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    INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY


    ENVIRONMENTAL HEALTH CRITERIA 7





    PHOTOCHEMICAL OXIDANTS






    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 154067 2

    (c) World Health Organization 1979

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    should be made to the Office of Publications, World Health
    Organization, Geneva, Switzerland. The World Health Organization
    welcomes such applications.

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    on the part of the Secretariat of the World Health Organization
    concerning the legal status of any country, territory, city or area or
    of its authorities, or concerning the delimitation of its frontiers or
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    CONTENTS

    ENVIRONMENTAL HEALTH CRITERIA FOR PHOTOCHEMICAL OXIDANTS

    1. SUMMARY AND RECOMMENDATIONS FOR FURTHER RESEARCH AND OTHER ACTION

         1.1. Summary
                1.1.1. Chemistry and analytical methods
                1.1.2. Sources of photochemical oxidants and their
                        precursors
                1.1.3. Environmental concentrations and exposures
                1.1.4. Effects on experimental animals
                1.1.5. Effects on man
                        1.1.5.1   Controlled exposures
                        1.1.5.2   Industrial exposure
                        1.1.5.3   Community exposure
                1.1.6. Evaluation of health risks
         1.2. Recommendations for further research and other action
                1.2.1. Health effects research
                1.2.2. Photochemical oxidant control

    2. CHEMISTRY AND ANALYTICAL METHODS

         2.1. Chemical and physical properties
                2.1.1. Ozone
                2.1.2. Peroxyacylnitrates
                2.1.3. Other oxidants
         2.2. Atmospheric chemistry
         2.3. Measurement of photochemical oxidant concentrations
                2.3.1. Sampling
                2.3.2. Analytical methods
                        2.3.2.1   Ozone
                        2.3.2.2   Total oxidants
                        2.3.2.3   Peroxyacetylnitrate

    3. SOURCES OF PHOTOCHEMICAL OXIDANTS AND THEIR PRECURSORS

         3.1. Natural sources
         3.2. Man-made sources of oxidant precursors
         3.3. Indoor sources
         3.4. Oxidant-precursor relationships

    4. ENVIRONMENTAL CONCENTRATIONS AND EXPOSURES

         4.1. Background concentrations
         4.2. Rural areas
         4.3. Urban areas
         4.4. Indoor concentrations

    5. EFFECTS ON EXPERIMENTAL ANIMALS

         5.1. Absorption of ozone
         5.2. Effects on the respiratory system
                5.2.1. Morphological changes
                        5.2.1.1   Short-term exposure (24 h or less)
                        5.2.1.2   Prolonged and repeated exposures
                5.2.2. Functional changes
                        5.2.2.1   Short-term exposure (24 h or less)
                        5.2.2.2   Prolonged and repeated exposures
                5.2.3. Biochemical changes
                        5.2.3.1   Effects indicating possible mechanisms
                                  of action
                        5.2.3.2   Biochemical effects at the subcellular
                                  level
                        5.2.3.3   Extracellular effects
                5.2.4. Carcinogenicity
                5.2.5. Tolerance to ozone
                5.2.6. Effects on the host defence system
                5.2.7. Interaction of ozone with bronchoactive and other
                        chemicals
         5.3. Systemic reactions and other effects
                5.3.1. Effects on growth
                5.3.2. Haematological effects
                        5.3.2.1   Short-term exposure (24 h or less)
                        5.3.2.2   Prolonged and repeated exposures
                5.3.3. Effects on reproduction
                5.3.4. Behavioural and related changes
                        5.3.4.1 Short-term exposure (24 h or less)
                        5.3.4.2 Prolonged and repeated exposures
                5.3.5. Miscellaneous systemic reactions to lung damage
         5.4. Mutagenicity
         5.5. Summary table

    6. EFFECTS ON MAN

         6.1. Controlled exposures
                6.1.1. In vitro effects on human tissues
                6.1.2. Sensory effects
                6.1.3. Effects on respiratory function
                        6.1.3.1   Exposure to ozone
                        6.1.3.2   Exposure to mixtures of ozone and other
                                  air pollutants
                        6.1.3.3   Exposure to peroxyacetylnitrate alone or
                                  in combination with carbon monoxide
                        6.1.3.4   Exposure to irradiated automobile
                                  exhaust
                        6.1.3.5   Exposure to ambient air containing
                                  elevated concentrations of oxidants
                6.1.4. Changes in electroencephalograms
                6.1.5. Chromosomal effects

         6.2. Industrial exposure
         6.3. Community exposure
                6.3.1. Mortality
                6.3.2. Annoyance and irritation
                6.3.3. Athletic performance
                6.3.4. Effects on children
                6.3.5. Effects on the incidence of acute respiratory and
                        cardiovascular diseases
                6.3.6. Effects on the prevalence of chronic respiratory
                        diseases and on pulmonary function
                6.3.7. Effects on patients with pre-existing diseases
                        6.3.7.1   Asthma
                        6.3.7.2   Chronic respiratory diseases
                6.3.8. Cancer
                6.3.9. Motor vehicle accidents
         6.4. Summary tables

    7. EVALUATION OF HEALTH RISKS FROM EXPOSURE TO PHOTOCHEMICAL
         OXIDANTS

         7.1. Exposure conditions
         7.2. Exposure-effect relationships
                7.2.1. Animal data
                7.2.2. Controlled human exposures
                7.2.3. Industrial exposure
                7.2.4. Community exposure
         7.3. Guidelines on exposure limits

    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-evaluating the conclusions contained in the criteria
    documents.

    WHO TASK GROUP ON ENVIRONMENTAL HEALTH CRITERIA FOR PHOTOCHEMICAL
    OXIDANTS

     Tokyo, 30 August-3 September 1976

    Participants

     Members

    Dr K. Biersteker, Medical Research Division, Municipal Health
         Department, Rotterdam, Netherlands  (Chairman)

    Professor K. A. Bustueva, Department of Community Hygiene, Central
         Institute for Advanced Medical Training, Moscow, USSR
          (Vice-Chairman)

    Dr R. G. Derwent, Environmental and Medical Sciences Division, Atomic
         Energy Research Establishment, Harwell, England  (Rapporteur)

    Dr D. E. Gardner, Biomedical Research Branch, Clinical Studies
         Division, Health Effects Research Laboratory, Environmental
         Protection Agency, Research Triangle Park, NC, USA

    Dr J. Jager, Centre of General and Environmental Hygiene, Institute of
         Hygiene and Epidemiology, Prague, Czechoslovakia

    Dr G. von Nieding, Laboratory for Respiration and Circulation,
         Bethanien Hospital, Moers, Federal Republic of Germany

    Mr E. A. Schuck, US Environmental Protection Agency, Environmental
         Monitoring and Support Laboratory, Las Vegas, NV, USA

    Professor N. Yamaki, Faculty of Engineering, Saitama University,
         Saitama, Japan

     Observers

    Dr J. Kagawa, Department of Medicine, Tokai University, Kanagawa,
         Japan

    Professor K. Maeda, Department of Medicine, Tokyo University, Tokyo,
         Japan

    Dr T. Nakajima, Division of Environmental Health Research, Osaka
         Prefectural Institute of Public Health, Osaka, Japan

    Dr T. Okita, Department of Community Environmental Sciences, The
         Institute of Public Health, Tokyo, Japan

    Dr H. Watanabe, Hyogo Prefectural Institute of Public Health, Kobe,
         Japan

    Dr N. Yamate, First Section of Environmental Chemistry, National
         Institute of Hygienic Sciences, Tokyo, Japan

     Secretariat

    Dr G. Freeman, Department of Medical Sciences, Stanford Research
         Institute, Menlo Park, CA, USA  (Temporary Adviser)

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

    Mr G. Ozolins, Scientist, Control of Environmental Pollution and
         Hazards, World Health Organization, Geneva, Switzerland

    Professor C. M. Shy, Institute for Environmental Studies and
         Department of Epidemiology, School of Public Health, University
         of North Carolina, Chapel Hill, NC, USA  (Temporary Adviser)

    Dr T. Suzuki, The Institute of Public Health, Tokyo, Japan  (Temporary
          Adviser)

    Professor T. Toyama, Department of Preventive Medicine, Keio
         University, Tokyo, Japan  (Temporary Adviser)

    ENVIRONMENTAL HEALTH CRITERIA FOR PHOTOCHEMICAL OXIDANTS

        A WHO Task Group on Environmental Health Criteria for
    Photochemical Oxidants met in Tokyo from 30 August to 3 September
    1976. Dr Y. Hasegawa, Medical Officer, Control of Environmental
    Pollution and Hazards, Division of Environmental Health, WHO, opened
    the meeting on behalf of the Director-General and expressed the
    appreciation of the Organization to the Government of Japan for acting
    as host to the meeting. The Task Group reviewed and revised the second
    draft criteria document and made an evaluation of the health risks
    from exposure to photochemical oxidants.

        The first and second drafts of the criteria document were prepared
    by Professor Carl M. Shy of the Department of Epidemiology, School of
    Public Health, University of North Carolina, Chapel Hill, NC, USA, and
    Dr Donald E. Gardner, Chief, Biomedical Research Branch, Clinical
    Studies Division, Health Effects Research Laboratory, US Environmental
    Protection Agency, Research Triangle Park, NC, USA. The comments upon
    which the second draft was based were received from the national focal
    points for the WHO Environmental Health Criteria Programme in
    Bulgaria, Canada, Czechoslovakia, the Federal Republic of Germany,
    Japan, New Zealand, Poland, Sweden, the USA, and the USSR; and from
    the Food and Agriculture Organization of the United Nations (FAO),
    Rome, the World Meteorological Organization (WMO), Geneva, and the
    International Union of Pure and Applied Chemistry (IUPAC). The
    collaboration of these national institutions and international
    organizations is gratefully acknowledged.

        The Secretariat also wishes to acknowledge the most valuable
    collaboration in the final phases of the preparation of this document,
    of Professor Shy, Dr Gardner, Dr R. G. Derwent of the Environmental
    and Medical Sciences Division, Atomic Energy Research Establishment,
    Harwell, England, and Professor K. Schaffner of the Institute of
    Radiation Chemistry at the Max-Planck-Institute for Carbon Research,
    Mulheim an der Ruhr, Federal Republic of Germany.

        This document is based primarily on original publications listed
    in the reference section. Much valuable information may also be found
    in other published criteria documents (US Department of Health,
    Education and Welfare, 1970; North Atlantic Treaty Organization, 1974;
    National Academy of Sciences, 1977).

        Because biological knowledge concerning many components of
    photochemical air pollution is limited, the Task Group agreed that a
    definition of photochemical oxidants should be given early in the
    criteria document.

        Photochemical oxidants can be formed as the result of the
    sunlight-induced oxidation of precursor pollutants emitted into the
    atmosphere. These precursor compounds include the oxides of nitrogen
    and a variety of hydrocarbons with different chemical reactivities
    with respect to the formation of photochemical oxidants. The principal
    oxidants are ozone, nitrogen dioxide, and the peroxyacylnitrates.
    However, until recently, measurement methods specific for each of
    these oxidants were not available and the most commonly employed
    methods were affected, to some extent, by interference from other
    atmospheric pollutants. Thus, when such studies are being considered,
    it is important to know whether some correction has been made for this
    interference, particularly in studies related to health effects in
    man.

        Although many other ingredients have been identified in
    photochemical air pollution, there is little information available at
    the moment concerning their biological significance, and they have not
    been referred to in this document. The biological significance of
    nitrogen dioxide has been reviewed and evaluated in another WHO
    environmental health criteria document (World Health Organization,
    1977).

        Details of the WHO Environmental Health Criteria Programme
    including some terms frequently used in the documents 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).a

        The following conversion factors have been used in the present
    documentb:

                  carbon monoxide (CO)           1 ppm = 1150 g/m3
                  nitric oxide (NO)              1 ppm = 1230 g/m3
                  nitrogen dioxide (NO2)         1 ppm = 1880 g/m3
                  nitrous oxide (N2O)            1 ppm = 1800 g/m3
                  ozone (O3)                     1 ppm = 2000 g/m3
                  peroxyacetylnitrate (PAN)      1 ppm = 5000 g/m3
                  sulfur dioxide (SO2)           1 ppm = 2600 g/m3

                 

    a Reprints available from the Division of Environmental Health,
      World Health Organization, 1211 Geneva 27, Switzerland.

    b When converting values expressed in ppm to g/m3, the numbers
      have been rounded up to 2 or, exceptionally 3 significant figures
      and, in most cases, concentrations higher than 10 000 g/m3, have
      been expressed in mg/m3.

    1.  SUMMARY AND RECOMMENDATIONS FOR FURTHER RESEARCH AND OTHER ACTION

    1.1  Summary

    1.1.1  Chemistry and analytical methods

        In the context of this report, photochemical oxidants are
    understood to include ozone, nitrogen dioxide, and peroxyacylnitrates.
    Many other compounds have been proposed as components of photochemical
    air pollution but, as little information is available concerning their
    biological significance, these substances have not been discussed in
    this document. As nitrogen dioxide is an important air pollutant in
    its own right, it is the subject of a separate document (World Health
    Organization, 1977). Thus this report deals mainly with ozone and
    "oxidants" as measured by the neutral buffered potassium iodide method
    (NBKI).

        Ozone and peroxyacylnitrates can be measured specifically by
    chemiluminescent reactions and by gas chromatography in conjunction
    with electron-capture detectors. These methods are highly sensitive
    and are not subject to interference from other atmospheric pollutants.
    To obtain the most reproducible data, sampling manifolds should be
    made entirely of teflon or glass as oxidants in the inlet stream may
    react with plastics or metal.

        The terms "oxidant" or "total oxidant" are used to describe the
    oxidizing property of sampled air as determined by its reaction with
    neutral phosphate-buffered potassium iodide. Nitrogen dioxide in
    sampled air enhances the reaction with potassium iodide, while sulfur
    dioxide inhibits it. The terms "corrected oxidant" or "adjusted
    oxidant" indicate that measurements have been corrected for the
    presence of nitrogen dioxide and sulfur dioxide. Interference from
    these substances is not entirely eliminated by the current systems
    used for removing them from the inlet stream. The accuracy of the
    analytical procedure for oxidant measurements also depends on the pH
    of the buffer solution, reagent concentrations, and other variables.

    1.1.2  Sources of photochemical oxidants and their precursors

        Ozone, a natural constituent of the stratosphere formed by the
    photolysis of molecular oxygen, can be transported by atmospheric
    circulation into the lower atmosphere. Natural hydrocarbons including
    terpenes from trees and vegetation are also subject to photochemical
    reactions producing oxidants. These two processes are the natural
    sources of background ozone concentrations.

        Ozone and peroxyacylnitrates are formed in the lower atmosphere by
    reactions between oxides of nitrogen and an array of photochemically
    reactive hydrocarbons. The chemical structure and reactivity of each
    organic hydrocarbon determines its importance in the formation of
    oxidants. Motor vehicles, space heating, power plants, and industrial
    processes are major sources of these oxidant precursors.

        A balance between oxidizing and reducing agents would be
    maintained in the atmosphere, thus avoiding accumulation of ozone and
    other photochemical oxidants, if it were not for the photochemical
    degradation of hydrocarbons into peroxy radicals. Peroxy radicals
    rapidly convert nitric oxide to nitrogen dioxide, thus shifting the
    equilibrium towards ozone production during daylight. At night,
    emissions of nitric oxide into the atmosphere serve as a sink for
    ozone.

        Welding and the manufacture of hydrogen peroxide are the main
    sources of occupational exposure to ozone. The use of ultraviolet
    lamps, electrostatic precipitators, or photocopying machines may also
    generate ozone.

    1.1.3  Environmental concentrations and exposures

        Since significant concentrations of oxidants in urban areas are
    generally restricted to a period of 4-6 h per day, oxidant or ozone
    data are most often reported in terms of maximum 1-h concentrations or
    in terms of the number of hours or days recorded with hourly
    concentrations exceeding a specified value. In isolated places far
    removed from sources of pollution, maximum hourly ozone concentrations
    of 100 g/m3 (0.05 ppm) have been recorded. Transport of oxidants
    from urban areas for distances of 100-700 kilometres appears to be a
    widespread phenomenon, and 1-h ozone concentrations of 120 g/m3
    (0.06 ppm) or more have been observed in rural areas. In some large
    cities, maximum 1-h oxidant concentrations exceed 200 g/m3 (0.1 ppm)
    on 5-30% of days, while in Los Angeles it is commonplace for maximum
    1-h oxidant values to exceed 200 g/m3 (0.1 ppm) on most days of the
    month between May and October.

        Diurnal patterns in oxidant levels are an important feature of the
    urban environment and result from hourly changes in solar radiation
    and pollutant emission intensity. Maximum hourly ozone levels
    frequently occur around noon and are often preceded by peak
    concentrations of nitrogen dioxide. Concentrations of
    peroxyacetylnitrate are typically between 1/50th and 1/100th of those
    of ozone and, in general, closely follow temporal variations in ozone
    levels.

        On a seasonal basis, oxidant concentrations tend to increase
    during the high temperature season, and the frequency of days on which
    oxidant concentrations exceed 200 g/m3 (0.1 ppm) is greatest during
    this period.

        Oxidant concentrations indoors tend to be lower than those
    outdoors, and are reduced by destructive reaction on material
    surfaces. They are also reduced by activities that generate nitric
    oxide such as smoking and cooking.

    1.1.4  Effects on experimental animals

        Ozone concentrations of 2000 g/m3 (1.0 ppm) or less with
    exposure periods of up to 24 h produced numerous morphological changes
    in the lung parenchyma in several animal species. With prolonged
    exposure (6-10 months), pulmonary damage such as emphysema,
    atelectasis, focal necrosis, bronchopneumonia, and fibrosis has been
    reported. The degree of morphological injury seems to be proportional
    to the product of the concentration and the duration of exposure.

        Disturbances in respiratory functions have been noted in
    experimental animals exposed for 2-5 h to ozone concentrations of
    520-2000 g/m3 (0.26-1.0 ppm).

        Biochemical studies that have been conducted to clarify the
    mechanisms of ozone toxicity at subcellular level have mainly been
    based on two hypotheses: (a) that oxidation of sulfhydryl groups by
    ozone causes changes in metabolism that result in toxic effects; and
    (b) that ozone reacts with unsaturated lipids to produce lipid
    peroxidation and consequent cell damage. However, the subcellular
    toxic action of ozone is still not fully understood. In other
    biochemical studies, changes have been reported in the mitochondrial
    oxygen consumption, the activities of lysosomal and microsomal
    enzymes, and in the synthesis of nucleic acids. Studies on the
    induction of oedema by lung histamine and on the effects of ozone on
    the surface active substance have been inconclusive.

        In small rodents, "tolerance" to oedematigenous effects caused by
    exposure to ozone concentrations of 2000-8000 g/m3 (1-4 ppm) has
    been obtained by pre-exposure to an ozone concentration of at least
    600 g/m3 (0.3 ppm). However, this did not seem to provide protection
    against effects that impair the phagocytic activity of macrophages.

        Resistance to artificially-induced respiratory infection was
    reduced in several animal species by 3-4 h exposure to ozone
    concentrations of 160-800 g/m3 (0.08-0.40 ppm). The effect of the
    ozone was further enhanced by a third stressor such as cold or
    exercise. Various mechanisms have been proposed for the enhanced
    infectivity including inactivation of a protective factor that favours

    survival of alveolar macrophages, inactivation of alveolar macrophage
    secretory enzymes, depression of bactericidal activity, and reduction
    in the phagocytic activity of alveolar macrophages.

        It has been shown that exposure of pregnant mice to ozone at
    200-400 g/m3 (0.1-0.2 ppm) for 7 h per day, for 15 days, increased
    neonatal mortality and that exposure of mice to ozone at 400 g/m3
    (0.2 ppm) for 6 h and rats to 1000 g/m3 (0.5 ppm) for 45 min
    resulted in significant losses in motor activity. However, it is not
    clear whether these effects are due to the direct action of ozone or
    oxidizing agents or whether they are secondary reactions to damage in
    the respiratory system caused by ozone.

        Data concerning other extrapulmonary effects and the
    carcinogenicity and mutagenicity of ozone are inadequate.

        Effects produced by exposure to various mixtures of ozone and
    other air pollutants, and ambient air containing elevated
    concentrations of oxidants are mainly similar to those seen from
    exposure to ozone alone. However, one study reported that a single
    exposure to a mixture of ozone and nitrogen dioxide produced an
    additive effect in reducing resistance to respiratory infection in
    mice, and that repeated exposure might give rise to a synergistic
    effect. Some effects such as those on growth have only been reported
    with exposure to mixtures of pollutants. A deficiency of vitamin E has
    also been reported to enhance the toxic effects of ozone.

    1.1.5  Effects on man

    1.1.5.1  Controlled exposures

        A large number of sensory effects in man have been studied under
    controlled conditions. The odour threshold for ozone has been shown to
    be 15-40 g/m3 (0.008-0.02 ppm) and the lowest oxidant concentration
    producing eye irritation has been suggested to be 200 g/m3
    (0.1 ppm). Various measures of visual perception were affected by a
    3-h exposure to ozone concentrations of 400-1000 g/m3 (0.2-0.5 ppm).

        Controlled exposure of healthy male subjects to ozone
    concentrations ranging from 200 to 2000 g/m3 (0.1-1.0 ppm) has been
    reported to cause increased airway resistance and decreased
    ventilatory performance. Effects at the lower end of this dose-range
    were elicited when test subjects carried out intermittent light
    exercise during a 2-h exposure period. One investigator failed to
    observe changes in airway resistance at an ozone exposure of
    500 g/m3 (0.25 ppm) for 2 h. Thus, not all investigators were in
    agreement concerning the lowest experimental ozone exposures that
    affect airway resistance; however, three investigators found increased
    airway resistance at an ozone concentration of 740 g/m3 (0.37 ppm).

        A 2-h exposure to a combination of ozone at 50 g/m3 (0.025 ppm),
    nitrogen dioxide at 100 g/m3 (0.05 ppm) and sulfur dioxide at
    260 g/m3 (0.1 ppm) did not have any effect on airway resistance.
    However, this combined exposure did enhance the bronchoconstrictor
    effect of acetylcholine. A combination of an ozone concentration of
    740 g/m3 (0.37 ppm) and a sulfur dioxide concentration of 960 g/m3
    (0.37 ppm) had a potentiated effect on the impairment of ventilatory
    performance compared with the effects of the same concentration of
    each gas administered singly. In other studies in which various
    mixtures of ozone and other air pollutants were used, sulfur dioxide
    seemed to potentiate the effect of ozone more than nitrogen dioxide.

        Studies were also performed on volunteer patients with chronic
    pulmonary disease. The respiratory function of these patients showed
    an improvement when they breathed filtered air for 40 h or more
    compared with unfiltered ambient air with an oxidant concentration of
    about 400 g/m3 (0.2 ppm).

        Exposures to a peroxyacetylnitrate concentration of 1350 g/m3
    (0.27 ppm) caused minor changes in variables that reflect
    cardiorespiratory and temperature regulation.

    1.1.5.2  Industrial exposure

        Several cases of severe ozone intoxication have been reported in
    welders using inert gas-shielded, consumable electrodes which greatly
    increased the ultraviolet irradiation of the work area. At ozone
    concentrations of 600-1600 g/m3 (0.3-0.8 ppm), an increasing number
    of welders complained of chest constriction and irritation of the
    throat, while acute symptoms disappeared when ozone levels were
    reduced to 500 g/m3 (0.25 ppm) or less.

        There are very few studies on long-term industrial exposure to
    ozone and in most of them the exposure-response relationship has
    either not been well evaluated or has been confounded by other
    coexistent pollutants.

    1.1.5.3  Community exposure

        So far, no evidence has been obtained to indicate an association
    between peak oxidant concentrations and variations in the daily
    mortality rate of the general population. On the other hand, the
    association of oxidant levels with eye and respiratory irritation has
    been well documented, and in one study made on student nurses in Los
    Angeles, a significant increase in the frequency of cough, eye, and
    chest discomfort, and headache was demonstrated when maximum hourly
    oxidant concentrations reached 100-580 g/m3 (0.05-0.29 ppm). Hourly
    oxidant levels were also correlated with decreased performance in high
    school cross-country runners, and the estimate of the lowest
    concentration at which this effect occurred was an hourly oxidant
    concentration of 240 g/m3 (0.12 ppm).

        Effects of oxidants on children have been extensively studied. A
    correlation was detected between the decreases in airway conductance
    and ventilatory performance of school children and increase in ozone
    levels over a range up to 560 g/m3 (0.28 ppm); other pollutants
    monitored at the same time included nitric oxide, nitrogen dioxide,
    sulfur dioxide, and particulate matter. Combinations of these
    pollutants may have been responsible for the observed effects. An
    attempt was also made to relate the rates of illness in school
    children during an influenza epidemic to the pollution gradient which
    existed during the season of peak oxidant concentrations, but there
    was no significant association. In Japan, a variety of respiratory and
    systemic symptoms were reported among school children on several smog-
    alert days. The systemic symptoms appeared to be attributable to a
    psychosomatic response among the students.

        In a study on the incidence of acute respiratory diseases, peak
    concentrations of oxidants and mean concentrations of sulfur dioxide
    and nitrogen dioxide were found to be correlated with acute episodes
    of pharyngitis, bronchitis, and upper respiratory infections among
    college students in the Los Angeles Basin. However, there was no
    association between the admissions to a hospital in Los Angeles for
    cardiovascular conditions and oxidant concentrations.

