very large increase in mortality to about 3
                                                 times normal, during 5-day fog. Pollution figures represent
                                                 means for whole area: maximum (central site) sulfur dioxide
                                                 3700 g/m3, smoke 4500 g/m3 (Ministry of Health, UK, (1954).

          710           750          --          London, 1958-59. Increases in daily mortality up to about 1.25
                                                 times expected value (Lawther, 1963; Martin & Bradley, 1960).

          500           500          --          London, 1968-60. Increases in daily mortality (as above) and
                                                 increases in hospital admissions, becoming evident when
                                                 pollution levels shown were exceeded (magnitude increasing
                                                 steadily with pollution) (Martin, 1964).

          500            --          --          New York 1962-66. Mortality correlated with pollution: 2%
                                                 excess at level shown (Buechley, 1973).

          500           250          --          London, 1954-68. Increases in illness score by diary technique
                                                 among bronchitic patients seen above pollution levels shown
                                                 (means for whole area) (Lawther et al., 1970).
                                                                                                                                

    Table 15  (cont'd).
                                                                                                                                

        concentration
          24-h mean
        values (g/m3)
                            
                                   Total
                                  suspended
    Sulfur dioxide    Smoke      particulates                         Effects
                                                                                                                                

          300           140          --          Vlaardingen, Netherlands, 1969-72. Temporary decrease in
                                                 ventilatory function (Van der Lende et al., 1975).

         200a            --         150b         Cumberland, WV, USA. Increased asthma attack rate among
                                                 small group of patients, when pollution levels shown were
                                                 exceeded (Cohen et al., 1972).
                                                                                                                                

    a  West-Gaeke method
    b  High volume sampling method

    Other measurements by Organization for Economic Cooperation and Development or British daily smoke/sulfur
    dioxide methods (Ministry of Technology, UK, 1966; Organization for Economic Cooperation and Development, 1965).

    

        Table 16.  Exposure-effect relationships of sulfur dioxide, smoke, and
               total suspended particulates: effects of long-term exposures
                                                                                             

            Concentration
          Annual means of 24-h
           mean values (g/m3)
                                              

                                   Total
                                 suspended
    Sulfur dioxide    Smoke      particulates            Effects
                                                                                             

         200           200           --          Sheffield, England. Increased
                                                 respiratory illnesses in children
                                                 (Lunn et al., 1967, 1970)

         --            --           180b         Berlin, NH, USA. Increased respiratory
                                                 symptoms, decreased respiratory
                                                 function in adults (Ferris et al., 1973)

         150           --            --          England & Wales. Increased respiratory
                                                 symptoms in children (Colley & Reid, 1970)

         125           170           --          Cracow, Poland. Increased respiratory
                                                 symptoms in adults (Sawicki, 1972)

         140d          140d          --          Great Britain. Increased lower
                                                 respiratory tract illnesses in
                                                 children (Douglas & Waller, 1966)

         60-140a       --         100-200c       Tokyo, Increased respiratory symptoms
                                                 in adults (Suzuki & Hitosugi, unpublished
                                                 data, 1970)
                                                                                             

    a  Automatic conductimetric method
    b  High volume sampler (2-month mean, possible underestimation of annual mean).
    c  Light-scattering method, results not directly comparable with others.
    d  Estimates based on observations after end of study; probable underestimation
       of exposures in early years of study.

    Other measurements by Organization for Economic Cooperation and Development or
    British daily smoke/sulfur dioxide methods (Ministry of Technology, UK, 1966;
    Organization for Economic Cooperation and Development, 1965).

