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
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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. Larré, 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 µg/m3
Ozone 1 ppm = 2140 µg/m3
Carbon monoxide 1 ppm = 1250 µg/m3
a When converting values expressed in ppm to µg/m3, the numbers
have been rounded up to 2 or, exceptionally, 3 significant figures,
and concentrations higher than 10 000 µg/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 µg/m3 (0.035-0.07 ppm) with the highest daily means from
300-900 µg/m3 (0.1.-0.3 ppm). For smoke, the corresponding values are
30-200 µg/m3 and 150-900 µg/m3 respectively, and for suspended
particulate matter, measured by the high volume sampler, the annual
arithmetic means are 60-500 µg/m3 with maximum daily means of about
150-1000 µg/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 µg/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 µg/m3 (0.07 ppm) and 150 µg/m3 (high volume sampler),
respectively. In long-term, joint exposure studies, effects were noted
at annual mean concentrations of 60-140 µg/m3 (0.02-0.05 ppm) for
sulfur dioxide and 100-200 µg/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 µg/m3 (0.18 ppm) for sulfur dioxide and 500 µg/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 µg/m3 (0.18 ppm) for sulfur dioxide and 500 µg/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 µg/m3 (0.09 ppm) and a smoke concentration of
250 µg/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 µg/m3
(0.035 ppm) for sulfur dioxide and 100 µg/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 µg/m3 for sulfur dioxide, and for smoke) and in terms of
annual means (40-60 µg/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 µg/m3 for annual arithmetic means and 150-230 µg/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 µg/m3
(0.35-1.05 ppm). At higher concentrations (above about 10 000 µg/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 µm, 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 µm, up to
about 5 to 10 µm, 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 µm, 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
µm 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
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 µm, 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 µm that is related to particles
formed by coagulation or condensation from smaller units, and a
further mode of the order of 10 µm 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 µm 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).
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 (µg/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 µg/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
µg/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
ß ray sampler and mass of material determined by of Germany, but not to any large extent
attenuation of ß 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 µg/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 µg 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 µm 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).
(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,
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).
Table 10. Concentrations of sulfur dioxide, smoke, and suspended particulate
matter (1974)a
Site Concentration (µg/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 µg/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 µg/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 µg/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 µg/m3 at eastern urban sites, within the sulfur
dioxide range of 100-200 µg/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 µg/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.
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
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).
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 o