        A few reports are available on the effects of oxidants on patients
    with pre-existing diseases. One study suggested a relationship between
    the proportion of asthmatics who experienced asthma attacks and daily
    peak oxidant levels, but results were confounded by concomitant
    seasonal changes.

        Studies on the effect of long-term exposure to photochemical
    oxidants are relatively few. To date, urban differences in lung cancer
    mortality rates in Californian cities do not suggest an influence of
    oxidant exposure on lung cancer risk. Similarly, studies have not
    revealed any relationships between the prevalence of chronic
    respiratory disease and geographical differences in oxidant
    concentrations.

        As in all studies of urban populations, epidemiological studies of
    oxidant exposure cannot yield results on health effects attributable
    only to oxidants, since photochemical air pollution typically consists
    of ozone, nitrogen dioxide, peroxyacylnitrates, nitrate and sulfate
    particulates, and other components. In general, however, observed
    health effects were found to be more closely correlated with ozone
    levels than with levels of other pollutants.

    1.1.6  Evaluation of health risks

        Although it is known that ozone is only one of a number of
    photochemical oxidants and that there are many other components of
    photochemical air pollution, it is the only substance for which a

    health protection guideline can be given, based on existing exposure-
    effect data.

        From controlled human and community exposure studies, ozone
    concentrations at which the first adverse effects in man appear have
    been reported to be 200-500 g/m3 (0.1-0.25 ppm). Experimental
    studies on animals support these estimates.

        The Task Group agreed that a 1-h exposure to ozone of
    100-200 g/m3 (0.05-0.10 ppm) (measured by chemiluminescence) should
    be used as a guideline for the protection of public health and that a
    safety factor could not be applied because of the relatively high
    natural concentrations of ozone.

        The Group also considered that a maximum 1-h oxidant concentration
    of 120 g/m3 (0.06 ppm) (measured by the NBKI method), which was
    recommended as the long-term goal by the WHO Expert Committee in 1972
    and is approximately equal to the highest natural background level of
    oxidants, would be the best estimate of the exposure limit for
    oxidants for the general population.

        In response to the question of whether the proposed guideline was
    realistic in view of natural exposure levels and the long-distance
    transport of ozone, the Group expressed the view that every effort
    should, nevertheless, be made to develop control strategies for
    achieving the proposed guideline or at least for not exceeding it more
    than once a month.

    1.2  Recommendations for Further Research and Other Action

    1.2.1  Health effects research

        The WHO Task Group was particularly concerned with the potential
    for enhanced biological response of combined or sequential exposure of
    human populations to nitrogen dioxide and ozone. The recommendations
    listed take into account this concern as well as some other gaps in
    knowledge concerning the health effects of photochemical oxidants.

    (a) The following information should be obtained by means of carefully
        controlled exposure studies on human volunteers:

        i.   data on the lowest concentration of ozone at which various
             lung function variables are affected;

        ii.  effects of sequential exposure to nitrogen dioxide and ozone;

        iii. effects of ozone pre-exposure on the sensitivity of airways
             to bronchoconstrictor agents;

        iv.  effects on airways of combined exposure to: ozone and sulfur
             dioxide; ozone and tobacco smoke; ozone and increased
             temperature or other stressors.

    (b) Epidemiological studies should be conducted to evaluate:

        i.   effects of oxidant exposure on the susceptibility of human
             populations to respiratory infections;

        ii.  comparative effects of exposure of urban populations to
             combined nitrogen dioxide and ozone (oxidants) versus
             exposure of rural populations to ozone alone. Measurements of
             lung function and other variables shown to be affected by
             photochemical oxidant and nitrogen dioxide peaks may be used
             for these studies.

    (c) Experimental animal studies should be conducted to evaluate:

        i.   effects of intermittent exposure to ozone, mimicking ambient
             air exposures;

        ii.  effects of the joint action of ozone and other pollutants
             and/or other environmental stressors;

        iii. mechanism of tolerance to oxidants;

        iv.  carcinogenic, cocarcinogenic, and mutagenic effects of ozone;

        v.   effects of ozone exposure on humoral and cellular immunity.

    1.2.2  Photochemical oxidant control

        In order to reduce exposure of the general population to
    photochemical oxidants, the ratio of reactive hydrocarbons to oxides
    of nitrogen must be carefully controlled as well as their absolute
    levels. Unilateral or unbalanced control may result in higher levels
    of ozone and/or nitrogen dioxide. The Task Group recommended that to
    achieve a balanced control of both reactive hydrocarbons and oxides of
    nitrogen, appropriate laboratory and field studies should be conducted
    to evaluate the effects that both groups of compounds may have on the
    control of photochemical oxidants.

    2.  CHEMISTRY AND ANALYTICAL METHODS

    2.1  Chemical and Physical Properties

    2.1.1  Ozone

        Ozone is one of the strongest oxidizing agents; only fluorine,
    atomic oxygen, and oxygen fluoride (OF2) have higher redox
    potentials. Ozone is an important constituent of the upper atmosphere.
    Although it is present in only small concentrations (a few parts per
    million), ozone is responsible for shielding the earth from
    ultraviolet radiation (UV-B) that is biologically harmful. Formation
    of ozone occurs predominantly at altitudes above 30 km where solar UV
    radiation with wavelengths of less than 242 nm slowly dissociates
    molecular oxygen (O2) into oxygen atoms (O). These oxygen atoms
    rapidly combine with molecular oxygen to form ozone. Ozone strongly
    absorbs solar radiation in the wavelength region of 240-320 nm. It is
    this absorption that shields the earth from harmful UV radiation (see
    for example National Academy of Sciences, 1977).

        Some of the physical properties of ozone, the most abundant
    ubiquitous atmospheric oxidant, are listed in Table 1.

    Table 1.  Physical properties of ozonea
                                                                 

    Chemical formula                       O3
    Physical state at NTPb                 colourless gas
    Relative molecular mass                48.0
    Melting point                          - 192.7C
    Boiling point                          - 111.9C
    Density relative to air                1.658
    Vapour density
      at 0C. 101 kPa (760 mmHg)           2.14 g/litre
      at 25C. 101 kPa (760 mmHg)          1.96 g/litre
    Solubility at 0C, 101 kPa (760 mmHg)  0.494 ml/100 ml water
                                                                 

    a  From: US Department of Health, Education and Welfare (1970).
    b  NPT = normal temperature and pressure. i.e. 25C and 101 kPa
       (760 mmHg).


        The absorption of electromagnetic radiation by ozone in the
    ultraviolet and infrared regions is used in analytical methods.

    2.1.2  Peroxyacylnitrates

        Photochemical processes produce other oxidizing species besides
    ozone. These include peroxyacylnitrates, which have the following
    general structure:

    FIGURE 01

    This class of compounds includes:

    R = CH3: peroxyacetylnitrate (PAN)
    R = C2H5: peroxyproprionylnitrate (PPN)
    R = C6H5: peroxybenzoylnitrate (PBzN)

    Although each of these species has received some attention, monitoring
    data are available only for peroxyacetylnitrate. The physical
    properties of this species are described in Table 2.

    Table 2.  Physical properties of peroxyacetylnitratea
                                                                

                                     O
                                     "
                                     "
    Chemical formula             CH3COONO2
    Physical state at NTP        colourless liquid
    Relative molecular mass      121
    Boiling point                No true boiling point, compound
                                   decomposes before boiling
    Vapour pressure at room      About 2 kPa (15 mmHg)
     temperature
                                                                

    a  From: US Department of Health, Education and Welfare (1970).


    Peroxyacylnitrates have two characteristics that help in their
    detection at low concentrations i.e., absorption in the infrared
    region of the spectrum and electron-capturing ability (Stephens,
    1969). The second of these characteristics is exploited in the
    electron-capture detector which, when used in conjunction with gas
    chromatography, provides the basis of an accepted method for the
    measurement of peroxyacetylnitrate levels in air.

    2.1.3  Other oxidants

        Hydrogen peroxide has been identified as a potential photochemical
    oxidant. However, it is an extremely difficult substance to detect
    specifically in the atmosphere and, at present, it is not possible to
    assess its significance as a photochemical air pollutant.

    2.2  Atmospheric Chemistry

        There  are no significant primary emissions of ozone into the
    atmosphere and all the ozone found has been formed by chemical
    reactions that occur in the air.

        In the upper atmosphere, ozone is mainly formed by the action of
    solar radiation on molecular oxygena:

    O2 + radiation (lambda < 175 nm) --> O(3P) + O(1D)               (1)

    O2 + radiation (lambda < 242 nm) --> 2 O(3P)                     (2)

    O(3P) + O2 + M --> O3 + M                                        (3)

        In the lower atmosphere, ozone-producing processes involve
    absorption of solar radiation by nitrogen dioxide:

    NO2 + radiation (lambda < 430 nm) ka NO + O(3P)                  (4)
                                      ->

    O(3P) + O2 + M --> O3 + M                                        (3)

    O3 + NO kb NO2 + O2                                              (5)
            ->

                 

    a O(3P) is the symbol for atomic oxygen in its lowest energy state
      ("triplet oxygen"); O(1D) represents the next higher energy state
      of atomic oxygen ("singlet oxygen"); M is another molecule that
      must be present for the reaction to take place ("third body"),
      usually oxygen or nitrogen. Square brackets e.g. [NO] represent
      the concentration of the chemical species inside the brackets.

        Thus, the mechanism of ozone production during the sunlight
    irradiation of polluted air is simple in outline (ozone formation by
    the interaction of molecular oxygen with the photoproducts of nitrogen
    dioxide) but complex in detail. It is based on reactions (3)-(5) which
    give the following expression for the ozone concentration:

               ka[NO2]
    O3 approx.                                                         (6)
               kb[NO]

        In polluted atmospheres, the most readily observed sink for ozone
    involves the emission of nitric oxide. At night-time, the equilibrium
    expressed by equation (6) is displaced by the rapid reaction (5), and
    continuous nitric oxide emissions rapidly reduce the ozone
    concentration to undetectable levels. This atmospheric chemical
    process is supplemented by the destruction of ozone at ground level by
    contact with soil and vegetation surfaces (Regener & Aldaz, 1969).

        In a hydrogen-free atmosphere, a fairly even balance between
    oxidizing and reducing agents would be maintained. However, peroxy
    radicals (RO2) produced by the photochemical degradation of
    hydrocarbons have the important property of reacting with nitric oxide
    thereby converting it to nitrogen dioxide. The significance of any
    process resulting in the conversion of nitric oxide to nitrogen
    dioxide is that during daylight the equilibrium expressed by equation
    (6) shifts in favour of ozone production.

        There are no significant primary emissions into the atmosphere of
    peroxyacylnitrates all of which are formed by atmospheric chemical
    reactions of the general type:

    RCO(O2) + NO2 --> RCO(O2)NO2                                       (7)

    2.3  Measurement of Photochemical Oxidant Concentrations

    2.3.1  Sampling

        Ozone is highly reactive with most materials including plastics,
    metals, and fabrics. The most reproducible ambient concentration data
    have been obtained using sampling manifolds made entirely of teflon or
    glass.

        As with oxides of nitrogen, residence time in these sampling
    manifolds requires specific consideration when sampling air containing
    nitric oxide, nitrogen dioxide, and ozone during daylight. Since the
    equilibrium shown in equation (6) is disturbed inside the sampling
    manifold, ozone concentrations can be underestimated if sampling times
    exceed 10 seconds (Butcher & Ruff, 1971).

        When measuring ozone, the site of sampling should be selected with
    extreme care as exhaust gases from motor vehicles and central heating
    appliances readily remove ozone.

    2.3.2  Analytical methods

        For measurement purposes, oxidants are generally divided into
    three categories: ozone, total oxidants, and peroxyacylnitrates. Ozone
    and peroxyacylnitrates can be measured specifically while total
    oxidants are usually determined as a class of compounds.

        The reagent employed to measure the oxidizing property of
    photochemical oxidants is a solution of neutral-buffered potassium
    iodide (NBKI). This reagent reacts with ozone, nitrogen dioxide, and
    peroxyacylnitrates. Reducing agents such as sulfur dioxide have an
    inhibiting effect on the reagent solution and must be removed, from
    the inlet stream. To this extent, reaction with the potassium iodide
    reagent is a measure of the net oxidizing capacity of a sample of
    ambient air.

        The terms "oxidant" or "total oxidant" are used to describe the
    net oxidizing capacity of the sampled air as determined by reaction
    with NBKI. The terms "corrected oxidant" or "adjusted oxidant"
    indicate that measurements have been corrected for the presence of
    reducing agents (sulfur dioxide) or other oxidizing agents (nitrogen
    dioxide) in the air.

        Long-term averaging values are usually meaningless for
    photochemical oxidants and attention is directed to 1-h values
    (section 4.3). Thus, the use of continuous instruments with automatic
    data collection systems has an advantage. Manual methods are available
    for the determination of photochemical oxidants and these may be
    automated to a certain extent.

    2.3.2.1  Ozone

        Continuous ozone analysers are usually based on the
    chemiluminescent reaction of ozone with ethylene (Nederbragt et al.,
    1965; Warren & Babcock, 1970). A chemiluminescence method based on the
    reaction of ozone with certain dyes (Regener, 1964) has been further
    developed in the Netherlands (Guicherit, 1975). These techniques are
    not subject to atmospheric interference, are highly sensitive
    (2 g/m3 or 0.001 ppm), and perform well under field conditions. The
    methods are not absolute and hence some form of calibration involving
    potassium iodide or UV absorption spectroscopy is necessary.

        Specific determination of ozone may be accomplished by ultraviolet
    absorption spectroscopy in the 200-300 nm wavelength range (Hodgeson,
    1972). The advantages of this method are that it is highly sensitive,

    does not require cylinders of explosive gases, and that it gives an
    absolute measurement (De More & Patapoff, 1976). Very high
    concentrations of certain hydrocarbons or mercury vapour may cause
    interference.

        The dihydroacridine method is a specific, inexpensive, manual
    method for ozone determination in which samples are taken every
    30 min. Interference effects can be eliminated by parallel sampling
    with an identical system fitted with an ozone scrubber. This is the
    only suitable method for short-term ozone measurements when a
    chemiluminescent or UV absorption instrument is not available (World
    Health Organization, 1976).

    2.3.2.2  Total oxidants

        Total oxidants are generally measured using acid- or neutral-
    buffered solutions of potassium iodide (Byers & Saltzman, 1958; US
    Department of Health, Education and Welfare, 1970; World Health
    Organization, 1976). There are indications that the accuracy of the
    method depends on the pH of the buffer solution, the concentration of
    the reagent, and other variables. In addition to these drawbacks, the
    method is basically unspecific. Any other oxidizing or reducing
    species can cause interference. Such interference results when
    sampling air containing sulfur dioxide, chlorine, nitrogen dioxide, or
    peroxides. Nitrogen dioxide and sulfur dioxide interfere most by
    giving erroneously high and low values, respectively.

        Interference from sulfur dioxide can be eliminated by passing the
    air sample through glass fibre filters impregnated with a suitable
    material such as chromium trioxide. Humidity often renders these
    scrubbers ineffective because under such conditions they oxidize
    nitric oxide to nitrogen dioxide, thus increasing nitrogen dioxide
    interference. As there is also ozone loss after extended use, these
    scrubbers are not entirely satisfactory (World Health Organization,
    1976).

        Interference from nitrogen dioxide is more difficult to eliminate.
    Some form of adjustment can be made to the total oxidant reading if
    continuous nitrogen dioxide measurements are also available. This
    correction depends to some extent on experimental conditions and, in
    view of the problems previously noted with measurements of oxides of
    nitrogen, may not be easy to make.

        Other methods are available for the determination of oxidants but
    none of them is commonly used. They include the ferrous ammonium
    sulphate, alkali potassium iodide, and phenolphthalein methods (World
    Health Organization, 1976).

    2.3.2.3  Peroxyacetylnitrate

        The infrared absorption and electron-capturing properties of
    peroxyacetylnitrate have been used for its measurement in simulated
    and real atmospheres (Stephens, 1969). Electron-capture detectors
    preceded by gas chromatographic separation offer a limit of detection
    of 0.5 g/m3 (0.0001 ppm) for the automatic determination of ambient
    concentrations of peroxyacetylnitrate. Water vapours may interfere
    with the passage of peroxyacetylnitrate along the chromatographic
    column (Farwell & Rasmussen, 1976) and trace contaminants in cylinder
    gases can be a problem because of their slow accumulation on
    chromatography columns used for continuous measurements.

        Peroxyacetylnitrate can also be measured by the hydrolysis of
    peroxyacetylnitrate solution to give nitrite ions that can be
    determined colorimetrically. However, this method suffers from
    interference by nitrogen dioxide (Konno & Okita, 1974; Stephens,
    1969).

    3.  SOURCES OF PHOTOCHEMICAL OXIDANTS AND THEIR PRECURSORS

    3.1  Natural Sources

        As mentioned previously, ozone is a natural constituent of the
    upper atmosphere. A small amount of ozone, which is formed by the
    photolysis of molecular oxygen, is carried by atmospheric circulation
    into the lower atmosphere (section 4). Natural sources of ozone are
    associated with the passage of cold fronts (Ripperton et al., 1971)
    and atmospheric electrical phenomena (US Department of Health,
    Education & Welfare, 1970).

        The photochemical oxidation of natural hydrocarbons including
    terpenes from trees and other vegetation takes place during daylight
    hours (Rasmussen, 1972; Went, 1966). Generally, these processes are
    difficult to study because of the low concentrations of the
    hydrocarbons and their short atmospheric residence. However, hazes
    often associated with certain forest regions may well be explained by
    photochemical aerosol production from natural airborne hydrocarbons
    (Grimsrud et al., 1975).

    3.2  Man-made Sources of Oxidant Precursors

        Emissions of oxides of nitrogen from man-made activities include
    important contributions from both stationary and mobile sources (World
    Health Organization, 1977).

        By comparison, the position with regard to hydrocarbon precursors
    is much more complex. In this instance, the term "hydrocarbon" refers
    to a class of pollutants that contain carbon atoms and produce
    oxidants under irradiation in the presence of oxides of nitrogen. This
    is a very wide definition that covers many hundreds of different
    organic compounds emitted into the atmosphere by man-made processes.

        Not all organic compounds play an equal role in oxidant formation.
    The relative importance of each organic compound in this process
    depends on its chemical structure and reactivity. The chemical
    structure determines the number of nitric oxide --> nitrogen dioxide
    conversions involved in the atmospheric degradation of each organic
    compound and ozone may be produced at each of these steps (Calvert,
    1976; Demerjian et al., 1974). Reactivity requires special attention
    because the time scale for ozone or peroxyacetylnitrate production is
    related to the time scale for hydrocarbon degradation. For the
    so-called "highly reactive" hydrocarbons this may be 1 h or less. It
    may take up to 3 h for the less reactive hydrocarbons and require
    several days for the so-called unreactive hydrocarbons. Even in the
    presence of reactive hydrocarbons, ozone production may only become
    significant 10 km downwind from a source, and peak ozone
    concentrations may be observed over 60 km downwind (White et al.,
    1976). Thus, the relationship between hydrocarbon precursors and
    observed oxidant concentrations in large urban areas may be obscured.
    There may be marked differences in the nature of the oxidants in air
    sampled during the early morning when peak concentrations of oxidant

    precursors occur and in that sampled after midday when ozone
    concentrations are elevated.

        In view of this complexity, it is clearly necessary to have some
    form of rational assessment of hydrocarbon reactivity (Darnall et al.,
    1976). Complete inventories of individual substances are only
    available for gasoline-engine exhaust emissions; for the storage,
    distribution, and use of organic substances such as petroleum
    products; and for specific industrial processes. An example of total
    emissions of oxides of nitrogen and hydrocarbons is given in Table 3.
    These figures are presented merely to show the wide diversity of the
    processes responsible for oxidant precursor emissions; it is not
    sufficient to consider motor vehicles as the only important source.

    Table 3.  Total emissions of hydrocarbons and oxides of nitrogen
              in the Federal Republic of Germany in 1971 in thousands
              of tonnesa
                                                                  

    Source                      Total emission (103 tonnes)
                                                                  

                             Hydrocarbons     Oxides of nitrogen
                                                                  

    Domestic heating             173                117
    Traffic exhaust              325                308
    Power plants                  14                373
    Industrial combustion         53                470
    Industrial processes         955                 30
                                                                  

    a From: Federal Republic of Germany, Ministry of Internal
      Affairs (1974).


    3.3  Indoor Sources

        The use of ultraviolet lamps, electrostatic precipitators,
    photocopying machines, and odour control equipment can lead to
    increases in indoor concentrations of ozone. Welding and the
    manufacture of hydrogen peroxide are important indoor sources of ozone
    in industry and pose problems in occupational health (section 6.2).

        However, other indoor activities such as smoking and cooking with
    gas stoves tend to produce elevated nitric oxide concentrations that
    destroy ozone and peroxyacylnitrates (Schuck & Stephens, 1969).

    3.4  Oxidant-precursor Relationships

        While potential synergistic health effects of oxidants and
    nitrogen dioxide are recognized, programmes for the control of these
    two pollutants are usually developed independently of one another.

    From a control point of view, however, such an approach is not
    justified because of the complexity of the photochemical reaction
    system. The only predictable way to control the formation of
    photochemical oxidants is to reduce the initial components in
    incremental steps. Decreasing the primary emission of oxides of
    nitrogen without reducing hydrocarbon emissions will lead to an
    increase in oxidant levels.

        A further example of the uncoordinated approach to air pollution
    abatement can be illustrated by examination of the current methods
    applied to hydrocarbon control. These methods have focused on
    increasing combustion efficiency, an action which does reduce the
    hydrocarbon concentrations in exhaust but has the side effect of
    causing an increase in atmospheric concentrations of nitrogen dioxide.
    As expected, changing the ratio of oxidant precursors has a complex
    effect on the formation of photochemical oxidants. For example, the
    effect of this change in ratio in the south coast air basin of
    California has been to produce substantial reductions in the
    concentrations of oxidants in Los Angeles town centre. However, the
    reductions become smaller at downwind sites and at 80 kilometres
    downwind, an increase in maximum daily values was observed
    (Dimitriades, 1976).

    4.  ENVIRONMENTAL CONCENTRATIONS AND EXPOSURES

    4.1  Background Concentrations

        Ozone concentrations in places far removed from sources of
    pollution show fairly constant values with some seasonal variations
    interspersed with irregular maxima due to specific meteorological
    events. Monthly mean concentrations of ozone, which vary considerably
    with both latitude and the month of the year, are illustrated in
    Fig. 1. The reported values range from 10 to 80 g/m3
    (0.005-0.04 ppm).

        Many observations indicate that hourly values range from 10 to
    100 g/m3 (0.005-0.05 ppm) (Berry, 1964; Haagen-Smit, 1952; US
    Department of Health, Education and Welfare, 1970). However, higher
    values have been observed on isolated occasions. In a study at Chalk
    River, Ontario, Canada, a maximum value was observed of 120 g/m3
    (0.06 ppm) for 4 h (US Department of Health, Education and Welfare,
    1970).

    4.2  Rural Areas

        Polluted air masses from urban and industrial areas can affect
    suburban and rural areas in the direction of the prevailing wind for
    considerable distances. Elevated oxidant concentrations have been
    measured in a number of downwind rural locations where local sources
    of oxidant precursors were insignificant. It has been suggested that
    long-distance atmospheric transport might be responsible for many
    cases of high oxidant concentrations found over rural areas and some
    specific examples are given in Table 4.

        A Midwest study in the USA in 1974 showed elevated ozone levels
    over an extensive rural area (radius of over 240 km) due to the
    combined effects of a number of urban areas. High ozone levels from
    one particular urban area extended as far as 48-80 kilometres downwind
    (US Environmental Protection Agency, 1976).

        Observations of a 1-h concentration of 120 g/m3 (0.06 ppm) in
    rural areas can generally be associated with the transport of man-made
    oxidants from distant sources.

    FIGURE 02

        Table 4.  Long-distance transport of photochemical oxidants
                                                                                      

    Rural region          Possible source           Trajectory  Reference
                                                    length
                                                    (km)
                                                                                      

    Mineral King Valley,  Fresno, California        < 100       Miller et al. (1972)
      California, USA
    Garrett County,       New York, Philadelphia,   > 100       US Environmental
      Maryland, USA       Baltimore, Washington DC                Protection Agency
                          or Pittsburgh                           (1973)
    Southern Eire         Continental Europe        100-700     Cox et al. (1975)
      & Southern UK
    New York State, USA   Buffalo, New York         100-300     Stasiuk & Coffey (1974)
    Tochigi and Gunma     Tokyo                     < 100       Environment Agency
      Prefectures, Japan                                        (1976)
    Midwest, USA          St. Louis                 > 150       White et al. (1976)
                                                                                      

    
    4.3  Urban Areas

        Since photochemical oxidants are the products of sunlight-induced
    photochemical reactions, elevated concentrations of oxidants in urban
    areas are generally restricted to a 4- to 6-h period within a day,
    representing only 15-25% of the 24-h interval. For this reason, the
    reporting of oxidant or ozone data as daily, monthly, or yearly means
    can be misleading when evaluating trends or comparing oxidant
    concentrations in different cities. Thus, oxidant or ozone data are
    usually reported in terms of highest 1-h concentrations or in terms of
    the number of days with hourly concentrations exceeding a specified
    value or the number of hours when a given range of concentrations
    occurred within a year. However, they may also be given as
    instantaneous or five minute peak concentrations or frequency
    distributions.