    
    9.  EVALUATION OF HEALTH RISKS FROM EXPOSURE TO SULFUR OXIDES, SMOKE,
        AND SUSPENDED PARTICULATE MATTER

        It is well established that respiratory diseases are important
    causes of disability and death, and that, for some of them, there is
    evidence of association with environmental factors in the ambient air.
    There is evidence that exposure to mixtures of urban air pollutants
    containing sulfur oxides and particulate matter is related to a
    variety of adverse effects on health, even when other factors are
    controlled (section 8 -- Tables 15 and 16). Although in most of the
    studies considered the levels of air pollution have been expressed in
    terms of sulfur dioxide, smoke, or suspended particulate matter, this
    does not necessarily imply that these are the causative agents. They
    provide only indices of pollution, and certain components, such as
    sulfuric acid or sulfates, may be of particular importance.
    Measurements of some of these components have now been made in a
    number of areas and used in some of the more recent epidemiological
    studies. The Task Group concluded, however, that, at present, there
    are not enough data on any of these other indices of pollution to
    allow exposure-response relationships to be established. Thus, the
    present discussion is confined to the effects of pollution expressed
    in terms of sulfur dioxide, smoke, and total suspended particulate
    matter.

    9.1  Exposure Levels

        Concentrations of sulfur dioxide, smoke, and suspended particulate
    matter vary greatly from place to place and from time to time. Many
    factors including sources, topography, and weather conditions may be
    involved in these variations.

        An annual arithmetic mean of sulfur dioxide concentrations in
    urban areas typically ranges from 100-200 g/m3 (0.035-0.070 ppm),
    whereas a maximum daily mean ranges from 300-900 g/m3
    (0.11-0.32 ppm). For smoke, an annual arithmetic mean ranges from
    30-200 g/m3 with a maximum daily mean of 150-900 g/m3; for total
    suspended particulates, these levels range from 60-500 g/m3 and
    150-1000 g/m3, respectively.

        Comparatively little information is available concerning indoor
    concentrations of sulfur dioxide and particulate matter, though, in
    general, these levels are known to be lower than those outdoors,
    except at work places.

        Levels in working environments are considerably higher than
    general community levels and are thought to have been much higher in
    the past.

        In evaluating exposure levels that have been used in the past in
    connexion with epidemiological studies, a serious question arises as
    to how far these measurements, often intended primarily for control or
    monitoring purposes, can be considered as providing adequate measures
    of exposure. As discussed in section 5, it must be recognized that all
    the figures quoted in subsequent sections are based on measurements
    that have been made outdoors, usually at only a limited and not wholly
    representative set of fixed sites, though most people spend much time
    indoors, where concentrations may differ substantially from those
    outside. However, this does not necessarily invalidate these data
    which are serving primarily as indices.

        So far, there has been little information on the particle size
    distribution and the chemical composition of selected particle size
    ranges that would make the nature of exposure more understandable.

        Comparable assessment is often difficult because of differences in
    the methods of measurement of particulate matter that have been used
    in the health effect studies. Smoke, assessed in terms of blackness,
    is a measure of pollution associated with the incomplete combustion of
    fuel, and total suspended particulates, determined by weight, is a
    wider concept that includes all material which, by virtue of its
    particle size, remains in suspension for long periods.

    9.2  Experimental Animal Studies

        Laboratory studies have been performed using a variety of test
    animals, of periods of exposure, of experimental designs in exposure,
    and of combinations of pollutants and other agents.

        Some of these studies have provided useful information on the
    mechanisms of the biological action of sulfur dioxide, sulfuric acid
    mist, or particulate matter. However, the results are of limited value
    in developing guidelines for the protection of human health from
    effects of these pollutants as they exist in urban areas, and for this
    reason it is necessary to turn to the available epidemiological
    evidence.

    9.3  Controlled Studies in Man

        Effects studied in volunteers under controlled conditions include
    those on the functions of the respiratory system, sensory organs, and
    cerebral cortex. Durations of exposure have been very short, usually
    less than a few hours.

        Slight effects on pulmonary function were observed with exposure
    to sulfur dioxide at a concentration of 2100 g/m3 (0.75 ppm), but
    not with exposure to a concentration of 1100 g/m3 (0.37 ppm).

        With exposure to sulfuric acid, a decrease in tidal volume was
    found at a concentration as low as 350 g/m3.

        Combined exposures to sulfur dioxide and ozone or hydrogen
    peroxide produced a greater effect on respiratory function than
    exposure to each compound alone.