        As shown in Table 5, the highest 1-h concentrations at 8 locations
    were of the order of 300-800 g/m3 (0.15-0.40 ppm). It is important
    to recognize that the data presented are only for one site in each of
    the cities and do not necessarily represent the maximum levels
    occurring in these urban areas or provide a good indication of human
    exposure levels. For this reason, frequency distributions or reporting
    of the number of days or hours when a given concentration was exceeded
    are helpful. Such data are presented in Tables 6, 7, and 8 for
    selected monitoring stations in Tokyo, Washington DC, and Delft,
    respectively, to illustrate the distribution of the concentrations
    recorded at these monitoring stations. These tables are not intended
    to indicate long-term trends since meteorological patterns that
    greatly influence the ambient concentrations of oxidants can vary
    considerably from year to year.

        The data for Tokyo show both the number of days when a given
    concentration was exceeded and the total number of hours in which
    concentrations falling within the specified ranges were observed. As
    shown in Table 6, an hourly concentration of 200 g/m3 (0.1 ppm) was
    exceeded on 10-30 of days in these years, and in approximately 1-4% of
    all the hours in the year.

        In Washington, DC, a concentration of 200 g/m3 (0.1 ppm) was
    exceeded, on average, on about 5% of the days, with most of the days
    having maximum 1-h concentrations of about 100 g/m3 (0.05 ppm).

        The data for Delft, Netherlands, show the number of hours in the
    year when given concentrations were observed. More than 90% of the
    hours exhibited concentrations of less than 100 g/m3 (0.05 ppm).
    However, in 1971 and 1973, there were 48 and 30 hours, respectively,
    when concentrations exceeded 200 g/m3 (0.1 ppm).

        Table 5.  Highest 1-h concentrations of ozone or total oxidants observed at
              selected sites in 1974
                                                                                      

    City                                     Concentration            Method
                                                              
                                             g/m3       (ppm)
                                                                                      

    Bonn, Federal Republic of Germany1      290        (0.145)      chemiluminescence
    Eindhoven, Netherlands2                 420        (0.210)      chemiluminescence
    London, UK3a                            294        (0.147)      chemiluminescence
    Los Angeles, USA4                       548        (0.274)      NBKI
    Osaka, Japan5                           320        (0.160)      NBKI
    Riverside, USA4                         744        (0.372)      NBKI
    Tokyo, Japan5                           380        (0.190)      NBKI
    Washington, USA4                        312        (0.156)      chemiluminescence
                                                                                      

    From:  1 Becker & Schurath (1975).
           2 Guicherit (1975).
           3 Ball (1976).
           4 US Environmental Protection Agency (1976).
           5 Environment Agency (1975b).
           a Data for 1975.


    Table 6.  Number of days and hours when hourly oxidant concentrations were in the
              range of indicated levels at a National Air Sampling Nelwork Station,
              Tokyo, Japana
                                                                                      

    Concentrationb                       Number of days (hours)
                                                                                      

    g/m3       (ppm)         1971         1972          1973         1974
                                                                                      

      0-100     (0 -0.05)    113 (7032)   185 (7863)     60 (7248)   184 (8122)
    120-180   (0.06-0.09)    135 (798)    111 (379)     199 (1050)   134 (495)
    200-280   (0.10-0.14)     60 (251)     31 (81)       73 (298)     40 (107)
    300-380   (0.15-0.19)     18 (42)       8 (14)       21 (59)       5 (10)
    400-480   (0.20-0.24)      6 (8)        1 (2)         7 (14)       0 (0)
       >500       (>0.25)      0 (0)        0 (0)         1 (2)        0 (0)
                                                                                      

    Total                    332 (8131)   336 (8339)    361 (8671)   363 (8734)
                                                                                      

    a From: Tokyo Metropolitan Government (1971-1974) (unpublished data).
    b Measured by NBKI method.

    Table 7.  Number of days when at least one hourly oxidant
              concentration was in the range of indicated levels in
              Washington, DC, USAa
                                                                  

    Concentrationb                       Number of days
                                                                  

    g/m3         (ppm)            1970        1971       1972
                                                                  

      0-100        (0   -0.05)         72         155        134
    120-200        (0.06-0.10)         85         127         39
    220-300        (0.11-0.15)          8          17          6
      >300             (>0.15)          2           0          0
                                                                 

    Total                             167         299        179
                                                                  

    a From: US Environmental Protection Agency (1964-1973).
    b Measured by the NBKI method.


    Table 8.  Number of hours when hourly ozone concentrations were in
              the range of indicated levels in Delft, Netherlandsa
                                                                      

    Concentrationb                 Number of hours
                                                                    

    g/m3           (ppm)         1971      1972      1973      1974
                                                                    

      0-100         (0 -0.05)     7799      8370     8364       6907
    120-150      (0.06 -0.075)     647       325      267        401
    152-200      (0.076-0.10)      130        34       81         39
       >200           (>0.10)       48         8       30          8
                                                                    

    Total                         8624    8737     8742     7355
                                                                    

    a From: Guicherit (1975).
    b Measured by the chemiluminescence method.
    
        When the maximum concentration for a given averaging time, e.g.,
    1 h, is known, maximum values for other averaging times can be
    estimated from the averaging time-concentration model of Larsen
    (1974). The concentrations for various averaging periods, calculated
    from the base data of 1-h concentrations of 300 and 800 g/m3, are
    shown in Table 9.

    Table 9.  Estimated ozone concentrations for various
              averaging periodsa
                                                           

    Averaging period       Maximum concentration (g/m3)
                                                           

    1-h (base data)            300               800
    3-h                      190-215           435-520
    1 day                     85-120           225-310
    1 year                    15-30             35-80
                                                           

    a Adapted from: Larsen (1969).


        These calculations are based on geometric standard deviations
    reported for several cities in the USA (Larsen, 1969).

        Caution should be exercised when applying Larsen's model. Although
    it is not applicable to averaging times of less than 1 h, the model is
    generally accurate when using observed annual means to predict
    concentrations between 1 h and 1 day. For periods longer than 1 day it
    is applicable only if the observed data closely follow a log-normal
    distribution.

        Seasonal and diurnal variations in oxidant values are important
    characteristics of the urban pattern of environmental concentrations
    of this group of pollutants. These temporal variations result from:
    (a) variations in oxidant precursors; (b) variations in atmospheric
    transport and dilution of pollutants, and (c) variations in
    meteorological conditions and other atmospheric variables involved in
    the photochemical reaction process. Because of diurnal variations in
    intensity of both solar ultraviolet radiation and of precursor
    emission rates, maximum daily ozone concentrations frequently occur
    around noon. However, such maxima have also been observed during the
    morning or afternoon hours, mostly in suburban areas.

        Examples of diurnal patterns of oxidants or ozone and nitrogen
    dioxide concentrations are shown in Figs. 2, 3, and 4. The close
    relationship between oxidants and nitrogen dioxide quite frequently
    leads to oxidant peaks following nitrogen dioxide peaks. However, this
    is not always, as shown in Fig. 3.

    FIGURE 03

    FIGURE 04

    FIGURE 05

        Seasonal variations in oxidant concentrations are manifested by
    increases in the diurnal maxima and by increases in the number of days
    per month that exhibit elevated oxidant values. For example, Fig. 5
    shows the number of days per month in which the oxidant concentration
    in Los Angeles, USA, equalled or exceeded 200 g/m3 (0.1 ppm).
    Similarly, Fig. 6 shows the number of days per month in Delft,
    Netherlands, that also exhibited ozone concentrations equal to or
    above 200 g/m3 (0.1 ppm).

        Peroxyacetylnitrate is generally formed simultaneously with ozone.
    However, comparatively few measurements have been made of
    peroxyacetylnitrate in the ambient atmosphere. The ratio of
    peroxyacetylnitrate to ozone observed at a maximum concentration of
    peroxyacetylnitrate was about 1:100 in rural England (Sandalls et al.,
    1974) and about 1:50 in Delft, Netherlands (Nieboer & Van Ham, 1976).
    Variations in concentrations of peroxyacetylnitrate often follow those
    of ozone, as shown in Fig. 7 (Sandalls et al., 1974). However, this is
    not always the case (Stephens, 1976).

    4.4  Indoor Concentrations

    Oxidant concentrations inside buildings tend to be lower than those
    outdoors because of destructive reactions that occur on most surfaces
    (Mueller et al., 1973; Sabersky et al., 1973). However, certain indoor
    sources of ozone (section 3.3) may increase indoor concentrations.
    Indoor concentrations at places of work are discussed in section 6.2.

    FIGURE 06

    FIGURE 07

    FIGURE 08

    5.  EFFECTS ON EXPERIMENTAL ANIMALS

        There is considerable evidence to show that even short exposure to
    high concentrations of ozone may endanger the health of experimental
    animals. In reviewing this evidence, emphasis has been placed on
    studies in which animals were exposed to concentrations of oxidants of
    2000 g/m3 (1.0 ppm) or less, since these studies are more relevant
    for predicting the health risk to man. However, some experiments
    conducted at higher concentrations have also been discussed when it
    was considered that they would contribute to a better understanding of
    the mechanism of the biological action of oxidants.

    5.1  Absorption of Ozone

        A number of factors can influence the transport and removal of
    ozone in the upper airways such as: (a) nasal morphology; (b) route,
    rate, and depth of breathing; and (c) biochemical composition and
    amount of mucus. The decomposition of ozone within the upper airways
    may protect the lower part of the respiratory tract against the
    irritant gas. Various attempts to determine or model the respiratory
    absorption of ozone in the upper airways have been made (McJilton et
    al., 1972; Vaughan et al., 1969; Yokoyama & Frank, 1972). This work
    has recently been reviewed by Miller (1977) who developed a
    mathematical model for the transport and removal of ozone in the
    respiratory tract of guineapigs, rabbits, and man, and predicted that
    whatever the initial concentration, the respiratory bronchioles would
    receive the highest dose of ozone. This agrees well with various
    experimental studies in animals. For all three species the
    relationship between respiratory bronchiolar concentration and the
    inhaled ozone concentration at the tracheal level is linear on a
    log-log scale at concentrations greater than 100 g/m3 (0.05 ppm),
    the respiratory bronchiolar dose for rabbits being 80% of that for man
    and twice that for guineapigs.

    5.2  Effects on the Respiratory System

    5.2.1  Morphological changes

    5.2.1.1  Short-term exposure (24 h or less)

        The primary target of ozone is the respiratory tract and
    particularly the pulmonary parenchyma. In small laboratory animals,
    exposure to ozone at acutely toxic concentrations results in pulmonary
    oedema, haemorrhage, and death. The LD50 is about 12 mg/m3 (6.0 ppm).
    At lower concentrations in the range of 400-2000 g/m3 (0.2-1.0 ppm),
    ozone causes numerous changes in both the epithelial and endothelial
    cells of the lung and ultrastructural effect indicate that the primary
    lesions are in the epithelial lining of the terminal bronchioles and
    the proximal alveoli.

        The sequence of degeneration, desquamation, and destruction of
    type I alveolar cells in rats following exposure to an ozone
    concentration of 400 g/m3 (0.2 ppm) for 2 h was demonstrated in
    electron microscopic studies by Stephens et al. (1974). The type II
    epithelial cells appeared to be more resistant. Freeman et al. (1974)
    also reported significant histological changes in type I epithelial
    cells after 4 h exposure to a concentration of 1800 g/m3. The loss
    of ciliated epithelium throughout the upper respiratory tract,
    swelling and denudation of type I cells, erythrocyte lysis within
    alveolar capillaries, and breakdown of capillary endothelium has been
    reported in cats exposed to ozone concentrations of 520, 1000, and
    2000 g/m3 (0.26, 0.5, and 1.0 ppm) for 4.7-6.6 h (Boatman et al.,
    1974). These effects appeared to be dose-related. Similar effects were
    noted by Bils (1970) in mice after a 7-h exposure to ozone at
    concentrations of 1200 and 2600 g/m3 (0.6 and 1.3 ppm).

        Cell renewal rate within the alveoli was studied by Evans et al.
    (1971) by injecting tritiated thymidine into 18 to 20-month-old mice
    before exposing them for 6 h to ozone concentrations of 1000, 2400,
    5000, or 7000 g/m3 (0.5, 1.2, 2.5, and 3.5 ppm). Immediately
    following exposure, the number of labelled alveolar cells (those
    synthesizing DNA) was significantly lower in all exposed groups
    compared with controls.

        Similar morphological studies have also been performed using
    complex mixtures containing oxidants. Ultrastructural alterations
    consisting of disrupted cytoplasm and abnormal mitochondria were seen
    in the alveolar tissue of mice exposed for 2-3 h to Los Angeles air
    that had a total oxidant concentration of 800 g/m3 (0.4 ppm) (Bils,
    1966). Exposure of mice for 3 h to an irradiated synthetic atmosphere
    that contained propylene, nitric oxide, carbon monoxide, and water
    vapour and simulated a heavy smog, gave rise to a similar pattern of
    ultrastructural changes (Bils & Romanovsky, 1967). Rats continuously
    exposed for 24 h to a mixture of ozone at a concentration of
    500 g/m3 (0.25 ppm) and nitrogen dioxide at a concentration of
    4700 g/m3 (2.5 ppm) exhibited increases in the numbers of alveolar
    macrophages and of free cells in the lung resembling desquamated type
    I cells, and hypertrophy of the epithelium were noted in rats
    continuously exposed for 24 h to a mixture of ozone at a concentration
    of 500 g/m3 (0.25 ppm) and nitrogen dioxide at a concentration of
    4700 g/m3 (2.5 ppm) (Freeman et al., 1974).

    5.2.1.2  Prolonged and repeated exposures

        Prolonged exposure to low levels of ozone causes more extensive
    and irreparable damage to the lung than the oedematigenous and acute
    inflammatory reactions observed following short exposure to high
    concentrations. Emphysema, atelectasis, focal necrosis, broncho-
    pneumonia, and fibrosis have been reported, often accompanied by a
    variety of cellular alterations. The degree of morphological injury
    appears to be proportional to the concentration and time of exposure.

        Freeman et al. (1974) described a number of pathomorphological
    changes in rats continuously exposed to ozone at concentrations of
    1100 and 1800 g/m3 (0.54 and 0.88 ppm) for as long as 6 months.
    After 48 h of exposure to the lower concentration, an influx of
    macrophages and an increase in mitotic figures were observed, and the
    alveolar ducts became demarcated by hypertrophic alveolar epithelium.
    Most of these changes became more obvious with increasing length of
    exposure with the exception of the terminal bronchioles which appeared
    to recover and return to normal. After exposure to the higher
    concentration of 1800 g/m3 (0.88 ppm) for 48 h, the bronchiolar
    epithelium exhibited both metaplasia and fibrosis. Adenoma-like
    structures containing large numbers of macrophages and fibrotic
    lesions appeared after 6 days and after 3 weeks of exposure, half of
    the rats had died and were found to have emphysema-like lesions.

        Using scanning electron microscopy, Ikematsu et al. (1976) found
    that continuous exposure to an ozone concentration of 2000 g/m3
    (1.0 ppm) for 10 days resulted not only in desquamation of epithelial
    cells but also in the appearance of inflammatory cells in the tonsils
    of rabbits. Similar pathological effects were observed with
    intermittent exposure indicating that the animals were unable to
    recover sufficiently during the periods of exposure to clean air.
    Emphysematous and vascular lesions in the lung of the rabbit described
    by P'an et al. (1972), were the result of repeated exposure to
    800 g/m3 (0.4 ppm) for 6 h per day, 5 days per week, for 10 months.
    Stokinger et al. (1957) also reported fibrotic changes and chronic
    bronchial and bronchiolar emphysema in the lungs of mice, rats,
    guineapigs, and hamsters exposed to an ozone concentration of
    2000 g/m3 (1.0 ppm) for 6 h per day, 5 days per week, for 433 days.
    However, under the same conditions of exposure, the effects in dogs
    were limited to the trachea and large bronchi. When dogs were exposed
    to ozone concentrations ranging from 2000 to 6000 g/m3 (1-3 ppm) for
    8, 16, or 24 h daily for 18 months, the morphological damage was
    roughly proportional to the product of the concentration and the time
    of exposure, the epithelial lining of the terminal airways and
    proximal alveoli being most adversely affected (Freeman et al., 1973;
    Stephens et al., 1973). Similarly, Castleman et al. (1973a) found that
    the walls of the terminal airways and the interalveolar septa of rat
    lung were most affected by continuous exposure to an ozone
    concentration of 1600 g/m3 for 7 days.

        Intermittent exposure for 8 h per day, for 7 days to an ozone
    concentration of 400 g/m3 (0.2 ppm) produced damage to the
    respiratory bronchioles in bonnet monkeys. In rats fed a normal diet,
    similar effects were seen at the same concentration of ozone, but, in
    vitamin E-deficient rats, an equivalent effect was caused by ozone at
    200 g/m3 (0.1 ppm) (Dungworth et al., 1975; Dungworth, 1976).

        Loosli et al. (1972) measured the cell turnover rate in mice and
    found it to be significantly higher in animals exposed to synthetic
    photochemical air pollution for 8-12 months than in control animals
    breathing filtered air. The synthetic air pollution contained ozone at
    600-840 g/m3 (0.30-42 ppm), carbon monoxide at 3.5-12 mg/m3
    (3-10 ppm), nitrogen dioxide at 1300-1600 g/m3 (0.70-0.85 ppm), and
    sulfur dioxide at 5700-6000 g/m3 (2.2-2.3 ppm). The effects were
    similar to those seen with exposure to pure ozone.

        Rats exposed for 1 month to a mixture of ozone at 1800 g/m3
    (0.9 ppm) and nitrogen dioxide at 1700 g/m3 (0.9 ppm), developed
    enlarged alveolar spaces (Freeman et al., 1974). In rats exposed for
    2 weeks to a combination of ozone at 500 g/m3 (0.25 ppm), and
    nitrogen dioxide at 4700 g/m3 (2.5 ppm), the bronchiolar epithelium
    became cuboidal. However, after 6 months of exposure, the tissue
    appeared to be normal (Freeman et al., 1974).

        Slightly inflammatory and proliferative changes were observed by
    Nakajima et al. (1972) in the bronchial membranes of mice exposed to
    irradiated auto exhaust that contained oxidant concentrations of
    200-300 g/m3 (0.1-0.15 ppm), for 2-3 h per day, 5 days per week, for
    30 days.

    5.2.2  Functional changes

    5.2.2.1  Short-term exposure (24 h or less)

        The first abnormal sign observed in various animal species during
    exposure to ozone is an irregular respiratory pattern. This usually
    appears within the first few minutes of exposure and, in most cases,
    the animal returns to normal, when allowed to recover in the clean
    air.

        Functional changes in the respiratory system have been observed in
    several species of animals at ozone concentrations of less than
    2000 g/m3 (1.0 ppm). Yokoyama (1972a) studied the ventilatory
    function of guinea-pigs before, during, and after a 2-h exposure to an
    ozone concentration of 1000 g/m3 (0.5 ppm). He reported increases in
    the frequency of respiration and airway resistance, but a decrease in
    tidal volume. In another investigation, Yokoyama (1973) exposed only
    the right lung of rabbits to ozone at 2000 g/m3 (1 ppm) for
    3 h and used the left lung as a control. The exposed lung of rabbits
    killed 1 and 3 days after exposure, had a reduced vital capacity.
    However, the reduction in vital capacity in those killed 7 days after
    exposure was only slight and was not significant. Other studies on
    lung function in guineapigs exposed to ozone for 2 h indicated that
    pulmonary flow resistance was not altered by exposure to
    concentrations of 700 and 1400 g/m3 (0.34 and 0.68 ppm), but that it
    increased significantly at exposure levels of 2000 and 2700 g/m3
    (1.08 and 1.35 ppm) (Murphy et al., 1964a).

        Scheel et al. (1959) exposed rats to an ozone concentration of
    4000 g/m3 (2 ppm) for 3 h. Decreases in minute ventilation, tidal
    volume, and oxygen uptake, that occurred immediately after exposure,
    reached minimum recorded values 8 h later. All measurements returned
    to normal levels 20 h after exposure.

        When cats were exposed for an average of 4.6 h to ozone
    concentrations of 520, 1000, and 2000 g/m3 (0.26, 0.5, and 1 ppm),
    Watanabe et al. (1973b) found that pulmonary flow resistance increased
    with increasing ozone concentrations. Dynamic compliance was reduced,
    but to a lesser extent, and vital capacity was unaffected. The
    proportion of animals that showed a reduction in diffusion capacity
    appeared to increase with increasing ozone concentrations.

        There are a few studies in which animals were exposed to ambient
    air and irradiated auto exhaust containing high concentrations of
    oxidants, including those of Swann & Balchum (1966) who measured the
    total expiratory flow resistance in guineapigs on days of unusual
    weather and smog conditions in Los Angeles. When the resistance was
    compared with routine monthly measurements on the same animals,
    significant increases were found at oxidant levels of approximately
    600 g/m3 (0.30 ppm) or more. Substantial increases in resistance
    were also observed, when relatively high concentrations of nitrogen
    dioxide (1700 g/m3; 0.92 ppm), carbon monoxide (30 mg/m3;
    26 ppm), and hydrocarbons (16 ppm) were present, but the oxidant level
    (80 g/m3; 0.04 ppm) was relatively low.

        The effects on guineapigs of a 4-h exposure to diluted,
    irradiated, or nonirradiated exhaust atmospheres were reported by
    Murphy et al. (1963). Marked, rapid increases in total expiratory flow
    resistance accompanied by a decrease in respiratory rate and a small
    increase in tidal volume occurred during exposure to irradiated
    exhaust. The reaction to nonirradiated exhaust was comparatively
    slight. Ranges of concentrations of the main pollutants in the
    exhaust-contaminated air used in this study were: for irradiated
    exhaust gases: total oxidants, 660-1640 g/m3 (0.33-0.82 ppm);
    nitrogen dioxide, 800-10 000 g/m3 (0.43-5.5 ppm); and carbon
    monoxide, 39-360 mg/m3 (34-310 ppm): for nonirradiated exhaust gases:
    total oxidants, less than 40 g/m3 (0.02 ppm); nitrogen dioxide,
    710-3000 g/m3 (0.38-1.58 ppm); and carbon monoxide, 98-345 mg/m3
    (85-300 ppm). In addition, increased levels of formaldehyde, acrolein,
    and olefin were present.

    5.2.2.2  Prolonged and repeated exposures

        The few available pulmonary function studies on animals exposed to
    ozone for extended periods of time include a study by Bartlett et al.
    (1975) who reported that continuous exposure of rats to an ozone
    concentration of 400 g/m3 (0.2 ppm) for 30 days caused a 16%
    increase in lung volume and an increase in alveolar dimension. A
    reduction in lung elasticity that was also reported was possibly an

    effect of ozone on collagen. Yokoyama (1974) exposed rabbits to an
    ozone concentration of 4000 g/ma (2 ppm), for 6 h per day, for 3-4
    days, and found that the pulmonary flow resistance was greater, and
    compliance lower, than in the controls. In animals exposed to
    2000 g/m3 (1 ppm), for 6 h per day, for 7-8 days, the values were
    between those of the controls and of the group treated with
    4000 g/m3 (2 ppm).

    5.2.3  Biochemical changes

    5.2.3.1  Effects indicating possible mechanisms of action

        The actual mechanism of ozone toxicity at the subcellular level is
    still obscure. Studies of the biochemical effect of ozone have mainly
    been based on two hypotheses: (a) that ozone interacts with readily
    oxidizable substances thus altering the course of metabolism and
    producing a toxic effect; and (b) that ozone interacts with
    unsaturated lipids to produce lipid peroxidation and consequent cell
    damage.

        Sulfhydryl systems appear in the cell not only as reducing
    substances but also as functional constituents of a variety of enzymes
    and proteins. Ozone is capable of oxidizing these substances causing
    inactivation of enzymes and alterations in the structure and function
    of the cell membrane.

        In studies on ozone oxidation of glutathione  in vitro, Mudd et
    al. (1969) showed that both fast and slow oxidation occurred. It has
    also been shown that the oxidation of reduced glutathione (GSH)
    results in oxidized glutathione (GSSG), though some of the sulfhydryl
    groups form higher oxidation products that are not reduced by the
    reductases available in the cells (Menzel, 1971). Mountain (1963)
    demonstrated  in vivo oxidation of reduced glutathione and a decrease
    in the sulfur-containing enzyme succinic dehydrogenase (1.3.99.1)
    activity in the lungs of mice. King (1961) also found a decrease in
    sulfhydryl content and of enzymatic function of partially purified
    glyceraldehyde-3-phosphate dehydrogenase (1.2.1.12) in rat lungs
    following exposure to an ozone concentration of 2400 g/m3 (1.2 ppm)
    for 4 weeks.

        When rats were exposed to an ozone concentration of 4000 g/m3
    (2.0 ppm) for 4-8h, the concentrations of both the protein and
    nonprotein sulfhydryls in the lungs decreased. The activities of lung
    enzymes containing sulfhydryl also decreased including those of
    glucose-6-phosphate dehydrogenase (1.1.1.49), glutathione reductase
    (1.6.4.2), and cytochrome  c reductase related to succinate and
    reduced nicotinamide adenine dinucleotide (NADH) (DeLucia et al.,
    1972). Expanding these studies, DeLucia et al. (1975) showed that the
    magnitude of the decrease in the nonprotein sulfhydryl groups in rats

    was dependent on the duration of exposure and the concentration of
    ozone. Significant decreases were not observed at an ozone
    concentration of 1600 g/m3 (0.8 ppm) for 24 h.