        Studies on sensory and cerebral cortical functions showed that
    sulfuric acid was more toxic than sulfur dioxide and that a
    combination of these two substances produced an approximately additive
    effect.

        There is a limit to the use of these studies for the development
    of criteria for community exposures, particularly when long-term
    exposure to complex mixtures of air pollutants is involved.

    9.4  Effects of Industrial Exposures

        Effects on the health of workers have been studied in relation to
    exposure to sulfur dioxide, particulate matter, or sulfuric acid mist
    arising from various manufacturing processes. In many of these
    studies, exposure levels were relatively high and, in some studies,
    adverse effects were detected only at a daily mean sulfur dioxide
    concentration as high as 70 000 g/m3 (25 ppm).

        It has also been reported that exposure to sulfuric acid at a mean
    concentration of 1400 g/m3 during working hours for 2 days did not
    produce any effect on lung function.

        The fact that adverse effects have not been reported at
    comparatively high levels of sulfur dioxide or surfuric acid aerosols
    may well be explained by the influence of biasing factors, such as
    that the workers remaining in jobs with exposures to pollutants
    consist of people especially resistant to their effects. Therefore, it
    should not be considered an indication that such concentrations are
    without effects for the average working population and particularly
    not for workers with pre-existing pulmonary diseases.

        In evaluating the effects from industrial exposures, due attention
    must also be given to the variation in chemical composition and size
    distribution of particulate matter.

    9.5  Effects of Community Exposures

        Many of the epidemiological studies in relation to community
    exposures that were considered by a WHO Expert Committee in 1972
    (World Health Organization, 1972), and which must still be relied on
    to a large extent today, were based on the measurement of smoke rather
    than of total suspended particulates. However, some new data on the
    effects of total suspended particulates have become available and have
    been included in Tables 15 and 16 (section 8).

        Tables 17 and 18 show the levels above which some effects on
    health might be expected among specified populations for short-term
    and long-term exposures, respectively. These are based on the critical
    evaluation of the results of studies reviewed in section 8.

        In developing Table 17, greater emphasis had to be placed on some
    earlier results related to major effects seen when concentrations of
    pollution were much higher than those commonly experienced today. With
    the effective control of at least some components in areas where
    pollution was, at one time, very high, most of the more recent studies
    have failed to isolate the effects of the pollutants in question from
    effects similar in nature but arising from other factors.

        The figures for sulfur dioxide and smoke are essentially the same
    as those proposed by the Expert Committee in 1972 (World Health
    Organization, 1972) the only change being the adoption of 250 g/m3
    (0.09 ppm) instead of 250-500 g/m3 (0.09-0.18 ppm) as the level at
    which the worsening of the condition of patients from short-term
    exposures to sulfur dioxide might be expected. It was recognized that
    the magnitude of responses at this level appeared to be small and
    difficult to separate from effects due to other factors such as
    weather or infections. It is possible that a lower figure for smoke
    could now be adopted, but, in view of uncertainties concerning the
    differences in composition of this component of pollution from one
    area to another, and the changes with time even within one locality,
    it was felt that any further modification of the figure determined on
    the basis of the older studies from London would need new evidence.

        Table 18 represents an overall assessment of the effects of
    long-term exposure to sulfur dioxide and smoke in terms of increases
    in the prevalence of respiratory symptoms among both adults and
    children, and in terms of the increased frequency of acute respiratory
    illnesses, that has been demonstrated particularly among children.

        Table 17.  Expected effects of air pollutants on health in selected segments of
               the population:  effects of short-term exposuresa
                                                                                             

                                                 24-h mean concentration (g/m3)
                                                                                             

           Expected effects                       Sulfur dioxide           Smoke
                                                                                             

    Excess mortality among the elderly
    or the chronically sick                             500                 500

    Worsening of the condition of patients
    with existing respiratory disease                   250                 250
                                                                                             

    a  Concentrations of sulfur dioxide and smoke as measured by OECD or British daily
       smoke/sulfur dioxide method (Ministry of Technology, UK, 1966; Organization for
       Economic Cooperation and Development, 1965). These values may have to be adjusted
       in terms of measurements made by other procedures.