        Further studies have shown that exposure to ozone tends to
    increase the activity of enzymes that protect against intracellular
    oxidation. When rats were continuously exposed to an ozone
    concentration of 1500 g/m3 (0.75 ppm) for 1, 3, 10, or 29 days, it
    was noted that the activities of glutathione peroxidase (1.11.1.9),
    glutathione reductase (1.6.4.2), glucose-6-phosphate dehydrogenase,
    6-phosphogluconate dehydrogenase (1.1.1.43), and pyruvate kinase
    (2.7.1.40) were lower than those in the controls after 1 day but
    higher after, 3, 10, and 29 days of exposure (Chow & Tappel, 1973). In
    another study by the same investigators, rats exposed to 400 g/m3
    (0.2 ppm) continuously for 8 days or intermittently (8 h per day) for
    7 days showed significantly increased glutathione peroxidase activity
    in the lung. Fukase et al. (1975a, 1975b) exposed mice to ozone
    concentrations of 400, 1000, or 2000 g/m3 (0.2, 0.5 or 1.0 ppm) for
    4 h per day, for 30 days, and found progressive increases in the
    levels of glutathione and vitamin C in the lung with increasing ozone
    concentration. The authors also reported significant increases in the
    activities of glutathione peroxidase, glutathione reductase, and
    glucose-6-phosphate dehydrogenase in the lungs of mice. Exposure of
    both rhesus and bonnet monkeys to levels of ozone ranging from 400 to
    1600 g/m3 (0.2-0.8 ppm) for 8 h per day for 7 days, resulted in
    increased activities of succinate oxidase and glutathione peroxidase
    in the lungs. Linear regression analysis showed a significant
    correlation between ozone concentration and the augmentation in
    activity of these enzymes (Dungworth et al., 1975).

        Many investigators have attributed the biological effect of ozone
    to lipid peroxidation (Fournier, 1973; Goldstein et al., 1969; Menzel,
    1970; Roehm et al., 1971b). Oxidation of unsaturated fatty acids by
    ozone has been demonstrated both  in vivo and  in vitro. The
    mechanism of such action is based on the proclivity of ozone to react
    with the ethylene groups of the acid to form peroxides. Their
    decomposition results in the further formation of free radicals
    capable of initiating peroxidation of other unsaturated fatty acids.
    The breakdown products (peroxides, carbonyl compounds) may themselves
    be cytotoxic.

        Evidence of lung lipid peroxidation during ozone exposure was
    suggested by Goldstein et al. (1969) who found conjugated diene bonds
    in an extract of lungs of mice exposed to 800-1400 g/m3
    (0.4-0.7 ppm) for 4 h. Another index of lipid peroxidation is an
    increase in malonaldehyde. In studies by Chow & Tappel (1972), the
    malonaldehyde concentration increased in the lungs of rats exposed
    continuously to 1400-1600 g/m3 (0.7-0.8 ppm) for 5 and 7 days.

        Further evidence of the role of peroxidation in ozone toxicity is
    the fact that animals deficient in vitamin E are more susceptible to
    ozone. This is discussed in detail in section 5.2.7.

    5.2.3.2  Biochemical effects at the subcellular level

        In morphological and ultrastructural studies, swelling and
    degenerative changes in lung mitochondria have frequently been
    reported following ozone exposure. Mitochondrial functions are
    critical to the cellular terminal substrate oxidation and energy
    production. These organelles in lung cells may be the target for ozone
    since many mitochondrial enzyme activities are sulfhydryl-dependent
    and mitochondrial membranes contain abundant unsaturated
    phospholipids. It has therefore been suggested that the lung
    mitochondria may be a sensitive test system for detecting and
    evaluating ozone toxicity.

        Mustafa et al. (1973) found a 45% increase in pulmonary
    mitochondrial oxygen consumption in rats continuously exposed to
    1600 g/m3 (0.8 ppm) for 10-20 days. A 3-fold increase in the number
    of type II cells was also reported. These cells are rich in
    mitochondria and may have been responsible for the increase in oxygen
    consumption. A 17% increase in oxygen consumption was noted with
    continuous exposure to an ozone concentration of 400 g/m3 (0.2 ppm)
    for 7 days. This effect appeared to be dose-related.

        Lysosomes are vitally important in the intra- and possibly
    extracellular destruction of inhaled matter. The hydrolytic enzyme
    system of lysosomes in the alveolar machrophage is crucial to maintain
    the sterility of the lung against inhaled microbes. Any inactivation
    of these enzymes would be expected to increase the risk of respiratory
    disease.

        Hurst et al. (1970) reported that, when rabbits were exposed for
    3 h to an ozone concentration of 500 g/m3 (0.25 ppm), there was a
    reduction in the activity of lysosomal hydrolases, i.e., acid
    phosphatase (3.1.3.2), lysozyme (3.2.1.17), and beta-glucuronidase
    (3.2.1.31).  In vitro exposure of rabbit alveolar macrophages
    produced a similar decrease in lysosomal hydrolases (Hurst & Coffin,
    1971).

        Ozone may also induce increases in the concentrations of these
    enzymes which may be related to eventual chronic lung disease.

        When the specific activities of a number of lysosomal hydrolases
    were measured in whole lung homogenates of rats, there was a
    significant increase in activities after the animals had been
    continuously exposed to an ozone concentration of 1400-1600 g/m3
    (0.7-0.8 ppm) for 5-7 days (Dillard et al., 1972). This increase could
    be attributed to an inflammatory reaction induced by the ozone (Coffin
    et al., 1968b).

        Castleman et al. (1973b) exposed rats continuously for 7 days to
    ozone concentrations of 1400-1600 g/m3 (0.7-0.8 ppm). Using histo-
    and cytochemical techniques, they observed increased acid phosphatase
    activity but no change in beta-glucuronidase activity.

        One of the structural alterations reported following exposure to
    ozone is a change in the appearance of the endoplasmic reticulum.
    Biochemical evidence of the effects of ozone on microsomal enzymes was
    reported by Palmer et al. (1971, 1972). After a 3-h exposure to ozone
    at 1500 g/m3 (0.75 ppm), the lung tissue of hamsters and the
    tracheobronchial mucosa of rabbits showed reductions of 33% and 53%,
    respectively, in the activity of benzopyrene hydroxylase (1.14.14.2),
    a mixed function oxidase that depends on cytochrome P-450 and is
    located in the endoplasmic reticulum. Similar results were obtained by
    Goldstein et al. (1975) who exposed rabbits for 90 rain to
    2000 g/m3 (1.0 ppm) and demonstrated a decrease in the rabbit lung
    cytochrome P-450 concentration. It is of interest that the maximum
    effect was observed a few days after exposure.

        Very little information is available describing alterations in
    nucleic acids related to ozone exposure. Most of the studies have been
    conducted at much higher concentrations than those found in the
    environment and have yielded conflicting results. Since DNA synthesis,
    cell division, and growth are closely linked, further studies
    clarifying the potential hazard would be of value.

        In studies on mice exposed to an ozone concentration of
    5000 g/m3 (2.5 ppm) for 2 h per day, for 120 days, Werthamer et al.
    (1974) noted reductions in both DNA and RNA syntheses and a
    concomitant increase in protein synthesis. Evans et al. (1971) exposed
    aging mice for 6 h to 1000-7000 g/m3 (0.5-3.5 ppm) and found that,
    regardless of the ozone concentration, there was inhibition of DNA
    synthesis. The authors believed that this indicated a reduction in the
    ability of the alveoli to act as a source of new cells and to maintain
    the integrity of lung tissue during ozone exposure.

    5.2.3.3  Extracellular effects

        Since the primary cause of death from high concentrations of ozone
    is pulmonary oedema, investigators have attempted to determine the
    role of histamine in the pulmonary toxicity of ozone. The lung is rich
    in histamine -- containing mast cells and among the many effects of
    histamine are oedematigenous alterations in vascular capillaries.
    Oedema, whatever the cause, reduces the number of alveoli
    participating in gas exchange, and produces conditions favourable for
    bacterial growth. The exact concentration of ozone required to produce
    oedema depends on the animal species. It is generally believed that
    gross oedema is probably not elicited in any species exposed to ozone
    at the concentrations found in the ambient air.

        Alpert et al. (1971a) used a radio labelled albumen technique to
    detect the presence of pulmonary oedema in rats. A significant
    increase in albumen levels in pulmonary lavage fluid appeared after
    6 h exposure to 1000 g/m3 (0.5 ppm).

        The available data on lung histamine are conflicting. Dixon &
    Mountain (1965) reported that, following a single exposure of mice to
    an ozone concentration of 2000 g/m3 (1.0 ppm) for 5 h, there was a
    release of histamine from the lungs that persisted for at least 4
    days. Pretreatment with an antihistamine (promethazine) reduced the
    amount of oedema following exposure to a sublethal dose of ozone. It
    should be noted that the antihistamine used is a phenothiazine
    derivative the action of which might stabilize membranes and trap free
    radicals. In contrast, Easton & Murphy (1967) were unable to
    demonstrate any reduction of lung histamine in guineapigs exposed to
    ozone concentrations of 10-12 mg/m3 (5-6 ppm) for 2 h. Cronin & Giri
    (1974) also failed to observe any differences in the lung histamine
    level in rats exposed to an ozone concentration of 8000 g/m3
    (4.0 ppm) for 4 h, although pulmonary oedema was evident.

        Surface tension is an important contributor to the elastic
    properties of the lung. Any alteration of the normal surface tension
    in the alveoli may be implicated in the development of chronic lung
    disease. Consequently, several investigators have examined the effect
    of ozone on pulmonary surface activity. Gardner et al. (1971) exposed
    rabbits to ozone levels as high as 20 mg/m3 (10 ppm) for 2.5 h and
    then isolated the surface active material by pulmonary lavage. They
    found that ozone did not alter the surface tension of this material
    and that  in vitro exposure did not affect the properties of
    dipalmitoyl lecithin, a principal component of this surface active
    substance. Similar results were reported by Huber et al. (1971) who
    showed that a 3-h exposure of rabbits to ozone at 10 mg/m3 (5 ppm)
    did not alter surface activity in the lavaged alveolar lining material
    nor in extracts of the whole lung. On the other hand Yokoyama (1972a)
    reported that  in vitro exposure of guineapig lung extracts to ozone
    concentrations of 1000 to 24000 g/m3 (0.5-12 ppm) for 25-60 min
    resulted in a rapid increase in surface tension. However, he, too, did
    not find any change when dipalmitoyl lecithin was exposed to ozone.

        The effect of ozone on the appearance of lung tissue lipids in
    saline lavage fluid was studied by Kyei-Aboagye et al. (1973). They
    proposed that ozone affected the lung by decreasing lecithin formation
    while simultaneously stimulating the release of surfactant lecithins
    (palmitoyl and oleyl).

        In the lung, there is a ground substance between the basement
    membrane of the alveolar epithelium and the capillary endothelium. Any
    destruction of the integrity of this substance could affect the
    elasticity of the lung. Buell et al. (1965) fractionated the lung
    tissue of rabbits, after a 1-h exposure to an ozone concentration of

    2000 g/m3 (1.0 ppm), into soluble, lipid, and protein fractions. The
    isolation of aldehydes and ketones from the protein fraction indicated
    structural changes. It was suggested that, once these compounds were
    formed, they might affect the intra-and intermolecular crosslinking of
    elastic protein molecules which would in turn cause a reduction in the
    elasticity of the lung.

    5.2.4  Carcinogenicity

        The possibility that ozone might be carcinogenic has been studied
    in experimental animals.

        Exposure to ozonized gasoline with ozone concentrations of
    2000-7600 g/m3 (1.0-3.8 ppm) for 52 weeks caused an increased
    incidence of lung tumours in strain A mice (350 in each experimental
    group). After 40 weeks, tumours were found in 21% of animals in the
    control group and in 63% in the test group. After 52 weeks, the
    incidence of tumour-bearing animals in the control group exposed to
    washed air was 41% compared with 80% in the test group (Kotin & Falk,
    1956). Additional studies using the same atmosphere of ozonized
    gasoline were conducted on C57BL mice (405 in each experimental group)
    (Kotin et al., 1958). After 92 weeks, the incidence of tumour-bearing
    animals in the control group was 1.6% compared with 9.6% in the
    exposed group.

        Although these studies indicate that ozone may be tumorigenic
    further work is necessary to confirm these results.

        A number of studies including that of Penha et al. (1972) have not
    been considered in this document because information concerning the
    numbers of animals tested, control groups, etc. was inadequate.

    5.2.5  Tolerance to ozone

        The term "tolerance" refers to the fact that exposure to a
    nonlethal dose of a specific toxic substance protects the host against
    subsequent exposure to higher dose of the same chemical or of
    different agents with similar toxicological properties (cross-
    tolerance). Stokinger et al. (1956) reported such a protective effect
    for ozone. Tolerance to ozone has been reviewed by Fairchild (1967)
    and has also been reported by many other investigators (Henschler,
    1960; Matzen, 1957; Mendenhall & Stokinger, 1959), In small rodents,
    tolerance can be initiated by a concentration as low as 600 g/m3
    (0.3 ppm), maximum protection being obtained with concentrations
    within the range of 2000-8000 g/m3 (1-4 ppm) (Stokinger & Scheel,
    1962). A single exposure to ozone can also induce a cross-tolerance
    against subsequent lethal doses of X-ray irradiation (Hattori et al.,
    1963), nitrogen dioxide, hydrogen peroxide, carbonic dichloride
    (phosgene), ethanone (ketene), nitrosyl chloride, or hydrogen sulfide
    (Fairchild, 1967).

        Although the mechanism of tolerance is still not well understood,
    several possibilities have been envisaged. Fairchild (1967) observed
    that the level of reduced glutathione was maintained in ozone-tolerant
    animals but not in the nontolerant group. This could be brought about
    either by directly blocking the oxidation of reduced glutathione or
    through some enzymatic pathway that would stimulate production of
    reduced glutathione in response to a second exposure to ozone.
    However, it is possible that the maintenance of these thiols in the
    tolerant animal may be a result of the tolerance rather than the
    cause. The studies of Chow & Tappel (1972, 1973) provide biochemical
    support for this hypothesis. In addition, it has been shown (Mountain
    et al., 1960) that the activities of serum alkaline phosphatase
    (3.1.3.1), adrenal succinate dehydrogenase (1.3.99.1), and glucose-6-
    phosphatase (3.1.3.9) either remain normal or are only slightly
    altered in tolerant animals.

        Fukase et al. (1975a) reported tolerance in mice to an ozone
    concentration of 20-58 mg/m3 (10-29 ppm) after pre-exposure to
    concentrations of 400-2000 [g/m3 (0.2-1.0 ppm) and proposed that the
    mechanism involved an increase in reduced glutathione and the
    elevation of the activity of the peroxidative metabolic pathway.
    Physical swelling of the intra-alveolar septa has also been proposed
    by Henschler et al. (1964) as a mechanism of protection against the
    oedematigenous effects of ozone.

        Unilateral lung exposure models showed that, in order to induce
    protection in a particular lung, the tissue must have actually come
    into contact with ozone (Alpert & Lewis, 1971; Frank et al., 1970a).
    There was no significant crossover effect from contralateral lung,
    suggesting that there was no basis for assuming a circulating humoral
    factor, and that the phenomenon was purely local. While reduction in
    oedema development could be induced by pre-exposure to ozone, no
    protection was obtained against the influx of polymorphonuclear
    leukocytes into the lung (Gardner et al., 1972). It was also apparent
    that tolerance did not influence the number of recoverable cells in
    pulmonary lavage, their phagocytic capability, or the loss of
    macrophage hydrolytic enzyme activity. This indicated the possibility
    that tolerance protects only against pulmonary oedema and not against
    other more subtle reactions.

        As previously mentioned, tolerance is initiated in small rodents
    at approximately 600 g/m3 (0.3 ppm) (Stokinger & Scheel, 1962).
    According to Alpert et al. (1971a) this approximates the lowest level
    at which oedema can be demonstrated in rats by means of recovery of
    labelled serum albumen via lung lavage. Thus, Coffin & Gardner (1972b)
    postulated that it is probably necessary to produce a minimal
    oedematigenous response before tolerance develops.

        Experiments designed to test the susceptibility to bacterial
    infection of "tolerant" animals have been conducted by Coffin &
    Gardner (1972b). These studies illustrated that the "tolerant" animals

    were only partially protected against the joint effects of ozone and a
    viable microorganism. Partial protection was afforded by tolerance
    only when the initiating dose of ozone was 600 g/m3 (0.3 ppm) for
    3 h (Coffin & Gardner, 1975). The authors considered that the
    tolerance to infection seen at levels of ozone above 600 g/m3
    (0.3 ppm) could be due to the prevention of the formation of oedema
    and that tolerance was only partial because tolerance to the action of
    ozone on cellular and noncellular specific defence systems could not
    be induced. This was shown by Gardner et al. (1972) who reported that
    there was no tolerance to ozone damage in the alveolar macrophages,
    which are the prime defence against infectious disease in the lower
    part of the respiratory tract.

        In their study on the effects of ozone on laboratory-induced
    allergic respiratory disease in guineapigs, Matsumura et al. (1972)
    found that animals that had received pretreatment with ozone at
    2000 g/m3 (1 ppm) for 1 h before a challenging exposure of
    4000 g/m3 (2 ppm) showed only slight respiratory reaction to
    acetylcholine inhalation compared with those in which tolerance had
    not been induced.

        A variety of neurohumoral factors resulting from exposure to ozone
    have been described by Fairchild (1963), who reported that
    thyroidectomy, adrenalectomy, and hypophysectomy would increase the
    resistance of rats to otherwise lethal doses of ozone by preventing
    pulmonary oedema.

        While the mechanism of tolerance in relation to ozone-induced
    oedema is still not well understood, it has been suggested that the
    thymus might play a role. Thymectomized mice were unable to develop
    tolerance to ozone although the sham-operated animals exhibited
    tolerance under the same experimental conditions (Gregory et al.,
    1967).

    5.2.6  Effects on the host defence system

        Animals that are exposed to ozone have an increased susceptibility
    to disease-producing biological agents, which can result in an
    increased incidence of pulmonary infectious disease and death. Animal
    models have been developed by several investigators to examine this
    phenomenon experimentally by exposing animals to the pollutants and to
    aerosolized viable microorganisms. The high sensitivity of this model
    system is probably due to the fact that it reflects a summation of all
    the subtle effects that ozone has on the lung. Any alteration in
    cellular defence and ciliary activity, oedema, and immunosuppression
    would allow the viable organism to multiply and cause disease. In mice
    exposed to ozone at concentrations ranging from 2600 to 8800 g/m3
    (1.3 to 4.4 ppm) for 3 h, or 1700 g/m3 (0.84 ppm) for 4 h, per day,
    5 days per week, for 2 weeks, and subsequently challenged with
     Klebsiella pneumoniae, resistance to respiratory infection was
    significantly reduced (Miller & Ehrlich, 1958). Similar results were

    obtained with hamsters. In a study to elucidate a relationship between
    the time of exposure to ozone and the time of challenge with the
    infectious agent, Purvis et al. (1961) found a decrease in resistance
    to infection in mice treated with the bacterial aerosol within 19 h of
    exposure to the gas or 27 h before exposure. A subsequent study by
    Coffin and co-workers (1968a) showed that the lowest concentration of
    ozone necessary to produce this effect in mice was 160 g/m3
    (0.08 ppm) for 3 h; Coffin & Gardner (1972) established a dose-
    response relationship.

        Further enhancement of toxicity could be demonstrated when a third
    interactant such as cold or exercise was added to this infectivity
    model. Housing mice at 6-9C for 3 h prior to exposure to ozone at
    1400-1800 g/m3 (0.7-0.9 ppm) for 2 h and to the microorganisms
    increased the mortality rate compared with that in animals housed at
    room temperature (Coffin & Blommer, 1965). Mice subjected to physical
    activity while being exposed to ozone at 200-600 g/m3 (0.1-0.3 ppm)
    were less resistant to infectious agents than animals that were at
    rest during exposure (Gardner et al., 1974b).

        The effects of low concentrations of ozone on mice with induced
    silicosis were investigated by Goldstein et al. (1972). Starting at an
    ozone concentration of 800 g/m3 (0.4 ppm) for 4 h, a progressive
    decrease in pulmonary bactericidal activity occurred with exposure to
    increasing concentrations of ozone. Silicosis itself did not inhibit
    bactericidal activity.

        Further research has sought to delineate the mechanisms by which
    ozone reduces the resistance of animals to bacterial infection. It
    appears that a number of factors may be responsible.

        The number of organisms initially deposited within the lung does
    not play a major role in the enhancement of mortality since the
    pulmonary deposition of the microorganisms is less in ozone-treated
    animals. However, regardless of the initial deposition, ozone-treated
    animals subsequently have more organisms within the lungs, mainly
    because of reduced bactericidal ability and the subsequent
    multiplication of the inhaled organism (Coffin & Gardner, 1972a).
    Goldstein et al. (1971b) found that the magnitude of the increase in
    bacterial numbers was correlated with an increase in ozone
    concentration up to 5200 g/m3 (2.58 ppm). In rabbits exposed to
    ozone at a concentration of 1000 g/m3 (0.5 ppm), for 16 h per day,
    for 7 months, Friberg et al. (1972) did not find any effect on the
    physical removal of inhaled particles or on the number of macrophages
    but, under the same conditions, a significant decrease was found in
    the clearance of viable  Escherichia coli in the lungs of guinea-
    pigs.

        The effect of ozone on antibacterial activity in the mouse lung
    was determined  in vivo by investigating the removal of bacteria by
    mucociliary activity and by bactericidal activity simultaneously
    (Goldstein, 1971a, 1971b). Mice were exposed to various concentrations
    or ozone including 1200, 1400, 1600, and 2200 g/m3 (0.62, 0.70,
    0.80, and 1.1 ppm), for 17 h prior to, or 4 h after, infection with
    aerosols of radiolabelled  Staphylococcus aureus. Inhibition of
    pulmonary bactericidal activity was shown at an ozone concentration as
    low as 1200 g/m3 (0.62 ppm) and activity decreased progressively
    with increasing levels of ozone. The authors proposed that the
    bactericidal defect was due to dysfunction of the alveolar macrophage.

        Rabbits exposed to an ozone concentration of 600 g/m3 (0.3 ppm)
    for 3 h showed an impairment of the phagocytic properties of the
    pulmonary alveolar macrophages (Coffin et al., 1968b). Furthermore,
    exposure to ozone at 10 mg/m3 (5 ppm) for 3 h resulted in a reduction
    in the total number of macrophages with a concomitant influx of
    polymorphonuclear leukocytes into the lower respiratory tract (Coffin
    & Gardner, 1972a). Enzyme activity was reduced in macrophages
    recovered by lavage after exposure of rabbits to ozone at levels as
    low as 500 g/m3 (0.25 ppm) for 3 h (Hurst et al., 1970). The enzymes
    studied were lysozyme (3.2.1.17), acid phosphatase, and beta-
    glucuronidase and since these enzymes are involved in the
    intracellular degradation of ingested bacteria, their reduction could
    contribute to the poor bactericidal effect seen in the studies
    mentioned previously.

        Other effects on the alveolar macrophage have also been reported.
    For example, alveolar macrophages from rabbits exposed to 4000 g/m3
    (2.0 ppm) for 8 h per day for 7 days exhibited an increased membrane
    fragility (Dowell et al., 1970) and a dose-related reduction in
    interferon was observed in the alveolar macrophages of rabbits exposed
    to ozone concentrations of 2000, 6000, and 10 000 g/m3 (1, 3, and
    5 ppm) for 3 h (Shingu et al., 1972).

        Morphological changes seen in rabbit alveolar macrophages after
    exposure to ozone at a concentration of 10 mg/m3 (5 ppm) for 3 h
    included dilatation of the endoplasmic reticulum and perinuclear
    envelope, swelling of the mitochondria, intracellular vacuolization,
    cell lysis, and the formation of myelin figures and autophagic
    vacuoles (Huber et al., 1971). The ultrastructure of pulmonary
    macrophages was also examined  in situ in the lung tissue of rats
    exposed to ozone at 6000 g/m3 (3 ppm) for 4 h (Plopper et al.,
    1973b). Immediately after exposure, the cells resembled those of
    unexposed animals, but 12 h later there were twice as many
    macrophages. Many of these had large granular cytoplasmic inclusions
    that indicated increased phagocytic activity.

        There is evidence that the effect seen on the alveolar macrophage
    may be mediated through the noncellular milieu of the lung. Within the
    lung, the macrophages are located close to the so-called pulmonary

    surfactant contained in the extracellular lining of the epithelial
    surface of the lung alveoli. It is possible that ozone might also
    react with this lining film, which in turn, might be deleterious to
    the cell. Gardner (1971) and Gardner et al. (1971) conducted
    experiments on lavage fluid containing this surface-active substance
    and found that, when isolated from rabbits exposed to ozone at
    20 mg/m3 (10 ppm) for 2.5 h, it could adversely affect the stability
    of the alveolar macrophage  in vitro. A similar effect was noted when
    ozone was bubbled through the lavage fluid  in vitro. The effect on
    the lavage fluid could be seen at levels as low as 200 g/m3
    (0.1 ppm) for a 2.5-h,  in vivo exposure or after only 30-min  in
     vitro exposure. Since no significant alteration in the surface
    tension of the lavage fluid was produced by ozone exposure, it is
    suggested that the effect might be on a nonlipid component, possibly a
    protein. The activity found in the cell-free pulmonary lavage fluid
    has been called the "protective factor".

        Ozone may also inactivate some opsinogenic factor within the lung
    since Holzman et al. (1968) showed that it has a protein-degrading
    property. Exposure of mice and rabbits to a concentration of
    10 mg/m3 (5 ppm) for 3 h reduced the activity of active lysozyme,
    obtained by bronchopulmonary lavage, by approximately 30%.

        Various components of the endogenous defence mechanism of the lung
    were studied through a unilateral lung exposure model (Alpert et al.,
    1971b). Exposure to ozone at concentrations ranging from 1000 to
    6000 g/m3 (0.5 to 3 ppm) for 3 h decreased cellular viability,
    depressed the intracellular hydrolytic enzymes and increased the
    absolute number of polymorphonuclear leucocytes in the pulmonary
    lavage fluid. All effects were dose-related and were found only in the
    lung under treatment and not in the contralateral lung that breathed
    ambient air.

        Heuter et al. (1966) reported that exposure to an irradiated
    automobile exhaust atmosphere for 15 months increased the
    susceptibility of mice to pulmonary infection during the latter half
    of the animal's lifetime. The oxidant concentrations in the irradiated
    exposure, chamber ranged from 400 to 2000 g/m3 (0.2-1.0 ppm). In a
    similar study using irradiated automobile exhaust, enhancement of
    mortality compared with that of control animals kept in filtered air
    was noted with exposure to total oxidants at 300 g/m3 (0.15 ppm) and
    carbon monoxide at 29 mg/m3 (25 ppm) for 4 h. Coexistent
    concentrations of nitrogen dioxide ranged from a trace to 1900 g/m3
    (1.0 ppm) (Coffin & Blommer, 1967).

        Goldstein et al. (1974) studied the bactericidal effect when mice
    were exposed to a combination of nitrogen dioxide and ozone, and
    concluded that the combined pollutants caused bactericidal dysfunction
    at concentrations that were approximately the same as the lowest
    concentrations that caused similar dysfunction when the animals were
    exposed to the gases individually.

        Erlich et al. (1977) observed that a single joint exposure of mice
    to nitrogen dioxide and ozone resulted in an addition of effects and
    that a synergistic action might result from repeated exposures to the
    mixture.

        The animals were exposed for 3 h to 16 different combinations of
    nitrogen dioxide at levels of 0, 2800, 3800, 6600, and 9400 g/m3
    (0, 1.5, 2.0, 3.5, and 5.0 ppm) and ozone at 0, 100, 200, and
    1000 g/m3 (0, 0.05, 0.1, and 0.5 ppm). Within 1 h of termination of
    exposure to the pollutants, the mice were infected with  Streptococcus
     pyogenes.  Excess mortality rates due to exposure to the mixture of
    the two gases were approximately equivalent to the sum of those
    induced by the inhalation of each individual pollutant. In mice
    exposed repeatedly for 3 h per day, for 20 days, to a mixture of
    nitrogen dioxide and ozone at concentrations of 3800 g/ma (2.0 ppm)
    and 100 g/m3 (0.05 ppm), respectively, and challenged with
     Streptococcus aerosol, the number of deaths was significantly higher
    than in the control group. On the other hand, repeated daily exposure
    to either nitrogen dioxide or ozone at the above-mentioned
    concentrations did not show any major effect on the mortality rate.
    The authors considered that this result might suggest a synergistic
    action of the two pollutants making them more effective in reducing
    resistance to respiratory infection.

    5.2.7  Interaction of ozone with bronchoactive and other chemicals

        The effects of pre-exposure to ozone on the sensitivity of
    guineapigs to inhaled acetylcholine were studied by Matsumura et al.
    (1972). Animals pre-exposed to an ozone concentration of 4000 g/m3
    (2 ppm) for 30 min manifested severe difficulty in breathing and many
    died. This effect persisted for as long as 2 h after exposure. In a
    similar study, Matsumura (1970a) sensitized guineapigs to albumen and
    found that repeated 30 min pre-exposures to an ozone concentration of
    10 mg/m3 (5 ppm) enhanced the sensitization. An ozone concentration
    of 2000 g/m3 (1 ppm) had no such effect.

        Ozone-exposed guineapigs were also more susceptible to the toxic
    action of injected histamine (Easton & Murphy, 1967). A 2-h exposure
    to ozone at 10 mg/m3 (5 ppm) caused severe lung function changes and
    increased mortality. This increased susceptibility to histamine was
    detectable for as long as 12 h after the end of the exposure.

        In order to study the effects of ozone on susceptibility to
    serotonin, Suzuki & Nagaoka (1973) injected the compound into the
    abdominal cavity of rats after exposing the animals to ozone at
    concentrations ranging from 2000 to 12 000 g/m3 (1-6 ppm) for 3 h.
    Mortality increased with increasing levels of ozone, but fatalities
    did not occur in a control group injected with serotonin and breathing
    filtered air.

        A report by Goldstein et al. (1970) that a deficiency of vitamin E
    increased the toxicity of ozone in the rat has been supported by the
    studies of Roehm et al. (1971b, 1972) and Menzel et al. (1972). These
    investigators observed a significantly shorter 50% lethal time and
    more pronounced signs of respiratory distress in ozone-exposed rats
    (2000 g/m3 (1.0 ppm) for 9 days continuously) that were fed vitamin
    E-depleted diets in comparison with those fed vitamin E-supplemented
    diets. When the ozone concentration was reduced to 1000 g/m3
    (0.5 ppm), pulmonary oedema became evident and mortality rates
    increased in animals fed vitamin E-deficient diets after 6 weeks of
    exposure. In rats continuously exposed to toxic levels of ozone
    ranging from 1400 to 1600 g/m3 (0.7-0.8 ppm), protection by dietary
    vitamin E against lung lipid peroxidation was proportional to the
    logarithm of the concentration of the vitamin in the diet (Fletcher &
    Tappel, 1973). In further studies by Mustafa (1975), rats were fed a
    basal diet containing vitamin E at either 66 mg/kg or 11 mg/kg for 5
    weeks, and then exposed continuously to an ozone concentration of
    200 g/m3 (0.1 ppm), for 7 days. Oxygen consumption was measured in
    lung homogenate using succinate as a substrate. Animals receiving the
    higher concentration of vitamin E (66 mg/kg) were relatively
    insensitive to this level of ozone, i.e., there was no significant
    increase in oxygen consumption, but those receiving the lower
    concentration of vitamin E (11 mg/kg) showed a significant increase in
    oxygen consumption.

    5.3  Systemic Reactions and other Effects

        The number of studies on the effects of ozone on a wide range of
    biological phenomena including growth, reproduction, and behaviour is
    increasing but many of these studies need further confirmation.
    Furthermore, even when a correlation is found between ozone exposure
    and effects, it is difficult to decide whether the effects are due to
    the direct oxidizing action of ozone or are secondary reactions to
    pulmonary injury caused by ozone.

    5.3.1  Effects on growth

        There does not seem to be any convincing study which shows that
    ozone, at the concentrations found in ambient air, has any detrimental
    effect on body growth. Nevskaja & Kocetkova (1961) who exposed rats to
    a mixture of ozone at 800 g/m3 (0.4 ppm) and sulfuric acid at
    7000 g/m3 for 5 h per day, 6 days per week, for 100 days, and Loosli
    et al. (1972) who exposed mice to synthetic photochemical air
    pollution containing ozone at 600-840 g/m3 (0.30-0.42 ppm) reported
    that exposed animals weighed less than the controls. However, Emik et
    al. (1971) did not find any significant differences between the growth
    of guineapigs breathing ambient air containing oxidants at a mean
    concentration of 110 g/m3 (0.057 ppm) for over two years, and that
    of the controls breathing filtered air.

    5.3.2  Haematological effects

        It is still questionable whether the haematological effects noted
    in ozone exposure are due to the direct action of ozone on the
    cellular and acellular components of the blood as it passes through
    the lung capillaries or whether they are caused by oxidizing
    intermediates, such as ozonides or peroxides, that might penetrate the
    alveolar basement membrane and enter the pulmonary circulation. A
    third possibility would be that these effects are secondary reactions
    induced by ozone perhaps causing the release of some mediating
    substance, yet to be identified.

    5.3.2.1  Short-term exposure (24 h or less)

        Short-term exposure to high concentrations of ozone ranging from
    6000 to 16 000 g/m3 (3.0-8.0 ppm) has been reported to cause
    increases in neutrophil-lymphocyte ratios in rats (Bobb & Fairchild,
    1967), and in the number of erythrocytes and leukocyte indices in mice
    (Kusumoto et al., 1976). Within this range of concentrations,
    Goldstein et al. (1968) and Goldstein (1973) found a reduction in
    acetylcholinesterase (3.1.1.7) in the erythrocytes of mice and
    induction of hydrogen peroxide formation in the circulating
    erythrocytes of rats and mice. Veninga (1970, unpublished data)a
    reported doubling in the number of binucleated lymphocytes and
    increased levels of serum glutamic pyruvic transaminase (2.6.1.2) but
    no changes in blood catalase (1.11.1.6) in mice exposed for 2 h to an
    ozone concentration of 400 g/m3 (0.2 ppm).

        Increased resistance to haemolysis of erythrocytes was reported in
    mice exposed to an ozone concentration of 2000 g/m3 (1 ppm) for
    30 min (Mizoguchi et al., 1973). Menzel et al. (1975) presented
    evidence that fatty acid ozonides produced Heinz bodies in
    erythrocytes in mice exposed to ozone at 1700 g/m3 (0.85 ppm) for
    4 h. Further exposure for 3 days resulted in a decline in the number
    of Heinz body positive cells. It is of interest to note that these
    bodies were not produced with ozone in the absence of serum containing
    unsaturated lipids. The authors postulated that this indicated an
    oxidation of the erythrocyte membrane and suggested that fatty acid
    ozonides might be the toxic intermediaries. At lower concentrations,
    Brinkman et al. (1964) noted increased sphering of erythrocytes of
    mice, rabbits, and rats, after an  in vitro exposure to ozone at
    400 g/m3 (0.2 ppm) for 1-2 h.

                 

    a  Veninga, T. S. Ozone-induced alterations in murine blood and liver.
       Paper presented at the 2nd International Clean Air Congress, Washington,
       DC, 6-11 December 1970 (No. MB-15E).

        A 3-h exposure to irradiated automobile exhaust containing an
    oxidant concentration of 740-1200 g/m3 (0.37-0.58 ppm) was found to
    increase the number of leukocytes in the blood of mice and reduce the
    level of serum alkaline phosphatase (3.1.3.1) (Kusumoto et al., 1976).

    5.3.2.2  Prolonged and repeated exposures

        Increasing the length of exposure provided further evidence for
    the oxidizing effect of ozone on the blood of rabbits and rats. After
    continuous exposure for 8 days to ozone at 1600 g/m3 (0.8 ppm), the
    lysozyme activity in the plasma and soluble fraction of lung of rats
    significantly increased (Chow et al., 1974). Continuous exposure of
    rats to an ozone concentration of 110 g/m3 (0.06 ppm) for 93 days
    resulted in a decrease in blood cholinesterase (3.1.1.8) activity
    which returned to normal 12 days after exposure ceased (Eglite, 1968).
    More recently, Jegier & P'an (1973) and P'an & Jegier (1972) reported
    a rise in serum trypsin protein esterase in rabbits exposed to
    800 g/m3 (0.4 ppm) for 6 h per day, 5 days per week, for 10 months.

        Long-term, combined exposure to ozone and carbon monoxide produced
    a reduced level of serum glutamic oxaloacetic transaminase (2.6.1.1)
    in rabbits. The animals were exposed for 1000 days to urban air
    containing oxidants and carbon monoxide at mean concentrations over 2
    years of 110 g/m3 (0.057 ppm) and 2000 g/m3 (1.7 ppm)
    respectively, (Emik et al., 1971).

    5.3.3  Effects on reproduction

        A few studies have been reported concerning the possible effects
    of ozone and photochemical oxidants on reproduction. The data indicate
    that the newborn animals may be more susceptible to exposure to
    oxidants than the parents.

        Brinkman et al. (1964) and Veninga (1967) exposed pregnant mice
    for 7 h per day, 5 days per week, for 3 weeks to ozone concentrations
    of 200-400 g/m3 (0.1-0.2 ppm). They found a 4-fold increase in
    neonatal mortality. The second author also observed increases in the
    incidence of both incisor growth and blepharophimosis in the new born.

        Similar studies using irradiated automobile exhaust were performed
    by Hueter et al. (1966) who found that mice exposed for 13 months
    before, during, and after gestation showed a marked decrease in the
    number and frequency of litters, the survival of infants, and the
    total number of pups born. Nonirradiated automobile exhaust did not
    produce any significant effects. The oxidant concentrations in the
    irradiated exposure chamber ranged from 400 to 2000 g/m3 (0.2 to
    1.0 ppm). In a follow-up study, Lewis et al. (1967) found that the
    number of female mice that did not become pregnant after mating with
    pretreated males was twice that found in mice mated with untreated
    controls.

    5.3.4  Behavioural and related changes

    5.3.4.1  Short-term exposure (24 h or less)

        Behavioural changes in response to acute ozone exposures have not
    been studied very extensively. The major effect that has been noted is
    a reduction in spontaneous activity. Murphy et al. (1964a) and
    Konigsberg & Bachman (1970) found significant losses in motor activity
    in mice exposed to 400 g/m3 (0.2 ppm) for 6 h and in rats exposed to
    1000 g/m3 (0.5 ppm) for 45 min, respectively.

        Using an evoked response technique, Xintaras et al. (1966)
    measured a reduction in the amplitude of response to a flash in the
    specific visual cortex and in the superior colliculus in rats exposed
    to ozone at 1000-2000 g/m3 (0.5-1.0 ppm) for 1 h. In studies on
    rhesus monkeys, Reynolds & Chaffee (1970) reported increases in both
    simple and choice reaction times after a 30-min exposure to an ozone
    concentration of 1000 g/m3 (0.5 ppm).

        Gardner et al. (1974) showed that the pentobarbital-induced
    sleeping time of mice significantly increased after 2 and 3 exposures
    each of 3 h, to an ozone concentration of 2000 g/m3 (1 ppm). After
    the third exposure, there was no change in the sleeping time compared
    with the controls unless in subsequent exposure the ozone
    concentration was raised to 10 mg/m3 (5 ppm), when the sleeping time
    again differed significantly. The authors suggested that ozone might
    be deactivating a liver microsomal enzyme that was responsible for the
    detoxification of the drug.

    5.3.4.2  Prolonged and repeated exposures

        Additional studies on the effects of ozone on the behaviour of
    animals have been conducted with longer exposures and complex mixtures
    of air pollutants. It appears that the reduction of voluntary running
    time may be an extremely sensitive indicator of ozone toxicity. The
    mechanism of this change remains obscure.

        Continuous exposure for 1 week to an ozone concentration of
    2000 g/m3 (1.0 ppm) caused an 84% reduction in voluntary activity in
    rats (Fletcher & Tappel, 1973). Decreased running activity has also
    been observed in mice exposed continuously to urban air with elevated
    oxidant levels and to irradiated motor vehicle exhaust (Emik et al.,
    1971; Heuter et al., 1966).

        According to Litt et al. (1968), ozone-exposed rats required a
    longer time to learn a specific task; learning was unstable and they
    exhibited a reduced ability for temporal discrimination. The animals
    were exposed intermittently for 2 months to ozone concentrations of
    600-1000 g/m3 (0.3-0.5 ppm). Nevskaja & Kocetkova (1961) exposed

    rats to a mixture of ozone at 800 g/m3 (0.4 ppm) and sulfuric acid
    at 7000 g/m3 for 5 h per day, 6 days per week, for 100 days, and
    found that a conditioned reflex was retarded.

    5.3.5  Miscellaneous systemic reactions to lung damage

        Other changes have been reported which indicate biochemical,
    physiological, and structural effects at sites distant from the lung
    that are possibly related to the inhalation of ozone. In most of these
    studies, the levels of ozone employed greatly exceeded those in the
    ambient air. After exposing rabbits to an ozone concentration of
    1500 g/m3 (0.75 ppm) for 4-8 h, Atwal & Wilson (1974) found
    histological and ultrastructural changes in the parathyroid glands.
    They reported hyperplasia of the chief cells, large numbers of
    secretion granules, proliferation and hypertrophy of the rough
    endoplasmic reticulum, Golgi complex, mitochondria, lipid bodies, and
    free ribosomes. These effects were seen up to 66 h after exposure.
    Atwal et al. (1975) expanded their short-term exposure study by
    exposing rabbits to 1500 g/m3 (0.75 ppm) for 48 h and found
    morphological changes in the parathyroid gland similar to those that
    they had reported for 4-8 h of exposure. The data suggested that ozone
    might trigger off an immune reaction which caused inflammatory injury
    to the parathyroid gland.

        It was demonstrated by Brinkman et al. (1964) that exposure to an
    ozone concentration of 400 g/m3 (0.2 ppm), for 5 h per day, for 3
    weeks, resulted in the rupture of nuclear envelopes and the extrusion
    of the contents of myocardial muscle fibres in rabbits and mice. This
    effect became reversible one month after the exposure.

        There are some data which suggest that ozone might accelerate the
    aging process. Stokinger (1965) reported premature aging in rabbits
    after 1 year of weekly 1-h exposures to ozone. The major changes found
    in the exposed animals were premature calcification of the
    sternocostal cartilage, severe depletion of body fat, dull cornea,
    sagging conjunctivae, and a general appearance of not thriving.

    5.4  Mutagenicity

        Chromosome aberrations (anaphase bridges) were found in 42% of
    root meristem cells of  Vicia faba exposed to an ozone concentration
    of 8000 mg/m3 (4000 ppm) for 1 h, but none were found in cells
    exposed to clean air (Fetner, 1958). When chick embryonic fibroblasts
    were exposed to ozone concentrations of 5000-10 000 mg/m3
    (2500-5000 ppm) for 24 h, a small number of cells were found with
    chromosome bridges in anaphase and telophase and with nuclear
    fragments (Sachsenmaier et al., 1965). Using a much lower
    concentration, Pace et al. (1969) exposed strain L cells continuously
    to an ozone concentration of 8000 g/m3 (4 ppm) for 30 h, and found a
    significant reduction in cell survival compared with that in the

    control group. The authors considered that this was due to ozone
    interference with mitotic activity.

         In vivo exposure studies on mammals by Zelac et al. (1971a,
    1971b) are of special interest. Female Chinese hamsters were exposed
    to an ozone concentration of 400 g/m3 (0.2 ppm) for 5 h and the
    number of circulating blood lymphocytes with chromosomal breaks was
    measured immediately after exposure and 6, and 15.5 days later. A
    significant increase in chromosomal breaks compared with pre-exposure
    values was still observed 15 days after exposure.

        Although further confirmation is required, these data and the
    results of studies on chromosomal changes in human tissues (see
    section 6.1.1) seem to indicate that ozone might be a mutagenic agent.

    5.5  Summary Table

        Table 10 is a summary of experimental animal studies that provide
    quantitative information useful for the establishment of guidelines
    for the protection of public health with respect to exposure to ozone
    at concentrations of up to 2000 g/m3 (1.0 ppm).



    
    Table 10.  Experimental animal studies
    Local effects on the respiratory system

     1.  Morphological changes
                                                                                                                                

        Ozone       Length of exposure
    concentration                                                                                      Number
                                                                                                         of
    g/m3   (ppm)   number     h/day    Effects                                    Responsea  Species  animals   Reference
                    of days
                                                                                                                                

    1800    (0.88)  180        24       Epithelial injury seen as early as 4 h     n.a.b      rat      n.a.      Freeman et al.
                                        after the beginning of exposure; after                                   (1974)
                                        3 weeks half of animals died and
                                        emphysema-like lesions observed.

    1600    (0.8)   7          24       Walls and interalveolar septa of terminal  --         rat      8 (8)     Castleman et al.
                                        airways thickened and infiltrated by                                     (1973a)
                                        mononuclear cells.

    1200    (0.6)   1          7        Swelling of epithelial alveolar lining     --         mouse    32 (13)   Bile (1970)
                                        cells & endothelium cells with
                                        occasional breaks in basement
                                        membrane.

    1100    (0.54)  180        24       Progressive changes in the airway          --         rat      n.a.      Freeman et al.
                                        epithelium after 6 days.                                                 (1974)

    1000    (0.5)   1          6        Immediately after the exposure the         --         mouse    16(12)    Evans et al.
                                        number of alveolar cells significantly                                   (1971)
                                        decreased.

    800     (0.4)   5 per      6        Emphysematous & vascular-type              --         rabbit   6 (6)     P'an et al.
                    week x              lesions.                                                                 (1972)
                    10 month
                                                                                                                                

    a  Number of animals showing effects/total number of animals; numbers in brackets refer to control groups.
    b  Not available.

    Table 10.  Experimental animal studies
    Local effects on the respiratory system

     1.  Morphological changes cont'd.
                                                                                                                                

        Ozone       Length of exposure
    concentration                                                                                      Number
                                                                                                         of
    g/m3   (ppm)   number     h/day    Effects                                    Responsea  Species  animals   Reference
                    of days
                                                                                                                                

    520-    (0.26   1          4.7-6.6  Dose-related loss of ciliated epithelium.  --         cat      14 (3)    Boatman et al.
    2000    -1.0)                                                                                                (1974)

    400     (0.2)   1          2        Degenerative changes in type I cells.      --         rat      n.a.      Stephens et al.
                                                                                                                 (1974)
                                                                                                                                

                                                        2.  Functional changes
                                                                                                                                

    1400    (0.68)  1          2        No significant increase in flow            --         guinea-  10 (10)   Murphy et al.
                                        resistance.                                           pig                (1964a)

    1000    (0.5)   1          2        Increase in airways resistance and         --         guinea-  10 (10)   Yokoyame
                                        breathing frequency with decrease in                  pig                (1972a)
                                        tidal volume.

    520-    (0.26   1          4.6      Increased flow resistance.                 --         cat      10 (4)    Watanabe et al.
    1000    -0.5)                                                                                                (1973b)

    400     (0.2)   30         24       Reduction in lung elasticity; increase in  --         rat      44 (44)   Bartlett et al.
                                        lung volume and in alveolar                                              (1975)
                                        dimensions.
                                                                                                                                

    a  Number of animals showing effects/total number of animals; numbers in brackets refer to control groups.
    b  Not available.

    Table 10.  Experimental animal studies
    Local effects on the respiratory system

     3.  Biochemical changes
                                                                                                                                

        Ozone       Length of exposure
    concentration                                                                                      Number
                                                                                                         of
    g/m3   (ppm)   number     h/day    Effects                                    Responsea  Species  animals   Reference
                    of days
                                                                                                                                

    1500    (0.75)  1          3        Reduction in activity of benzopyrene       --         hamster  25 (95)   Palmer et al.
                                        hydroxylase (1.14.14.2).                                 &     8 (15)    (1971. 1972)
                                                                                              rabbit

    1400-   (0.7    7          24       Increased acid phosphatase activity.       --         rat      14 (12)   Castleman et al.
    1600    -0.8)                                                                                                (1973b)

    1400    (0.7)   5          24       Indication of lipid peroxidation;          --         rat      33 (20)   Chow & Tappel
                                        increase in lysosomal hydrolase                                          (1972); Dillard
                                        activity.                                                                et al. (1972)

    1000    (0.5)   1          6        Increased albumen recovery from            --         rat      10 (18)   Alpert et al.
                                        alveolar spaces.                                                         (1971a)

    800-    (0.4    1          4        Evidence of formation of lipid peroxides   n.a.b      mouse    n.a.      Goldstein et al.
    1400    -0.7)                       in the lung.                                                             (1969)

    500     (0.25)  1          3        Reduced activity of several lysosomal      --         rabbit   6 (6)     Hurst et al.
                                        hydrolases.                                                              (1970)
                     
    400     (0.2)   7          24       Increase in pulmonary mitochondrial        --         rat      5-8       Mustafa et al.
                                        oxygen consumption.                                            (5-8)     (1973)
                                                                                                                                

    a  Number of animals showing effects/total number of animals; numbers in brackets refer to control groups.
    b  Not available.

    Table 10.  Experimental animal studies
    Local effects on the respiratory system

     4.  Effects on the host defence system
                                                                                                                                

        Ozone       Length of exposure
    concentration                                                                                      Number
                                                                                                         of
    g/m3   (ppm)   number     h/day    Effects                                    Responsea  Species  animals   Reference
                    of days
                                                                                                                                

    1200-   (0.62   1          17       Inhibition of pulmonary bactericidal       --         mouse    20 (29)   Goldstein et al.
    1600    -0.80)                      activity.                                                                (1971b)

    1000    (0.5)   210        16       No effect on physical clearance of         --         rabbit   8 (8)     Friberg et al.
                                        inhaled particles or on the number of                                    (1972)
                                        macrophages.

    1000    (0.5)   60         16       Decrease in the clearance of viable        --         guinea-  18 (18)   Friberg et al.
                                        Escherichia coli.                                     pig                (1972)

    800     (0.4)   1          4        Inhibition of bactericidal activity; no    --         mouse    38 (37)   Goldstein et al.
                                        additive role of the induced silicosis.                                  (1972)

    600     (0.3)   1          3        Impairment of phagocytic properties of     --         rabbit   n.a.b     Coffin et al.
                                        pulmonary alveolar macrophages.                                          (1968b)

    500     (0.25)  1          3        Diminished enzyme activities of alveolar   --         rabbit   6 (6)     Hurst et al.
                                                                                                                 (1970)
                                        macrophages.