    Table 18.  Expected effects of air pollutants on health in selected segments of
               the population: effects of long-term exposuresa
                                                                                             

                                                  Annual mean concentration (g/m3)
                                                                                  

        Expected effects                          Sulfur dioxide           Smoke
                                                                                             

    Increased respiratory symptoms among
    samples of the general population                   100                 100
    (adults and children) and increased
    frequencies of respiratory illnesses
    among children
                                                                                             

    a  Concentrations of sulfur dioxide and smoke as measured by OECD or British daily
       smoke/sulfur dioxide method (Ministry of Technology, UK, 1966; Organization for
       Economic Cooperation and Developments, 1965). These values may have to be adjusted
       in terms of measurements made by other procedures.

    
        For each pollutant there remains much uncertainty about the
    minimum levels associated with demonstrable effects. Where populations
    exposed to different levels of pollution have been compared, it cannot
    necessarily be assumed that even those who were exposed to a level
    lower than the known lowest effect level are entirely unaffected by
    pollution. There must also be doubts as to whether the effects
    observed in some studies were due in part to other pollutants, or to
    socioeconomic or other factors that had not been adjusted for
    completely.

        No firm conclusion was reached on the effects of total suspended
    particulates as a component of air pollution, together with sulfur
    dioxide, because of the limited amount of information available. For
    the effects of long-term exposure, a tentative figure of 150 g/m3
    (annual arithmetic mean) was suggested, based on the 2 entries in
    Table 16, recognising that one of these was based on light-scattering
    observations, and the other on high volume sampler measurements, but
    for a 2-month period only. It was noted that the one study on the
    effects of short-term exposure to total suspended particulates had
    been included in Table 15, but it was felt that this could not provide
    satisfactory information for an assessment of these effects.

        The figures in Table 18 have been expressed in terms of annual
    arithmetic mean concentrations, although it is not known whether the
    effects are related to extended exposures to these average levels, or
    more particularly to days of high pollution within each series. In
    view of the limited quantitative information on effects of pollution
    in terms of annoyance, this aspect has been omitted from Tables 17 and
    18.

    9.6  Guidelines for the Protection of Public Health

        Tables 17 and 18 present two different sets of criteria, one
    relating to effects of short-term exposure, in terms of 24-h average
    concentrations, and the other to effects of long-term exposure in
    terms of annual means. These effects may be interrelated; the gradual
    development of respiratory symptoms may, for example, be a reaction to
    repeated short-term exposure to peak 24-h values, or even to transient
    peaks lasting for still shorter periods, but in the absence of any
    substantial evidence on this point, the two criteria must, for the
    time being, be considered separately.

        With the present state of knowledge, it was considered that a
    safety factor of two below the figures given in Tables 17 and 18 would
    be reasonable to ensure the protection of public health, and,
    accordingly, Table 19 was developed, still considering the effects of
    short-term and long-term exposures separately. As an indication of the
    uncertainty surrounding these estimates, the figures have further been
    expressed with a range of  20%.

        The values proposed in Table 19 are in general agreement with
    those suggested as long-term goals in the earlier report (World Health
    Organization, 1972). In the case of sulfur dioxide, there has been
    some reduction in the 24-h figure (if this is regarded as a level not
    to be exceeded on more than 7 days a year), and this is in line with
    the revision of the "effect" level from a range of 250 to 500 g/m3
    in the 1972 report, to the present figure of 250 g/m3 (Table 17).

        Table 19 requires careful interpretation, for none of the figures
    can be considered as absolute limits. In the first place, day-to-day
    variations in the concentration of smoke and sulfur dioxide are
    determined largely by weather conditions, and occasional peaks far
    beyond the usual daily values may well occur, even with careful
    control of emissions.