    160     (0.08)  1          3        Increased susceptibility to                15/40      mouse    40 (40)   Coffin et al.
                                        Streptococcus.                             (6/40)                        (1968a)
                                                                                                                                

    a  Number of animals showing effects/total number of animals; numbers in brackets refer to control groups.
    b  Not available.

    Table 10.  Experimental animal studies--continued
    II.  Systemic reactions and other effects

     1.  Haematological effects
                                                                                                                                

        Ozone       Length of exposure
    concentration                                                                                      Number
                                                                                                         of
    g/m3   (ppm)   number     h/day    Effects                                    Responsea  Species  animals   Reference
                    of days
                                                                                                                                

    1700    (0.85)  1          4        Formation of Heinz bodies in red cells.    n.a.b      mouse    n.a.      Menzel et al.
                                                                                                                 (1975)
    1600    (0.8)   8          24       Increase of lysozyme activity in plasma    --         rat      8 (8)     Chow et al.
                                        and soluble fraction of lung.                                            (1974)

    800     (0.4)   5 per      6        increase in serum trypsin protein          --         rabbit   6 (6)     P'an & Jegier
                    week x              esterase.                                                                (1972); Jegier
                    10 months                                                                                    & P'an (1973)

    600     (0.3)   1          1        Inhibition of acetylcholine esterase       --         ox (in   --        P'an & Jegier
                                        (3.1.1.7) activity.                                   vitro)             (1970)

    400     (0.2)   1          1-2      Increased sphering of red blood cells.     --         mouse,   --        Brinkman et al.
                                                                                              rabbit,            (1964)
                                                                                              rat (in
                                                                                              vitro)

    400     (0.2)   1          2        Doubling in number of binucleated          --         mouse    n.a.      Veninga (1970,
                                        lymphocytes.                                                             unpublished)

    110     (0.06)  93         24       Decrease in blood choline esterase         --         rat      15 (15)   Eglite (1968)
                                        activity.
                                                                                                                                

    a  Number of animals showing effects/total number of animals; numbers in brackets refer to control groups.
    b  Not available.

    Table 10.  Experimental animal studies--continued
    II.  Systemic reactions and other effects

     2.  Effects on reproduction
                                                                                                                                

        Ozone       Length of exposure
    concentration                                                                                      Number
                                                                                                         of
    g/m3   (ppm)   number     h/day    Effects                                    Responsea  Species  animals   Reference
                    of days
                                                                                                                                

    400     (0.2)   5 per      7        Increase in neonatal mortality.            n.a.       mouse    n.a.      Veninga (1967)
                    week x
                    gestation
                    period +
                    1st 3
                    weeks of
                    life

    200-    (0.1    5 per      7        Increase in neonatal mortality.            n.a.       mouse    n.a.      Brinkman et al.
    400     -0.2)   week x                                                                                       (1964)
                    3 weeks

    1000-   (0.5    1          1        Decrease in the amplitude of evoked        --         rat      3 (3)     Xintaras et al.
    2000    -1.0)                       response to flash.                                                       (1966)
                                                                                                                                

    a  Number of animals showing effects/total number of animals; numbers in brackets refer to control groups.
    b  Not available.

    Table 10.  Experimental animal studies--continued
    II.  Systemic reactions and other effects

     3.  Behavioural changes
                                                                                                                                

        Ozone       Length of exposure
    concentration                                                                                      Number
                                                                                                         of
    g/m3   (ppm)   number     h/day    Effects                                    Responsea  Species  animals   Reference
                               of days
                                                                                                                                

    1000    (0.5)   1          0.5      increase in simple and choice reaction     --         rhesus   4 (4)     Reynolds &
                                        time.                                                 monkey             Chaffee (1970)

    1000    (0.5)   1          0.75     Significantly reduced motor activity.      --         rat      12 (12)   Konigsberg &
                                                                                                                 Bachman
                                                                                                                 (1970)
    600-    (0.3    60  intermittently  Increase in time to learn specific tasks.  --         rat      6 (6)     Litt et al.
    1000    -0.5)          (variable                                                                             (1966)
                          intervals)

    400     (0.2)   1          6        Reduction in spontaneous running           --         mouse    9 (9)     Murphy et al.
                                        activity.                                                                (1964a)
                                                                                                                                

     4. Miscellaneous extrapulmonary changes
                                                                                                                                

    1500    (0.75)  1-2        4-8, 24  Histological changes in parathyroid        --         rabbit   16 (16)   Atwal & Wilson
                                        gland.                                                                   (1974); Atwal
                                                                                                                 et al. (1975)
    400     (0.2)   21         5        Structural changes in myocardial           --         rabbit   n.a.      Brinkman et al.
                                        muscle fibres.                                        & mouse            (1964)

    400     (0.2)   1          5        Increase in chromosomal breaks in          --         hamster  8 (4)     Zelac et al.
                                        circulating lymphocytes.                                                 (1971a,b)
                                                                                                                                

    a  Number of animals showing effects/total number of animals; numbers in brackets refer to control groups.
    b  Not available.
    

    6.  EFFECTS ON MAN

    6.1  Controlled Exposures

        A considerable number of studies have been performed, under
    controlled conditions, on the effects of ozone on both healthy
    subjects and patients, with their consent. Some of these studies have
    provided useful information for the evaluation of exposure-effect
    relationships. A few  in vitro studies using human tissues have also
    helped to clarify the mechanisms of the biological actions of ozone.
    There have been few human studies on other oxidants.

    6.1.1  In vitro effects on human tissues

        Brinkman et al. (1964) found that  in vitro exposure of human red
    blood cells to an ozone concentration of 0.5 mg/m3 (0.25 ppm) for
    30 min accelerated sphering of these cells by X-ray irradiation,
    compared with unexposed cells. Chromosome breakages in human cell
    (epidermoid carcinoma cell) cultures exposed to an ozone concentration
    of 16 mg/m3 (8 ppm) for 5 or 10 min were equivalent to those produced
    by X-rays (200R, 250 kV) (Fetner, 1962).

        The production of interferon was suppressed when human tonsil
    lymphocytes were exposed  in vitro to an ozone concentration of
    10 mg/m3 (5 ppm) for 3 h (Watanabe et al., 1973a).

        Goldstein (1976) measured the combined effects of ozone at
    4-82 mg/m3 (2-41 ppm) and nitrogen dioxide at 6.8-190 mg/m3
    (3.6-102 ppm) on human red cells,  in vitro, and found that the
    absolute and relative concentrations of the pollutants as well as the
    sequence of administration could affect the interaction. In general,
    the effects of the two pollutants on the variables measured (osmotic
    fragility, acetylcholin-esterase (3.1.1.7) activity, lipid
    peroxidation, reduced glutathione, and methaemoglobin levels) were
    additive. At lower pollutant doses, a synergistic effect on the
    increase in lipid peroxides was reported. Ozone also potentiated the
    formation of methaemoglobin due to the action of nitrogen dioxide.

    6.1.2  Sensory effects

        The effects of ozone or oxidants on sensory organs have been
    studied in terms of eye irritation, changes in visual parameters, and
    olfactory thresholds.

        Eye irritation in two groups of 20 female telephone company
    employees working in identical adjacent rooms was evaluated by
    Richardson & Middleton (1957, 1958) in relation to oxidant
    concentrations from May to November 1956. Activated-carbon and dummy
    air-filter media were switched periodically between the two rooms so
    that the groups were alternately exposed to test and control

    conditions. In all cases, differences in eye irritation between the
    activated-carbon filtered and nonfiltered test conditions were highly
    significant (p < 0.01). The scatter diagram (Fig. 8) suggests the
    existence of a threshold for eye irritation at an oxidant
    concentration of approximately 200 g/m3 (0.10 ppm).

    FIGURE 09

        The index of eye irritation increased progressively as oxidant
    concentrations exceeded this value. No significant correlations
    between eye irritation and concentrations of nitrogen dioxide or
    suspended particulates were observed. However, in interpreting these
    results, care must be exercised in drawing conclusions regarding
    cause-effect relationships, as other controlled exposure studies have
    shown that ozone is not an eye irritant. By comparison,
    peroxyacylnitrates, acrolein, and peroxybenzoyl-nitrates have all been
    shown to be strong eye irritants (Heuss & Glasson, 1968; Schuck &
    Doyle, 1959; Stephens et al., 1961). Each of these compounds is a
    product of the photochemical reaction system and thus is highly
    correlated in time with measured levels of oxidants or ozone.

        Lagerwerff (1963) measured the effect of exposure to ozone on
    visual parameters in 22 male and 6 female volunteers. The subjects
    were exposed to ozone at 400, 700, and 1000 g/m3 (0.20, 0.35, and
    0.50 ppm) for 3 h, and again for 6 h, with 10 days rest between
    exposures. Visual acuity, depth perception, lateral and vertical
    phoria, divergence and convergence, near vision, and peripheral,
    colour, and night vision were tested. Considerable decreases in visual
    acuity in scotopic and mesopic ranges, increases in peripheral vision,
    changes in extra ocular muscle balance, and decrease in night vision
    were observed in the majority of subjects. A few subjects also
    developed a nonproductive cough at the highest ozone concentration,
    and several complained of difficulties with mental concentration at
    these levels.

        Studies of the olfactory threshold in 10-14 male volunteer test
    subjects exposed for 30 min to a series of different ozone
    concentrations were performed by Henschler et al. (1960). The lowest
    ozone concentration used, 40 g/m3 (0.02 ppm), was recognized
    immediately by 9 out of 10 subjects. The subjects reported that the
    odour diminished rapidly, and that it was no longer perceptible within
    -12 min. When exposed to an ozone concentration of 100 g/m3
    (0.05 ppm), 13 out of 14 subjects indicated that the odour was
    considerably stronger and that it lasted longer (2-30 min, with an
    average duration of 13 min). In a study by Eglite (1968), it was found
    that the threshold of smell for ozone was 15 g/m3 (0.008 ppm) for
    the most sensitive subject in a group of 20 persons.

    6.1.3  Effects on respiratory function

    6.1.3.1  Exposure to ozone

        A considerable number of controlled studies on exposure to ozone
    have been reported. However, with one exception, all these studies
    were concerned with short-term exposures of less than 6 h.

        A highly significant fall in steady state diffusion capacity and
    0.75-second forced expiratory volume (FEVo.75) was noted in each of 16
    test replicates on 10 men and 1 woman aged 20-45 years, exposed to
    ozone concentrations of 1200-1600 g/m3 (0.6 to 0.8 ppm) for 2 h
    (Young et al., 1964).

        Goldsmith & Nadel (1969) exposed 4 healthy males, for 1 h, to
    ozone concentrations of 200, 800, 1200, and 2000 g/m3 (0.1, 0.4,
    0.6, and 1.0 ppm). Consistent increases in airway resistance (Raw)
    were demonstrated only with exposure to 2000 g/m3 (1 ppm). Lower
    concentrations caused an increase in Raw in some subjects, but a
    clear dose-effect relationship could not be established at levels
    below 2000 g/m3 (1 ppm). When exposed to an ozone concentration of
    1500 g/m3 (0.75 ppm) for 2 h, 10 healthy males exhibited significant
    increases in Raw, reductions in maximum static elastic recoil
    pressure of the lung, and a fall in maximum flow at 50% of vital
    capacity (Bates et al., 1972). Exercise on a bicycle ergometer during
    ozone exposure accentuated these pulmonary function changes, and most
    subjects complained of substernal soreness and cough at the end of a
    2-h period. These observations were extended by Hazucha et al. (1973)
    to include an ozone concentration of 740 g/m3 (0.37 ppm) for 2 h and
    subjects were intermittently exercised during exposure. After exposure
    to 740 and 1500 g/m3 (0.37 and 0.75 ppm) for 2 h, both smokers and
    non-smokers (6 subjects per group) revealed significant decreases in
    forced vital capacity (FVC), one-second forced expiratory volume
    (FEV1.0), mid-maximal expiratory flow rate (MMFR), and maximum
    expiratory flow rate at 50% of vital capacity (MEFR 50%), but the
    effects were greater at 1500 g/m3 (0.75 ppm). These effects were
    closely related to the changes measured in the closing volume and
    indicated an early effect in the small airways.

        Seven healthy males, exposed to an ozone concentration of
    1000 g/m3 (0.50 ppm) for 2 h while performing intermittent light
    exercise, showed decreases in lung function measurements and oxidative
    changes in erythrocytes; 3 subjects had symptoms such as cough,
    substernal discomfort, and malaise. When 2 healthy and 3 sensitive
    subjects (those with a prestudy history of cough, chest discomfort, or
    wheezing associated with allergy or air pollution exposure, but with
    normal base line pulmonary function studies) were exposed to an ozone
    concentration of 740 g/m3 (0.37 ppm) for 2 h under the same
    conditions of exercise, there was a significant increase in the total
    respiratory resistance compared with the pre-exposure resistance.
    Oxidative biochemical changes were detected in the blood of this
    group, but were not as severe as in those exposed to an ozone
    concentration of 1000 g/m3 (0.5 ppm). Three healthy and 3 sensitive
    subjects exposed to 500 g/m3 (0.25 ppm) for 2 h did not show any
    consistent physiological changes attributable to exposure (Hackney et
    al., 1975). All the subjects of the above study were southern
    Californians. In order to study possible adaptation of these subjects
    to long-term ambient ozone exposure, the same investigators compared

    the effects of ozone exposure on southern Californians with those on
    Canadians and found that the latter were more responsive (Hackney et
    al., 1977).

        Ohmori (1974) found increased breathing frequency and volume in 4
    healthy males who were exposed under exercise to ozone concentrations
    of 200-500 g/m3 (0.1-0.25 ppm) for 30 min. Four normal male subjects
    exposed for 5 min to a combination of ozone at 1800 g/m3 (0.9 ppm)
    and light exercise showed a highly significant decrease in specific
    airway conductance (Kagawa & Toyama, 1975a). Folinsbee and co-workers
    (1975) tested the reaction of 28 normal adults (20 males, 8 females)
    exposed to ozone at concentrations of 740, 1000, or 1500 g/m3 (0.37,
    0.50, or 0.75 ppm) for 2h: the subjects were either at rest or
    exercising intermittently with sufficient intensity to increase lung
    ventilation by a factor of 2.5. There was a gradation of complaints
    depending on the ozone concentration. Under exercise, increase in
    respiratory frequency was closely correlated with the total dose of
    ozone and, at any given concentration, the effect was greater with the
    subject who had exercised. Statistically significant decreases were
    found in FVC at ozone concentrations of 1000 g/m3 (0.50 ppm) or
    more. Decrease in MEFR occurred in 50% of subjects at concentrations
    of 740 g/m3 (0.37 ppm) or more.

        The effects of ozone on pulmonary function were studied in 22
    young, healthy, male, nonsmokers who were exposed to 800 g/m3
    (0.4 ppm) for 2 and 4 h. Subjects were seated during the exposure
    except for 2 exercise periods of 15 min (bicycle), beginning after 1
    and 3 h of exposure, respectively. Significant changes in FVC, MMFR
    and Raw occurred after 2 h of exposure. Borderline effects were
    reported for FEV1.0, V50 (expiratory flow rate at 50% FVC) and V25
    (expiratory flow at 25% FVC). After 4 h of exposure, changes in all of
    these lung function variables became statistically significant (Rummo
    et al., 1975, unpublished data).a In studies by Kerr et al. (1975),
    20 lightly exercising subjects (19 males and 1 female) were exposed to
    an ozone concentration of 1000 g/m3 (0.5 ppm) for 6 h. Subjects,
    particularly nonsmokers, commonly reported dry cough and chest
    discomfort. Significant changes were produced in airway conductance,
    pulmonary resistance, and forced expiratory volumes, but not in
    diffusing capacity.

                 

    a  Rummo, N.J., Knelson, I. H., Lassiter, S., Cram, J. J., & House,
       D. (1975). Effects of ozone on pulmonary function in healthy young
       men. Research Triangle Park, NC, US Environmental Protection Agency,
       17 pp. (In-house Technical Report).

        Changes in the pulmonary function of 11 male subjects, aged 24-38
    years, exposed to an ozone concentration of 200 g/m3 (0.1 ppm) for
    2 h, were compared with a 1-h pre- and post-control period without
    ozone and with a control series without ozone. Under test conditions,
    which included intermittent light exercise, a significant increase in
    Raw and in the alveolar to arterial oxygen pressure difference
    (AaDO2) was observed in 7 out of 11 subjects (yon Nieding et al.,
    1977).

        In an attempt to study the effects of long-term repeated exposures
    to ozone, Bennet (1962) exposed 2 groups of 6 healthy males to
    400 g/m3 (0.2 ppm) or 1000 g/m3 (0.5 ppm) respectively, for 3 h
    per day, 6 days per week for 12 weeks. The 400 g/m3 (0.2 ppm)
    exposure group did not experience any symptoms or changes in the
    forced expiratory volume. While the second group was also
    asymptomatic, the FEV1.0 showed a significant decrease during the last
    few weeks of exposure.

    6.1.3.2  Exposure to mixtures of ozone and other air pollutants

        Bates & Hazucha (1973) demonstrated a synergistic action of ozone
    at 740 g/m3 (0.37) and sulfur dioxide at 960 g/m3 (0.37 ppm). With
    4 normal subjects engaged intermittently in light exercise, a 2-h
    exposure to sulfur dioxide, only, at 960 g/m3 (0.37 ppm) did not
    produce any changes in FVC, FEV1.0, MMFR, and MEFR 50%, while exposure
    to an ozone concentration of 740 g/m3 (0.37 ppm) caused a 10-15%
    reduction in these lung function variables. However, exposure to a
    combination of these gases at the same concentrations resulted in a
    20-45% decline in lung function measurements and this effect was even
    greater than the changes caused by a 2-h exposure to an ozone
    concentration of 1500 g/m3 (0.75 ppm). On the other hand, addition
    of nitrogen dioxide at a concentration of 560 g/m3 (0.30 ppm) to
    ozone at 500 g/m3 (0.25 ppm) did not result in any decrement in the
    lung functions of 3 healthy and 3 sensitive (section 6.1.3.1) subjects
    (Hackney et al., 1975a).

        In a series of studies already mentioned in section 6.1.3.1, von
    Nieding et al. (1977) found that the effects of exposure to an ozone
    concentration of 200 g/m3 (0.1 ppm) were not enhanced by combination
    with nitrogen dioxide at 9400 g/m3 (5 ppm), or by combination with
    nitrogen dioxide at 9400 g/m3 (5 ppm) and sulfur dioxide at
    13 000 g/m3 (5 ppm), though recovery time was delayed in the last
    experiment. Exposure for 2 h to a combination of ozone at 50 g/m3
    (0.025 ppm), nitrogen dioxide at 100 g/m3 (0.05 ppm), and sulfur
    dioxide at 260 g/m3 (0.1 ppm) did not have any effect on Raw, or on
    AaDO2. However, there was a dose-dependent increase in the
    sensitivity to acetylcholine of the bronchial tree compared with the
    controls. This effect was observed by measuring the increase in Raw
    after inhalation of 2% acetylcholine alone and in combination with the
    previously mentioned mixture of gases at the same concentrations. The

    effect became more pronounced when the acetylcholine was combined with
    ozone at 200 g/m3 (0.1 ppm), nitrogen dioxide at 9400 g/m3
    (5 ppm), and sulfur dioxide at 13 000 g/m3 (5 ppm).

    6.1.3.3  Exposure to peroxyacetylnitrate alone or in combination with
             carbon monoxide

        Thirty-two male college students were exposed to a peroxyacetyl-
    nitrate concentration of 1500 g/m3 (0.3 ppm) for 5 min while
    exercising on a bicycle ergometer (Smith, 1965). A statistically
    significant increase in oxygen uptake was observed during exercise, in
    comparison with that observed when filtered air was breathed. MEFR was
    significantly decreased by exposure to this gas during the recovery
    phase following exercise. However, peroxyacetylnitrate did not have
    any effect when the subjects were at rest.

        The metabolic, temperature, and cardiorespiratory reactions of 20
    healthy males (10 smokers and 10 nonsmokers) were monitored while
    working to their maximum and breathing filtered air or 3 different gas
    mixtures at 25  0.5C and a relative humidity of 20  2% (Raven et
    al., 1974). The mixtures were carbon monoxide in filtered air at
    57 500 g/m3 (50 ppm), peroxyacetylnitrate in filtered air at
    1350 g/m3 (0.27 ppm), and a combination of the two. Maximum aerobic
    capacity did not decrease significantly in either group during
    exercise and exposure to any of the pollutant gas mixtures compared
    with filtered air. For both smokers and nonsmokers exposure to
    peroxyacetylnitrate and carbon monoxide alone or in combination, while
    exercising to maximum aerobic capacity, produced minor alterations in
    cardiorespiratory and temperature regulating variables.

    6.1.3.4  Exposure to irradiated automobile exhaust

        Fourteen college students were exposed on 2 occasions to
    irradiated automobile exhaust and measurements were made of reaction
    time, vital capacity, and submaximum work performance on the bicycle
    ergometer. The individuals were exposed to a mixture of the following
    gases: carbon monoxide, carbon dioxide, nitric oxide, nitrogen
    dioxide, hydrocarbons, aldehydes, formaldehyde, and oxidants (Table
    12, section IID). It appeared that exposure to this mixture of
    pollutants had little effect on the types of human motor performance
    chosen for this study (Holland et al., 1968).

    6.1.3.5  Exposure to ambient air containing elevated concentrations of
             oxidants

        The effect of oxidant air pollution on patients with chronic
    obstructive lung disease was studied by Motley et al. (1959). Twenty
    normal individuals and 46 patients with chronic lung disease were
    evaluated by measuring lung volumes, forced expiratory volumes, and
    nitrogen washout before and after removal from ambient Los Angeles air
    into a room with clean filtered air. No significant changes were

    detected in the pulmonary function of normal subjects or in patients
    with chronic lung disease who entered the room on nonsmoggy days.
    However an improvement in lung function, particularly a decrease in
    residual lung volume and nitrogen washout time, occurred among
    patients who entered the filtered room on smoggy days and remained for
    40 or more hours. Simultaneous measurements of oxidants were not
    obtained at the study site, but during the study, ambient hourly
    oxidant concentrations at nearby monitoring stations ranged from
    400-1400 g/m3 (0.2-0.7 ppm). Subjects with chronic lung disease, who
    remained in the filtered air for only 18-20 h, did not experience any
    significant improvement in lung function.

        In a similar study, Balchum (1973) observed the pulmonary function
    of 15 patients with moderately severe chronic obstructive lung
    disease, who spent one week in a room without air filtration and a
    second week in clean filtered air. During the first week, when
    unfiltered ambient air was drawn into the room, 1-h oxidant
    concentrations averaged 220 g/m3 (0.11 ppm) and ranged up to
    400 g/m3 (0.2 ppm). During the second week, when air filters were
    activated, oxidant exposures ranged from 40 to 60 g/m3 (0.02 to
    0.03 ppm). Comparison of the effects of unfiltered and filtered air
    revealed a decrease in airway resistance and an increase in the
    arterial pO2 during the week of filtered air breathing. Changes were
    observed both at rest and during exercise in about 75% of the
    subjects. Decreases in airway resistance first became apparent after
    breathing filtered air for 48 h.

    6.1.4  Changes in electroencephalograms

        Changes (decrease in the alpha rhythms) in electroencephalograms
    were studied by Eglite (1968) in relation to exposure to ozone for
    3 min. All 3 healthy subjects studied showed changes when exposed to
    an ozone concentration of 20 g/m3 (0.01 ppm) and 1 subject reacted
    at a concentration at 10 g/m3 (0.005 ppm).

    6.1.5  Chromosomal effects

        In a recent study by Metz et al (1975), 6 human volunteers were
    exposed to an ozone concentration of 1 mg/m3 (0.5 ppm) for 6-10 h,
    and the circulating lymphocytes were examined for chromosomal changes.
    Although no chromosome aberrations were found, there was a significant
    increase in the number of minor chromosomal abnormalities (chromatid
    deletions) compared with pre-exposed lymphocytes.

    6.2  Industrial Exposure

        Several studies on the effects of industrial exposure have been
    reported, but in most of them, the effects of ozone have been
    confounded by the coexistence of other pollutants and a threshold
    concentration has not been determined.

        Kleinfield et al. (1957) reported several cases of severe ozone
    intoxication in welders using an inert gas-shielded consumable
    electrode which greatly increased the ultraviolet radiation. Ozone was
    measured at the breathing zone in 3 plants using this welding
    technique. No complaints or clinical findings were associated with
    ozone concentrations of 500 g/m3 (0.25 ppm) or less. At
    concentrations of 600-1600 g/m3 (0.3-0.8 ppm), an increasing number
    of welders complained of chest constriction and irritation of the
    throat.

        Similar findings using this welding technique were reported by
    Challen et al. (1958), when 11 out of 14 workers complained of
    respiratory symptoms at ozone concentrations in the range of
    1600-3400 g/m3 (0.8-1.7 ppm). Symptoms disappeared when ozone levels
    were reduced to 400 g/m3 (0.2 ppm). Young et al. (1963) failed to
    detect any significant changes in lung function tests (vital capacity,
    functional residual capacity, MMFR, FEV0.75, or diffusing capacity) in
    7 men engaged in argon-shielded electric arc welding when ozone
    concentrations were 400-600 g/m3 (0.2-0.3 ppm).