    Table 19.  Guidelines for exposure limits consistent with
               the protection of public healtha
                                                               

                                    Concentration (g/m3)
                                                               

                                Sulfur dioxide          Smoke

    24-h mean                      100-150             100-150

    Annual arithmetic mean          40-60               40-60
                                                               

    a  Values for sulfur dioxide and smoke as measured by OECD or
       British daily smoke/sulfur dioxide method (Ministry of
       Technology, UK, 1966; Organization for Economic Cooperation
       and Development, 1965). Adjustments may be necessary where
       measurements are made by other methods.


        Although there are two sets of conditions specified in Table 19,
    determined independently from evidence on the effects of short- and
    long-term exposures, they can generally be considered to be consistent
    with one another. If the proportion of days with 24-h values above
    those in Table 19 is small (e.g., of the order of 7 days per year),
    then the annual means may well fall within or below the ranges
    specified in the second row of the table. Annual means are specified
    here in terms of arithmetic means: the corresponding geometric means
    would generally be a little lower (see section 5).

        Much consideration was given to the possibility of extending
    Table 19 to include guidelines for total suspended particulates as
    measured by the high volume sampler but it was concluded that the
    available evidence on the effects associated with exposure to
    suspended particulate matter was highly unsatisfactory.

        The Task Group felt, however, that some recommendations for a
    guideline should be made. A tentative annual mean value of 150 g/m3
    has been suggested in section 9.5 as a level beyond which effects of
    long-term exposure to total suspended particulates might be observed.
    Applying a safety factor of two and introducing a  20% range, as in
    the case of smoke and sulfur dioxide, would then provide a range of
    levels from 60 to 90 g/m3 as a possible guideline.

        For short-term exposures, no satisfactory, direct evidence
    relating concentrations of total suspended particulates to effects is
    available. Because of this, a guideline for short-term exposure levels
    can only be inferred. Assuming that the same ratio of 24-h mean
    concentrations to the annual mean (each derived independently, see
    beginning of section) given in the guidelines for smoke, is applicable
    to suspended particulate matter, then a very approximate 24-h
    guideline for suspended particulate matter, as measured by the high
    volume sampler, would be in the order of 150 to 230 g/m3.

        While the Group felt that it was reasonable and prudent to
    consider the above figures as interim guidelines consistent with the
    protection of public health, it stressed the very urgent need for
    additional information on the effects of exposure to suspended
    particulate matter (measured by the high-volume sampler). Furthermore,
    it recognized the fact that the toxicological significance of total
    suspended particulates might vary depending on their chemical
    composition and particle size, and that under certain circumstances,
    the suggested guidelines might need to be reconsidered. It should be
    noted that the discussion above does not imply any preference for
    smoke measurements over those of total suspended particulates; indeed
    it is highly desirable to develop more appropriate methods for the
    measurement of suspended particulates, especially those limited to the
    measurement of respirable particles.

        It was recognized that in many urban and industrial areas existing
    levels of pollution by sulfur dioxide and suspended particulate matter
    were substantially above these guidelines. Furthermore, there was the
    problem that the long-distance transport of these pollutants from
    major sources could, in some circumstances, result in comparatively
    high background concentrations in rural areas, and high levels in the
    incoming air in towns striving to meet their own air quality
    standards. It was considered, however, that every effort should be
    made to develop control procedures that would allow these guidelines
    to be met.

        These guidelines are based on observations among populations in
    the community exposed to a mixture of sulfur dioxide and smoke or
    total suspended particulates and they may not apply to situations
    where only one of the components is present. On grounds of prudence,
    however, it is recommended that the levels of each pollutant should be
    below the values stated. It should be stressed again, however, that
    the data on which the guidelines are based are uncertain and each of
    the guidelines is tentative and subject to review when further
    information becomes available.

        It was the opinion of the Group that there is not yet sufficient
    information available on the effects of community exposures to
    sulfuric acid aerosols or suspended sulphates to develop guidelines
    for these air pollutants.

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    See Also:
       Toxicological Abbreviations