        Kudrjavceva (1963) described a complex of symptoms that included
    headache, weakness, increased muscular excitability, and decreased
    memory among workers engaged in the manufacture of hydrogen peroxide
    and exposed to ozone concentrations ranging from 500-800 g/m3
    (0.25-0.40 ppm) for 7-10 years. The investigator suggested that these
    reactions might be due to prolonged exposure to ozone under working
    conditions. Increased prevalence of bronchitis and emphysema,
    accompanied by decreased expiratory flow rate, was also reported in
    workers involved in the manufacture of hydrogen peroxide by Nevskaja &
    Diterihs (1957). Ozone concentrations of 80-1000 g/m3
    (0.04-0.50 ppm) were measured; sulfuric acid aerosol was also present.
    Control groups were not included in either of these studies.

        In several countries, occupational exposure limits for ozone have
    been set as maximum, time-weighted averages for an 8-h workday or 40-h
    week (ILO, 1977). In some countries, the exposure limit is 100 g/m3
    (0.05 ppm), in others, 200 g/m3 (0.1 ppm). In addition, a short-term
    exposure limit of 600 g/m3 (0.3 ppm), for periods up to 15 min, has
    been tentatively proposed by the USA provided that there are not more
    than 4 such periods per day with at least 60 min between each period,
    and that the daily time-weighted average is not exceeded. The German

    Democratic Republic has set a limit of 200 g/m3 for exposure periods
    not exceeding 30 min and the USSR has proposed an 8-h mean value of
    100 g/m3 (0.05 ppm).

    6.3  Community Exposure

        Epidemiological studies on the association between human health
    effects and exposure to photochemical oxidants have largely been
    carried out in the Los Angeles air basin. For most of these studies,
    investigators made use of measurements of photochemical oxidants
    obtained from a network of air monitoring stations operated by the Los
    Angeles County Air Pollution Control District. Oxidants were measured
    by the unbuffered potassium iodide method which yields oxidant values
    15-25% lower than the values obtained with the 1 or 2% neutral-
    buffered potassium iodide method more commonly used in regions outside
    the Los Angeles air basin.

        As in all studies on urban populations, the observed health
    effects of photochemical oxidant exposure cannot be attributed only to
    oxidants. Photochemical smog typically consists of ozone, nitrogen
    dioxide, peroxyacylnitrates and other nitrate compounds, sulfates,
    other particulate aerosols, and reducing agents. In combination, these
    pollutants may have an independent, additive, or synergistic effect on
    human health. In general, however, ozone appears to be the most
    biologically active pollutant and the correlations between health
    effects and pollutant exposure were found when results concerning
    ozone, rather than other identified pollutants, were statistically
    analysed. Since controlled exposure studies in man and animals confirm
    the greater biological reactivity of ozone compared with other
    components of photochemical smog, it seems reasonable to conclude, for
    the purpose of developing health protection guidelines, that ozone is
    the principal agent responsible for the exposure-response
    relationships observed in epidemiological studies on photochemical
    oxidants.

    6.3.1  Mortality

        Studies conducted by the California State Department of Public
    Health (1955, 1956) over the periods July-November 1953 and 1954, and
    July-December 1955 revealed that, during these months, the daily
    mortality rate of Los Angeles residents, aged 65 or over, was strongly
    influenced by a heat wave but was not consistently altered either by
    variations in oxidant concentrations or by the occurrence of smog-
    alert days (ozone concentrations of 600 g/m3 (0.3 ppm) or more).

        No significant correlations were found by Massey et al. (1961,
    unpublished data) between daily mortality rates and daily oxidant
    levels in an analysis of two areas of Los Angeles County, selected for
    similarities in temperature and for differences in air pollution
    levels (values not given). Hechter & Goldsmith (1961) also failed to

    find a significant correlation between monthly mortality rates due to
    cardiorespiratory diseases and pollutant levels (monthly means of
    daily maxima that ranged from 80 to 400 g/m3; 0.04-0.2 ppm) in Los
    Angeles County for the years 1956-58. The authors made Fourier curve
    analyses of the data in order to remove the major effect of season of
    year on mortality rate and pollutant levels. An association between
    respiratory and cardiac deaths and Los Angeles smog episodes with 1-h
    concentrations exceeding 400 g/m3 (0.2 ppm) was observed by Mills
    (1957) but he failed to take into account seasonal fluctuations in
    deaths and pollutants. However, it is possible that the method of
    adjustment for seasonal effects might mask a real effect of pollutant
    concentrations on mortality. Therefore no conclusive statement can be
    made concerning the lack of an association between mortality rate and
    short-term variations in oxidant levels.

        A model of daytime wind flow over Los Angeles was constructed by
    Mahoney (1971) to divide the city into 5 wind zones each about
    10 kilometres in width and representing distance downwind along the
    path of air flow. The mortality rate of the white population, adjusted
    for age, sex, and income level, from noncancerous respiratory diseases
    during 1961 increased in successive downwind zones affected by the Los
    Angeles sea breeze. The adjusted mortality rate in the wind zone
    immediately adjacent to the Pacific Ocean was 53 per 100 000, while
    mortality in the afferent wind zone most remote from the ocean was
    111 per 100 000. The author suggested that this geographical
    difference in mortality might be consistent with an effect of
    temperature, humidity, or oxidant air pollution.

    6.3.2  Annoyance and irritation

        In Los Angeles, 75% of the population complained that they were
    "bothered" by air pollution either at home or at work compared with
    24% in San Francisco and 22% in the "rest of the State". Eye
    complaints, which were relatively high (33%) in Los Angeles County and
    in the "rest of the State", were low (14%) in the San Francisco Bay
    area. However, fewer cases of hayfever and sinus trouble were reported
    in Los Angeles County compared with other areas (Hausknecht, 1960).

        The association of oxidant levels with eye irritation was
    investigated (Renzetti & Gobran, 1957) in a panel of office and
    factory workers and a second panel of scientists residing in the Los
    Angeles Basin. The data demonstrated increasing eye irritation with
    increasing maximum instantaneous oxidant concentrations over a range
    of values from 100 to 900 g/m3 (0.05-0.45 ppm).

        Daily symptoms associated with eye and respiratory irritation were
    studied by Hammer et al. (1974) in a group of Los Angeles student
    nurses in relation to daily oxidant levels. Headache, eye discomfort,
    cough, and chest discomfort were all found to be related to daily
    maximum hourly oxidant concentrations. On days when maximum hourly
    concentrations were 800-1000 g/m3 (0.40-0.50 ppm), students reported

    48% more cough and 100% more chest discomfort compared with days when
    oxidant levels were below the US air quality standard of 160 g/m3
    (0.80 ppm). Using "hockey stick" functions,a the threshold levels
    have been determined as maximum 1-h concentrations of 100 g/m3
    (0.05 ppm) for headache, 300 g/m3 (0.15 ppm) for eye irritation,
    530 g/m3 (0.27 ppm) for cough, and 580 g/m3 (0.29 ppm) for chest
    discomfort. Exposure-response relationships are shown in Table 11.

    6.3.3  Athletic performance

        The effect of oxidant concentrations on athletic performance was
    studied in Los Angeles by Wayne et al. (1967) in 21 competitive team
    events for High school cross-country runners from 1959-64. Since
    running times tended to improve throughout the season, team
    performance at a meeting was evaluated by determining the percentage
    of individual athletes who failed to improve when their running time
    was compared with that of their previous performance on the same
    course. Oxidant levels in the hour before the meeting, which ranged
    from below 100 g/m3 (0.05 ppm) to 600 g/m3 (0.3 ppm), were highly
    correlated ( r=0.88) with decreased performance, as shown in Fig. 9.
    Consistently high correlations were obtained when the observations
    were divided into the periods 1959-1961 ( r=0.945) and 1962-64
    ( r=0.945). Correlations with other pollutants (carbon monoxide and
    particulates) and with meteorological variables (temperature, relative
    humidity, wind velocity, wind direction) were considerably lower and
    usually not significant. The authors speculated that the observed
    oxidant-athletic performance relationship could be due to a direct
    effect on oxygen use or to respiratory discomfort associated with
    exercise in an atmosphere containing a high concentration of oxidants.
    A statistical test for threshold values, based on segmental regression
    analysis ("hockey stick" functions) applied to these data (Barth et
    al., 1971; Hasselblad et al., 1976) gave a threshold estimate of
    240 g/m3 (0.12 ppm) (1-h value) with a 95% confidence interval of
    134 to 326 g/m3 (0.067-0.163 ppm).

                 

    a The term hockey stick function has been used by Hammer et al.
      (1974) and Hasselblad et al. (1976) to describe a function
      consisting of a curve with zero slope up to a certain point and
      increasing monotonically from that point.


    
    Table 11.  Relative increase in headache, eye discomfort, cough, and chest discomfort in relation to photochemical oxidant
               exposure of student nurses in Los Angelesa
                                                                                                                                     

                                                                Relative increase in symptom
                                                                                                                                     

    Daily maximum 1-h oxidant  Headache                   Eye discomfort          Cough                    Chest discomfort
    level                                                                                                                            
                            
    g/m3     (ppm)            Simple        Adjustedb    Simple      Adjustedb   Simple       Adjustedb   Simple      Adjustedb
                                                                                                                                     

    <160      (<0.08)          1.00 (16.4)c  1.00 (10.6)  1.00 (8.9)  1.00 (5.2)  1.00 (13.0)  1.00 (9.5)  1.00 (3.5)  1.00 (1.8)
    180       (0.09)           0.97          1.00         1.01        1.07        1.02         1.07        0.91        1.05
    200-280   (0.10-0.14)      0.95          1.03         0.96        1.13        0.91         0.99        0.97        1.00
    300-380   (0.15-0.19)      0.95          1.07         1.12        1.33        0.95         1.02        1.05        0.94
    400-480   (0.20-0.24)      0.95          1.08         1.37        1.77        0.89         0.96        0.89        0.89
    500-580   (0.25-0.29)      1.01          1.07         1.67        2.15        0.95         1.01        1.03        1.11
    600-780   (0.30-0.39)      1.03          1.26         2.37        3.42        1.16         1.23        1.17        1.27
    800-1000  (0.40-0.50)      1.02          1.41         3.93        6.11        1.48         1.77        2.00        3.22
                                                                                                                                     

    a  From: Hammer et al. (1974).
    b  Excludes all days when the symptom was reported in conjunction with either "feverish", "chilly", or "temperature".
    c  Bracketed figure gives baseline symptom rate as mean daily percentage of symptom reported.
    

    FIGURE 10

    6.3.4  Effects on children

        Ventilatory performance, measured twice a month for 11 months by
    the Wright Peak Flow Meter, in 78 elementary school children in two
    cities in the Los Angeles Basin was unaffected by differences in the
    daily mean oxidant levels which were 320 g/m3 (0.16 ppm) and
    220 g/m3 (0.11 ppm) respectively (McMillan et al., 1969).

        Kagawa & Toyama (1975b) and Kagawa et al. (1976) studied the
    weekly variations in lung function (airway conductance and ventilatory
    performance) of 20, normal, 11-year-old, school children in Tokyo in
    relation to variations in temperature and ambient concentrations of
    ozone, nitrogen dioxide, nitric oxide, sulfur dioxide, hydrocarbons,
    and particulate matter. Students were tested from June 1972 to October
    1973. Ozone levels were determined by the ethylene-chemiluminescence
    method. Temperature was the factor most highly correlated with
    variations in specific airway conductance (negative correlation) and
    maximum expiratory flow rate (Vmax) at 25 and 50% FVC (positive
    correlation). Significant negative correlations between ozone and
    specific airway conductance, and between nitrogen dioxide, nitric
    oxide, sulfur dioxide, and particulate matter and Vmax at 25 or 50%
    FVC were observed in some children. The ranges of hourly pollutant
    levels, at the time of the lung function test (13h00), that were used
    for correlation during the study period were: 0-560 g/m3
    (0-0.28 ppm) for ozone, 40-550 g/m3 (0.02-0.29 ppm) for nitrogen
    dioxide, 30-340 g/m3 (0.01-0.13 ppm) for sulfur dioxide, and
    50-490 g/m3 for particulate matter (Toyama et al., 1977).

        The effect of oxidant air pollution exposure on the incidence and
    duration of A2 influenza in an epidemic that occurred in 1968-69 was
    studied retrospectively by Pearlman et al. (1971) in 3500 elementary
    school children from 5 southern California communities. These areas
    were selected to represent an exposure gradient for photochemical
    oxidants, although no difference in community oxidant exposure was
    present at the time (December 1968-January 1969) of the epidemic.
    Information from school absenteeism, questionnaires on illness, and
    haemagglutination-inhibition titres did not reveal any statistically
    significant morbidity differences corresponding to the pollution
    gradient that existed during the season of peak oxidant levels.

        Several Japanese investigators have reported acute reactions among
    school children exposed to moderately elevated concentrations of
    oxidants of 200-400 g/m3 (0.10-0.20 ppm), on smoggy days in a number
    of urban areas. Not only eye and respiratory irritation, but systemic
    symptoms such as paraesthesia, prostration, and convulsions were
    observed. In reviewing these studies the WHO Task Group was aided by
    several investigators and observers from Japan. In the opinion of the
    Task Group, eye and respiratory effects observed during these episodes
    may well have been caused by reported oxidant concentrations. However,
    systemic symptoms could not be explained only by the observed oxidant

    concentrations, but were more likely to be attributable to individual
    psychosomatic reactions among the students. This judgement is
    supported by the observation that acute systemic reactions were
    observed in only some of the students, most of whom were exercising at
    the time of peak oxidant levels. Thus, the students may well have
    experienced acute irritation of the pharynx and trachea, enhanced by
    their active status. However, the general population in the same
    neighbourhood, including young children in primary grades and school
    teachers, did not appear to have had these systemic symptoms. These
    comments are based on the following studies.

        Fujii (1972) reported the acute effects of an episode of
    photochemical smog in Osaka, Japan, on 27 August 1971. The oxidant
    concentration measured by the NBKI method rose to an hourly value of
    360 g/m3 (0.18 ppm) by 14h00. At this time, the peak ozone
    concentration, determined by the chemiluminescent method was
    480 g/m3 (0.24 ppm) and the maximum sulfur dioxide concentration was
    180 g/m3 (0.07 ppm). A total of 249 school children reported
    symptoms including pain in the throat, headache, coughing, breathing
    difficulties, eye irritation, and numbness in the limbs. On 14
    September 1971, maximum hourly ozone concentrations reached 480 g/m3
    (0.24 ppm) and a total of 290 persons complained of similar symptoms.
    These data do not provide a basis for estimating symptom frequency in
    the general population, because the total population at risk was not
    reported.

        In an evaluation of the clinical status of 15 high school children
    who were hospitalized after the onset of acute symptoms that began
    during photochemical smog episodes in Osaka, Japan, Adachi & Nakajima
    (1974) stated that "they were astonished by the seriousness of the
    conditions which surpassed by far the symptoms previously known in
    connection with the relation between photochemical oxidant
    concentration and its effect on man in the case of Los Angeles".
    Affected students were not only found to have irritation of the eyes
    and respiratory tract, but, systemic symptoms such as paraesthesia,
    muscle spasms as well as an increased leukocyte count and borderline
    elevation of serum alkaline phosphatase (3.1.3.1).

        Based upon complaints found in schools that were registered with
    local health centres located in Japanese cities, the Japan Public
    Health Association (1976) reported a significant increase in
    subjective complaints, particularly eye irritation, when photochemical
    oxidant concentrations exceeded hourly values of 300 g/m3
    (0.15 ppm), as determined by the 10% NBKI method.

        Mikami & Kudo (1973) described various local and systemic symptoms
    in 82 school children that were attributed to 3 photochemical air
    pollution episodes in Tokyo and Osaka in 1970, 1971, and 1972 with
    maximum 1-h concentrations ranging from 300 to 580 g/m3 (0.15 to
    0.29 ppm). A large proportion of cases were reported to have eye
    irritation, throat irritation, cough, breathlessness, headache, and
    chills. The authors pointed out that many of these symptoms had not
    been reported under Los Angeles smog conditions.

    6.3.5  Effects on the incidence of acute respiratory and
           cardiovascular diseases

        In studies on college students in the Los Angeles Basin, Durham
    (1974) showed that acute episodes of pharyngitis, bronchitis, and
    upper respiratory infections were associated with peak concentrations
    of oxidants and mean concentrations of sulfur dioxide and nitrogen
    dioxide. Oxidants, sulfur dioxide, and nitrogen dioxide
    (concentrations not given) were, in this order, consistently
    associated with the various episodes of illness. A comparison of
    selected high and low pollution days indicated that photochemical air
    pollution might have been responsible for a 16.7% increase in acute
    respiratory symptoms seen in Los Angeles schools situated in areas of
    highest concentration compared with those in areas of lowest
    concentration. The author's method of data presentation did not permit
    the estimation of dose-response relationships or of threshold
    concentrations.

        Brant & Hill (1964) and Brant (1965) did not find any correlation
    between admissions for cardiovascular conditions to Los Angeles County
    Hospital and oxidant levels (170 g/m3; 0.08 ppm as a mean value of
    daily 7-h measurements for the study periods) either on the day of
    admission or for 2 weeks prior to admission, for the period between 8
    August and 25 December 1954. However, Sterling et al. (1966, 1967)
    found statistically significant but very low correlations between
    admissions to Los Angeles hospitals for respiratory episodes during
    the period 17 March-26 October 1961 and daily mean levels of oxidants
    (74 g/m3; 0.037 ppm), ozone (84 g/m3; 0.042 ppm), and carbon
    monoxide (11 mg/m3; 9.5 ppm). Because of the limited time period and
    the extremely low correlation coefficients, it is difficult to
    conclude that a real relationship was demonstrated by these studies.

    6.3.6  Effects on the prevalence of chronic respiratory diseases and
           on pulmonary function

        Deane et al. (1965) conducted a survey on the prevalence of
    chronic respiratory disease in outdoor male telephone workers in San
    Francisco and Los Angeles. Symptoms of persistent cough and phlegm
    were slightly less prevalent in Los Angeles than in San Francisco in
    the 40-49-year group but more prevalent in the 50-59-year group (31.4%
    compared with 16.3% in San Francisco). These data were adjusted for
    cigarette smoking. No differences in pulmonary function test results
    were found.

        The prevalence of chronic respiratory symptoms and the results of
    pulmonary function tests were compared in 2 similar groups of
    nonsmoking, adult, male and female Seventh Day Adventists aged 45-64
    years residing in Los Angeles and San Diego, respectively. Annual mean
    oxidant values (94 g/m3; 0.047 ppm and 76 g/m3; 0.038 ppm), were
    essentially the same in the 2 cities, but a mean of maximum daily
    concentrations in Los Angeles (290 g/m3; 0.144 ppm) was twice as
    high as that in San Diego (148 g/m3; 0.074 ppm). The survey was
    performed when oxidant levels were low and similar in both cities, to
    minimize effects attributable to acute oxidant exposures. No
    differences were found in symptom prevalence or in several measures of
    ventilatory function. Prevalence rates for chronic respiratory disease
    were uniformly low (less than 4%) in both groups (Cohen et al., 1972).

    6.3.7  Effects on patients with pre-existing diseases

    6.3.7.1  Asthma

        The association between reported asthma attacks in 137 patients
    from Pasadena, California, and oxidant levels during the period 3
    September-9 December 1956 was studied by Schoettlin & Landau (1961).
    Daily records of the time of onset of asthma attacks were kept by each
    patient and collected weekly. The daily number of patients afflicted
    with asthma was moderately well correlated (r = 0.37) with concurrent
    maximum hourly oxidant readings. Asthma attacks were more weakly
    correlated with temperature, relative humidity, and water vapour
    pressure than with oxidants. The number of patients having attacks on
    days when maximum 1-h oxidant levels were higher than 500 g/m3
    (0.25 ppm) was significantly greater (p = 0.05) than the number of
    those having attacks on days with lower oxidant levels. Unfortunately,
    the analysis failed to isolate the pronounced seasonal variation of
    asthma. In the northern hemisphere, asthma attack rates tend to reach
    a peak in October and November (Booth et al., 1965), and decline
    sharply in late November and early December. This pattern corresponds
    closely to seasonal declines in peak oxidant levels. Hence, it is
    possible that observed asthma-oxidant correlations have been secondary
    to simultaneous seasonal changes in asthma frequency and oxidant
    levels.

    6.3.7.2  Chronic respiratory diseases

        Studies over 18 months on 25 patients with severe, chronic,
    obstructive lung disease at a chronic disease centre in Los Angeles
    County showed that the prevailing levels of oxidants, oxidant
    precursors, temperature, and relative humidity did not produce any
    effects on lung function (Rokaw & Massey, 1962).

        In studies on the effects of ambient air pollution exposure on
    armed forces veterans with chronic respiratory disease living in the
    Domiciliary Unit and Chronic Disease Annex of the Los Angeles Veterans
    Administration Center, subjects were evaluated once a week by

    pulmonary function tests and respiratory symptom questionnaires
    (Schoettlin, 1962). Analysis of variance did not show any
    statistically significant effects of air pollution (concentrations not
    reported) on respiratory symptoms or function, although maximum
    concentrations of oxidant and oxidant precursors consistently
    accounted for more of the variation in frequency of symptoms and
    clinical signs of disease than maximum temperature, relative humidity,
    or pollen counts.

    6.3.8  Cancer

        A 5-year prospective study of lung cancer from 1958 to 1963 was
    conducted by Buell et al. (1967) among 69 160 members of the
    California American Legion. Long-term residents of Los Angeles County
    had slightly lower age-smoking adjusted lung cancer rates (95.4 per
    100 000) than residents of the San Francisco Bay area counties and San
    Diego County (102 per 100 000). These urban groups, in turn, had
    higher rates than all other California counties (75.5 per 100 000).
    Nonsmokers followed the same geographical pattern: 28.1 per 100 000
    for Los Angeles, 43.9 per 100 000 for San Francisco and San Diego,
    11.2 per 100 000 for all others. Smokers of more than one packet of 20
    cigarettes a day in Los Angeles had the highest lung cancer rates,
    241.3 per 100 000, compared with San Francisco-San Diego, 226 per
    100 000, and with other counties, 137.5 per 100 000. The duration of
    exposure necessary to induce lung cancer may have been longer than the
    actual ozone exposures in this population study. Thus, it would appear
    worthwhile to extend these observations to subsequent years.

    6.3.9  Motor vehicle accidents

        The association between automobile accidents and days of elevated
    oxidant levels (120-480 g/m3; 0.06-0.24 ppm) in Los Angeles was
    studied from August to October in 1963 and 1965. Applying a sign-test
    and nonparametric correlation analysis to the data, Ury (1968) found a
    statistically significant relationship between oxidant levels and
    automobile accidents. Concentrations of carbon monoxide, oxides of
    nitrogen, and other pollutants would also be relatively elevated when
    oxidants were high and may have contributed to the results reported.
    Furthermore, the association of accidents with oxidant levels may be
    confounded by the fact that traffic jams produce both more accidents
    and a greater output of oxidant precursors.

    6.4.  Summary Tables

        Studies on the health effects of controlled, industrial, and
    community exposures that provide quantitative information useful for
    establishing guidelines for the protection of public health with
    respect to photochemical oxidants are summarized in Tables 12, 13 and
    14.


    
    Table 12. Controlled human studies
    I. Sensory effects
                                                                                                                                     

    Ozone concentration  Length of      Effects                              Responsea       Subjects             Reference
                         exposure
                                      
    g/m3   (ppm)        days  h/day
                                                                                                                                     

    400     (0.2)        1     3 and 6  Diminution in various measures of    Not applicable  22 healthy males, 6  Lagerwerff (1963)
    700     (0.35)       1     3 and 6  visual perception; noneffect dose                    healthy females
    1000    (0.5)        1     3 and 6  not determined; no mention of
                                        dose-response relationship.
    > 200   (> 0.1)      1     working  Increasing eye irritation with       Not applicable  20 women office      Richardson &
                         hours (study   increase in oxidant exposures;                       workers              Middleton
                         repeated for   no apparent effects below                                                 (1957, 1958)
                         123 days)      200 g/m3 (0.1 ppm).

    40-100  (0.02-                      Immediate odour perception after     9/10 at         10-14 healthy males  Henschler et al.
            0.05)                       beginning of exposure; odour         40 g/m3                             (1960)
                                        perception disappeared in -12       13/14 at
                                        min; odour considerably stronger at  100 g/m3
                                        higher dose.
    15-40   (0.008                      Odour perception immediately after   Not available   20 healthy subjects  Eglite (1968)
            -0.02)                      beginning of exposure; the most
                                        sensitive person perceived odour
                                        at the level of 15 g/m3
                                        (0.008 ppm).
                                                                                                                                     

    II. Effects on respiratory function
     A. Exposure to ozone
                                                                                                                                     

    Ozone concentration  Length of      Effects                                          Subjects                 Reference
                         exposure
                                      
    g/m3   (ppm)        days  h/day
                                                                                                                                     

    2000    (1.0)        1     1        Consistent increase in airway                    4 healthy males          Goldsmith et Nadel
                                        resistance; exposures at 200, 800,                                        (1969)
                                        and 1200 g/m3 (0.1, 0.4, 0.6 ppm)
                                        caused effect in some subjects,
                                        but no dose-response pattern.
    1800    (0.9)        1     5 min    Highly significant decrease of airway            4 healthy males          Kagawa & Toyama
                                        conductance after inhalation with exercise.                               (1975a)
    1200-   (0.6-0.8)    1     2        Significant reduction in diffusing capacity of   10 healthy men and       Young et al.
    1600                                lung and in FEV0.75b; substernal soreness        one healthy woman        (1964)
                                        present in all subjects.
    1000    (0.5)        6/wk  3        Significant decrease in FEV1.0c; no effect at    12 healthy males         Bennet (1962)
                         x 12           400 g/m3 (0.2 ppm).
                         wk5
    1000    (0.5)        1     6        Significant change in airway conductance         20 healthy subjects      Kerr et al. (1975)
                                        and pulmonary resistance (dry cough and          (19 men and 1
                                        chest discomfort).                               woman)
    1000    (0.5)        1     2        Decrease in pulmonary function                   7 healthy males          Hackney et al.
                                        measurements.                                                             (1975a)
                                                                                                                                     

                 
    a Response = number of subjects showing effect described
                          total number of subjects

    Table 12. Controlled human studies--continued
    II. Effects on respiratory function
     A. Exposure to ozone (cont'd)
                                                                                                                                     

    Ozone concentration  Length of      Effects                                          Subjects                 Reference
                         exposure
                                 
    g/m3   (ppm)        days  h/day
                                                                                                                                     

    800     (0.4)        1     2 and 4  After 2-h exposure: Rawd increase, FVCe          22 healthy male          Rummo et al.
                                        decrease MMFRf decrease; after 4-h               subjects                 (1975.
                                        exposure: Rawd increase, FVCe decrease.                                   unpublished
                                        MMFRf decrease and additionally FEV1.0c                                   data)
                                        decrease; RVg, FRCh, and TCLi did not
                                        change.
    740 &   (0.37 &      1     2        Significant decrease in ventilatory function     6-10 healthy males       Bates et al.
    1500    0.75)                       and in closing volume under intermittent                                  (1972).
                                        exercise; effect more pronounced at higher                                Hazucha et al.
                                        dose.                                                                     (1973)
    740,    (0.37,       1     2        Dose dependent change of ventilatory             28 healthy subjects      Folinsbee et al.
    1000 &  0.50 &                      pattern; significant reduction in MEFRj at       (20 males and 8          (1975)
    1500    0.75)                       50% of vital capacity at 740 g/m3               females)
                                        (0.37 ppm) or more, under exercise.
    740     (0.37)       1     2        Significant increase in total respiratory        2 healthy and 3          Hackney et al.
                                        resistance under intermittent light exercise     sensitively males.       (1975a)
    500     (0.25)       1     2        No consistent changes in lung function.          3 healthy and 3          Hackney et al.
                                        sensitively males                                (1975a)
    200-    (0.1-        1     30 min   Tendency towards increase in breathing           4 healthy male           Ohmori (1974)
    500     0.25)                       frequency and volume under exercise.             subjects
    200     (0.1)        1     2        Increase of Rawd and AaDO2k in 7 of 11 test      11 healthy male          von Neiding et al.
                                        subjects under intermittent light exercise.      subjects                 (1977)
                                                                                                                                     

     B. Exposure to mixtures of ozone and other air pollutants
                                                                                                                                     

    Ozone concentration               Length of     Effects                                   Subjects            Reference
                                      exposure
                                                   
    g/m3           (ppm)             days    h/day
                                                                                                                                     

    740 + 960       (0.37 + 0.37      1       2     More pronounced decrease in ventilatory   4 healthy males     Bates & Hazucha
    sulfur          sulfur                          function end in closing volume than in                        (1973)
    dioxide         dioxide                         single exposure to ozone at 740 g/m.3
                                                    (0.37 ppm).
    200 + 9400      (0.1 + 5          1       2     Increases in Rawd and AaDO2k similar      11 healthy male     von Nieding et al.
    nitrogen        nitrogen                        to those observed with ozone alone at     subjects            (1977)
    dioxide         dioxide                         200 g/m3 (0.1 ppm).

    200 + 9400      (0.1 + 5          1       2     Increases in Rawd and AaDO2k similar      11 healthy male     von Nieding et al.
    nitrogen        nitrogen                        to those observed with ozone at           subjects            (1977)
    dioxide +       dioxide + 5                     200 g/m3 (0.1 ppm) + nitrogen dioxide
    sulfur          sulfur                          at 9400 g/m3 (5 ppm); recovery time
    dioxide         dioxide)                        delayed.

    50 + 100        (0.025 + 0.05     1       2     No effect on Rawd and AaDO2k;             11 healthy male     von Nieding et al.
    dioxide +260    nitrogen                        sensitivity of the respiratory tract      subjects            (1977)
    sulfur dioxide  dioxide + 0.1                   to acetylcholine increased.
                    sulfur dioxide)
                                                                                                                                     
     C.  Exposure to peroxyacetylnitrate
                                                                                                                                     

    Ozone concentration         Length of       Effects                                     Subjects              Reference
                                exposure
                                             
    g/m3           (ppm)       days  h/day
                                                                                                                                     

    1500            (0.3)       1     5 min     Increase in oxygen uptake with exercise;    32 college males      Smith (1965)
                                                no effect at rest; significant decrease
                                                in MEFRj during the recovery phase
                                                following exercise.
    1350            (0.27)      1     42 min    Minor changes in cardiorespiratory and      20 healthy male       Raven et al.
                                                temperature regulation parameters.          subjects              (1974)
                                                                                                                                     

    II.  Effects on respiratory function
     D.  Exposure to irradiated automobile exhaust
                                                                                                                                     

    Oxidants       (0.22              Short term    No significant changes in reaction   14 healthy     Holland et al.
    440-540        -0.27)             (no details   time, vital capacity, and submaximum                subjects(1968)
                                      given)        work performance on the bicycle
                                                    ergometer.
    Carbon         (15-29)
    monoxide
    17-
    33 mg/m3
                                                                                                                                     

    Table 12.  Controlled human studies--continued
    II.  Effects on respiratory function

     E.  Exposure to ambient air with an elevated concentration of oxidants
                                                                                                                                     

    Ozone concentration         Length of           Effects                              Subjects             Reference
                                exposure
                                             
    g/m3          (ppm)        days  h/day
                                                                                                                                     

    Carbon         (800-1400)
    dioxide
    1520-
    2660 mg/m3
    Nitric         (0.38-0.58)
    oxide
    470-710
    Nitrogen       (0.7-1.0)
    dioxide
    1300-1900
    Hydrocarbons
    traces
    Aldehydes      (0.2-0.7)
    Formaldehyde   (0.2-0.24)
    250-300

    400-1400       (0.2-0.7)          2-90 h        Improvement in lung function upon    46 patients with     Motley et al.
                                                    residence in clean filtered room     chronic lung         (1959)
                                                    for 40 h or longer; threshold        disease
                                                    concentration not determined.

    100-460        (0.05-0.23)  21    24            Decrease in airway resistance and    15 patients with     Balchum (1973)
                                                    increase in arterial partial oxygen  moderately severe
                                                    pressure during week of residence    chronic lung
                                                    in clean filtered air; threshold     disease
                                                    concentration not determined.
                                                                                                                                     

    Table 12.  Controlled human studies
    III.  Changes in the electroencephalogram
                                                                                                                                     

    Ozone concentration   Length of         Effects                          Responsea   Subjects              Reference
                          exposure
                                       
    g/m3   (ppm)         days  h/day
                                                                                                                                     

    10      (0.005)       1     3 min       Decrease in alpha rhythm.        1/3         3 healthy subjects    Eglite (1968)
    15      (0.008)       1     3 min       Decrease in alpha rhythm.        2/3         3 healthy subjects    Eglite (1968)
    20      (0.01)        1     3 min       Decrease in alpha rhythm.        3/3         3 healthy subjects    Eglite (1968)
                                                                                                                                     

    a  Response = number of subjects showing effect described
                           total number of subjects
    b  FEV0.75   = 0.75 second forced expiratory volume
    c  FEV1.0    = one second forced expiratory volume
    d  Raw       = airway resistance
    e  FVC         = forced vital capacity
    f  MMFR      = mid-maximal expiratory flow rate
    g  RV        = residual volume
    h  FRC       = functional residual capacity
    i  TLC       = total lung capacity
    j  MEFR      = maximum expiratory flow rate
    k  AaDO2     = alveolar to arterial oxygen pressure difference
    l  sensitive (subjects) = those with a prestudy history of cough, chest discomfort, or wheezing associated with
                              allergy or air pollution exposure, but with normal base-line pulmonary function studies

    Table 13.  Studies on the effects of industrial exposures
                                                                                                                                     

    Ozone concentration      Averaging   Effects                                                             Reference
                             time
    g/m3       (ppm)
                                                                                                                                     

    1600-3400  (0.8-1.7)     1 h         11 of 14 welders complained of respiratory symptoms; symptoms       Challen et al. (1958)
                                         disappeared when ozone levels were reduced to
                                         400 g/m3 (0.2 ppm); nitrogen dioxide probably present.
    600-1600   (0.3-0.8)     1 h         Increased frequency of chest tightness and throat irritation among  Kleinfeld et al. (1957)
                                         welders; no complaints at 500 g/m3 (0.25 ppm); welders also
                                         exposed to nitrogen dioxide and particulates but concentration
                                         levels not given.
    500-800    (0.25-0.40)   long-term   Increased frequency of headache, weakness, change in neuromuscular  Kudrjavceva (1963)
                                         sensitivity, and decrease in memory among workers manufacturing
                                         hydrogen peroxide and exposed to ozone for 7-10 years; threshold
                                         concentration not determined.
    400-600    (0.2-0.3)     1 h         No evidence for changes in vital capacity or functional residual    Young et al. (1963)
                                         capacity in welders; nitrogen dioxide probably present.
    80-1000    (0.04-0.50)   long-term   Increased prevalence of bronchitis and emphysema in workers         Nevskaja & Diterihs
                                         engaged in manufacture of hydrogen peroxide for many years;         (1957)
                                         sulfuric acid aerosols also present; threshold concentration not
                                         determined.
                                                                                                                                     

    Table 14.  Studies on the effects of community exposures
                                                                                                                                     

    Hourly oxidant concentration       Effects                                                  Subjects           Reference
                                    
    g/m3            (ppm)
                                                                                                                                     

    500 and above    (0.25 and above)  Increased frequency of asthma; possible confounding of   137 patients with  Schoettlin
                                       asthma-oxidant association with seasonal effects; other  asthma             & Landau (1961)
                                       pollutants present but not reported.

    approx. 1000     (approx. 0.50)    Increased symptoms began at hourly concentrations of     102 student        Hammer et al.
                                       (headache) 100 g/m3 (0.05 ppm), (eye irritation)        nurses             (1974)

                                       300 g/m3 (0.15 ppm), (cough) 530 g/m3 (0.265 ppm).
                                       (chest discomfort) 580 g/m3 (0.29 ppm); threshold
                                       levels determined by "hockey stick" functions.

    240 and above    (0.12 and above)  Impaired performance determined by failure to improve    116 high school    Wayne et al.
                                       running times; threshold determined by "hockey stick"    cross-country      (1967)
                                       functions.                                               runners

    > 300            (0.15)            Increased frequency of complaints, particularly eye      7440 school        Japan Public
                                       irritation.                                              children           Health
                                                                                                                   Association (1976)
    0-560            (0-0.28)          Specific airway conductance of sensitive subjects was    20 healthy         Kagawa et al.
                     (range at time    significantly decreased with increasing hourly oxidant   11-year-old        et al. (1977)
                     of lung           concentrations; temperature, nitric oxide, nitrogen      school children
                     function test)    dioxide, sulfur dioxide, and particulate matter also
                                       showed significant correlations with various
                                       respiratory function tests of highly reactive children;
                                       threshold concentration not determined.
                                                                                                                                     
    

    7.  EVALUATION OF HEALTH RISKS FROM EXPOSURE TO PHOTOCHEMICAL OXIDANTS

        There appears to be sufficient information from experimental and
    epidemiological studies to justify an attempt to establish guidelines
    on the exposure limits for ozone and to review those for "oxidants"
    (as measured by the neutral-buffered potassium iodide method (NBKI)),
    proposed by a WHO Expert Committee in 1972. The Task Group appreciated
    the fact that photochemical air pollution contains other substances
    besides ozone, such as nitrogen dioxide, peroxyacetylnitrate, and
    possibly many other gaseous and particulate products of atmospheric
    photochemical reactions. However, present knowledge about the
    composition of photochemical pollution, the concentrations of
    individual components, and their possible impact on human health is so
    limited that no attempt can be made to estimate exposure limits for
    any single compound other than ozone. The Task Group was, of course,
    aware that some sensory effects of photochemical air pollution (such
    as eye irritation) might be due to a large extent, to these poorly
    defined components of the photochemical oxidant mixture. As nitrogen
    dioxide is an important air pollutant in its own right, it has been
    discussed in a separate criteria document (World Health Organization,
    1977).

    7.1  Exposure Conditions

        Exposure of man to ozone must have occurred for millions of years,
    as ozone, present naturally in higher tropospheric layers, is also
    found regularly in the lower atmosphere even in completely uninhabited
    regions like the Antarctic. These natural concentrations have been
    reported to have values ranging from 10 to 100 g/m3
    (0.005-0.05 ppm). It is difficult to determine the proportions of
    natural to man-made oxidants (including ozone) that occur in rural
    areas in most countries. In general, ozone concentrations greater than
    120 g/m3 (0.06 ppm) are considered to be related to man-made
    activities. Ozone may be transported over hundreds of kilometres and
    rural populations may be exposed to the pollutant, which earlier was
    considered to exist only in Urban areas. A characteristic of such
    exposures is that the precursors have vanished and ozone can therefore
    persist for days in succession, since it does not come into contact
    with other pollutants that act as its scavengers.

        In large urban areas with strong sunshine and dense traffic or
    other sources of precursors, photochemical air pollution is a daylight
    phenomenon with maximum 1-h ozone concentrations sometimes as high as
    300-800 g/m3 (0.15-0.4 ppm), occurring around noon or somewhat
    later. Such peak concentrations are preceded by nitrogen dioxide peaks
    and accompanied by concurrent rises in peroxyacetylnitrate
    concentrations. In contrast to oxides of sulfur and smoke, ozone
    exposures are always intermittent, the peak concentrations rarely
    lasting for more than 2-3 h. In the low temperature season,
    photochemical reactions are much less likely to occur at rates
    sufficient to produce large quantities of ozone.

        Unless certain industrial technological processes (e.g., welding)
    are in operation, ozone concentrations indoors tend to be considerably
    lower than those outdoors due to the presence of reactive surfaces,
    air conditioning, and indoor smoking.

    7.2  Exposure-effect Relationships

        Information presented in sections 5 and 6 is sufficient to
    evaluate the relationship between exposure and the associated effects,
    at least for some biological changes observed in man and experimental
    animals.

    7.2.1  Animal data

        There is a considerable amount of evidence that short-term,
    prolonged, or repeated exposure to ozone concentrations ranging from
    200 to 400 g/m3 (0.1-0.2 ppm) can cause a variety of biological
    changes in several animal species and that these effects become more
    pronounced with higher concentrations and increased exposure time (see
    Table 10). These effects are, of course, also influenced by other
    factors such as the animal species, the length of the interval between
    exposures, the presence of other pollutants, low temperature, and
    physical activity.

        The host's pulmonary defence mechanisms against infectious
    microorganisms are affected in several animal species by exposure to
    ozone (section 5.2.6). This may result in rapid multiplication of
    infectious microorganisms  in situ, causing disease and eventually
    death. An increase in mortality, resulting from the joint action of
    infectious microorganisms and ozone, has been demonstrated in
    artificially infected mice after a 3-h exposure to ozone at 160 g/m3
    (0.08 ppm). These effects were dose-related.

        Pathomorphological changes in the respiratory tract of various
    animal species, such as the rat, cat, rabbit, and mouse, have been
    observed at ozone concentrations of about 400 g/m3 (0.2 ppm) and
    higher (section 5.2.1). Short-term exposures (up to 24 h) produce
    oedema, degeneration and destruction of type I alveolar cells, loss of
    ciliated epithelium, and breakdown of capillary endothelium. When the
    exposure is repeated or its length extended, the biological changes
    become more severe and include emphysema, atelectasis, vascular
    lesions, bronchopneumonia, and fibrosis.

        Various functional changes in the respiratory tract begin at
    levels of about 520 g/m3 (0.26 ppm) (section 5.2.2). The activities
    of several enzymes in the lung tissue are also influenced at exposure
    levels of about 500 g/m3 (0.25 ppm) (section 5.2.3).

        After pre-exposure to ozone, animals appear to become tolerant to
    ozone concentrations that would otherwise cause pulmonary oedema
    (section 5.2.5). The development of tolerance in small rodents is
    related to pre-exposure levels of at least 600 /m3 (0.3 ppm). This
    tolerance does not protect animals from such effects of ozone as
    inflammation, alterations in alveolar macrophage functions, and
    impairment of respiratory functions, that can occur at concentrations
    lower than those that produce oedema.

        Although the primary target for ozone is the respiratory system, a
    number of studies have indicated that exposure to ozone may also
    result in some extrapulmonary effects, but the mechanisms of such
    action are not clear (section 5.3). For example, ozone exposure at
    400-500 g/m3 (0.2-0.25 ppm) for less than 2 h produced changes in
    the circulating lymphocytes (increasing the number of binucleated
    cells) and increased the number of spherocytes (section 5.3.2.1). It
    has also been shown that exposure of pregnant mice to ozone at
    200-400 g/m3 (0.1-0.2 ppm) for 7 h per day, 5 days per week, for 3
    weeks significantly increased neonatal mortality (section 5.3.3).

        The available information concerning the carcinogenicity and
    mutagenicity of ozone is inadequate for the definite evaluation of
    such effects (sections 5.2.4 and 5.4).

        Biological effects produced by the exposure of experimental
    animals to a combination of ozone and nitrogen dioxide or by exposure
    to complex pollutant mixtures containing oxidants, such as irradiated
    automobile exhaust, are generally similar to those produced by
    exposure to pure ozone. However, one study in which mice were exposed
    to a mixture of ozone and nitrogen dioxide indicated that the effect
    (reduction in resistance to respiratory infection) of a single
    exposure to this mixture was additive, and that repeated exposure to
    this mixture might result in a synergistic action.

    7.2.2  Controlled human exposures

        Although limited in number, human volunteer studies with
    short-term, controlled exposure to oxidants have proved useful for
    establishing exposure-effect relationships for ozone at levels ranging
    from about 200-700 g/m3 (section 6.1). Some of these studies provide
    evidence of changes in the respiratory function of healthy subjects
    that are related to exposure. Physical exercise tends to enhance the
    respiratory effects of ozone (section 6.1.3).

        Three investigators found a significant increase in airway
    resistance with exposure to an ozone level of 740 g/m3 (0.37 ppm)
    for 2 h. One of the investigators did not find any effect with a 2-h
    exposure to a level of 500 g/m3 (0.25 ppm). However, this particular
    study was conducted on subjects from southern California who were
    later found to be less sensitive to ozone. Another investigator, using
    similar test conditions, found a significant increase in airway

    resistance in 7 out of 11 subjects at an ozone level of 200 g/m3
    (0.1 ppm) for 2 h (section 6.1.3.1).

        In two studies, an improvement in lung function was noted when
    patients with chronic pulmonary disease breathed filtered air for 40 h
    or more, as compared with unfiltered ambient air. The ambient air
    concentrations of oxidant in the two studies ranged from
    400-1400 g/m3 (0.2-0.7 ppm) and up to 400 g/m3 (0.2 ppm)
    respectively (section 6.1.3.5).

        Result of a 7-month study in which female employees were exposed
    to unfiltered ambient air during office hours suggested that the
    lowest 1-h oxidant level that could be associated with eye irritation
    was about 200 g/m3 (section 6.1.2).

        Although  in vitro studies using human cells have shown some
    evidence of a joint action of ozone and nitrogen dioxide, this has not
    been clearly demonstrated in  in vivo studies. For example, whereas a
    potentiated increase in airway resistance was observed with exposure
    to a combination of ozone at 740 g/m3 (0.37 ppm) and sulfur dioxide
    at 960 g/m3 (0.37 ppm) for a period of 2 h, it was not observed with
    exposure to a combination of ozone at 500 g/m3 (0.25 ppm) and
    nitrogen dioxide at 560 g/m3 (0.30 ppm). Exposure to ozone at
    50 g/m3 (0.025 ppm) combined with nitrogen dioxide at 100 g/m3
    (0.05 ppm) and sulfur dioxide at 260 g/m3 (0.1 ppm) did not have any
    effect on airway resistance; however, this combined exposure resulted
    in enhancement of the bronchoconstrictor effect of acetylcholine
    (section 6.1.3.2).

    7.2.3  Industrial exposure

        Acute symptoms of chest tightness, irritation of the throat, and
    coughing have been documented in welders exposed to 1-h ozone levels
    of 600-1600 g/m3 (0.3-0.8 ppm). These symptoms disappeared when
    ozone concentrations fell to 500 g/m3 (0.25 ppm) or less. Possible
    chronic effects of repeated occupational exposure to ozone are not
    well documented, although one investigator reported that workers
    exposed to ozone levels in the range of 500-800 g/m3 (0.25-0.40 ppm)
    for 7-10 years had an increased frequency of headaches and weakness,
    increased muscle excitability, and impaired memory (section 6.2).

    7.2.4  Community Exposure

        Several studies have shown more frequent eye irritation, reduced
    athletic performance, changes in lung function of children, and
    increased frequency of asthma attacks, all of which have been
    associated with changes in hourly oxidant levels (sections 6.3.2,
    6.3.3, 6.3.4, 6.3.7). As in all studies of the effects of community
    exposures, it is difficult to determine precisely the lowest level at
    which adverse effects become manifest. However, most of these effects
    were observed when 1-h oxidant levels were in the range of about

    200-500 g/m3 (0.1-0.25 ppm). Although other pollutants such as
    nitrogen dioxide, particulate matter, and sulfur dioxide were
    simultaneously present, the strongest correlation of the observed
    effects was with hourly levels of photochemical oxidants.

        On the other hand, there is no evidence, so far, that long-term
    exposure to photochemical oxidants at levels currently present in
    urban air is associated with increased mortality (section 6.3.1), and
    there is no evidence that chronic respiratory diseases such as
    bronchitis, emphysema, and lung cancer are more prevalent in
    communities with high oxidant exposures (section 6.3.6). However, it
    should be pointed out that the number of epidemiological studies
    concerned with such associations is small.

    7.3  Guidelines on Exposure Limits

        The exposure-effect relationships discussed in section 7.2 make it
    possible to draw the following conclusions concerning the exposures to
    oxidants and ozone at which the effects in man begin to appear:

        (a) There is presumptive evidence from one controlled exposure
    study that some effects on the lung function of healthy human subjects
    might occur with exposure to an ozone level of 200 g/m3 (0.1 ppm)
    for 2 h.

        (b) There is also evidence from general population studies that
    suggests that 1-h ambient oxidant levels in the range of about
    200-500 g/m3 (0.1-0.25 ppm) may affect lung function in children,
    increase the frequency of asthma attacks, cause more frequent eye
    irritation, and reduce athletic performance.

        (c) There is limited evidence from controlled exposure studies
    that living in an environment with 1-h oxidant levels within the range
    of 400-1400 g/m3 (0.2-0.7 ppm) may exert additional stress on
    patients with chronic pulmonary disease.

        (d) There is convincing evidence from controlled human exposure
    studies that airway resistance may be increased in healthy human
    subjects following exposure to ozone levels of 700-800 g/m3
    (0.35-0.40 ppm) for 2h.

        Animal data generally support the results of human studies.
    However, some effects have been observed in animals at an ozone level
    of about 200 g/m3 (0.1 ppm) or even less, which have not yet been
    demonstrated in man. For example, in animals, short-term exposures to
    such concentrations appear to reduce resistance to pulmonary
    infections.

        The role of ozone and other photochemical oxidants in the etiology
    of cancer is not clear. The only available epidemiological study did
    not indicate any association between exposure to oxidants and the risk
    of lung cancer, and experimental studies on the carcinogenicity and
    mutagenicity of ozone in animals are not adequate for evaluation.
    Nevertheless, the Task Group felt that there may be reason for concern
    about the possible carcinogenicity of ozone (based primarily on some
    biochemical considerations regarding the mechanism of the biological
    effects of ozone). This aspect of its toxicity should be kept under
    continual surveillance.

        On the basis of all these considerations, the Task Group agreed
    that 1-h levels of ozone of 100-200 g/m3 (0.05-0.1 ppm) (measured by
    the chemiluminescence method) could be used as a guideline for the
    protection of public health. The relatively high natural
    concentrations of ozone precluded the use of any safety factor.

        The Task Group also agreed that a 1-h maximum level of 120 g/m3
    (0.06 ppm), which is approximately the highest natural background
    concentration of oxidants, would be the best single value estimate of
    the exposure limit for oxidants in the ambient air. This level is in
    agreement with the long-term goal for photochemical oxidants (as
    measured by the NBKI method) proposed by a WHO Expert Committee (World
    Health Organization, 1972).

        The issue was raised as to whether the proposed guideline was
    realistic in view of natural exposure levels and the long-distance
    transport of ozone. In response to this question, the Group expressed
    the view that every effort should, nevertheless, be made to develop
    control strategies for achieving the proposed guideline or at least,
    for not exceeding it more than once a month.

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    See Also:
       Toxicological Abbreviations