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    INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY



    ENVIRONMENTAL HEALTH CRITERIA 4





    OXIDES OF NITROGEN





    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 1977

    ISBN 92 4 154064 8

    (C) World Health Organization 1977

        Publications of the World Health Organization enjoy copyright
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    should be made to the Office of Publications, World Health
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    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 OXIDES OF NITROGEN

    1.   SUMMARY AND RECOMMENDATIONS FOR FURTHER RESEARCH
         1.1   Summary
               1.1.1   Chemistry and analytical methods
               1.1.2   Sources of oxides of nitrogen
               1.1.3   Environmental levels 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   Accidental and industrial exposures
                       1.1.5.3   Community exposures
               1.1.6   Evaluation of health risks
         1.2   Recommendations for further research

    2.   CHEMISTRY AND ANALYTICAL METHODS
         2.1   Chemical and physical properties
         2.2   Atmospheric chemistry
         2.3   Analytical methods
               2.3.1   Sampling
               2.3.2   Evaluation of analytical methods
                       2.3.2.1   Manual methods
                       2.3.2.2   Automatic methods
               2.3.3   Calibration procedures

    3.   SOURCES OF OXIDES OF NITROGEN
         3.1   Natural sources
         3.2   Man-made sources
               3.2.1   Stationary sources
               3.2.2   Mobile sources
               3.2.3   Non-combustion sources
               3.2.4   Other sources

    4.   ENVIRONMENTAL LEVELS AND EXPOSURES
         4.1   Background concentrations
         4.2   Urban concentrations
         4.3   Indoor exposure
         4.4   Smoking

    5.   EFFECTS ON EXPERIMENTAL ANIMALS
         5.1   Local effects on the respiratory system
               5.1.1   Morphological changes
               5.1.2   Functional changes
               5.1.3   Biochemical effects

         5.2   Other effects
               5.2.1   Effects on growth and body weight
               5.2.2   Immunological effects
               5.2.3   Haematological effects
               5.2.4   Miscellaneous biochemical effects
               5.2.5   Effects on reproduction
               5.2.6   Effects on the central nervous system
               5.2.7   Behavioural changes
               5.2.8   Carcinogenicity, mutagenicity and teratogenicity
         5.3   Interaction of nitrogen dioxide and infectious agents
         5.4   Summary table

    6.   EFFECTS ON MAN
         6.1   Controlled exposures
         6.2   Accidental and industrial exposures
         6.3   Community exposures
               6.3.1   Effects on pulmonary function
               6.3.2   Effects on the incidence of acute respiratory
                       disease
               6.3.3   Effects on the prevalence of chronic respiratory
                       disease
         6.4   Summary tables

    7.   EVALUATION OF HEALTH RISKS FROM EXPOSURE TO OXIDES OF NITROGEN
       
         7.1   Exposure levels
         7.2   Experimental animal studies
         7.3   Controlled studies in man
         7.4   Effects of accidental and industrial exposures
         7.5   Effects of community exposures
         7.6   Evaluation of health risks

    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 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 OXIDES OF NITROGEN
     Tokyo, 23-27 August 1976

    Participants

     Members

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

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

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

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

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

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

    Dr T. Nakajima, Division of Environmental Health Research, Osaka
        Prefectural Institute of Public Health, Osaka, Japan
         (Vice-Chairman)

    Dr G. von Nieding, Laboratorium fur Atmung und Kreislauf,
        Krankenhaus Bethanien, Moers, Federal Republic of Germany

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

     Observers

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

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

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

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

    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

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

    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 OXIDES OF NITROGEN

        A WHO Task Group on Environmental Health Criteria for Oxides of
    Nitrogen met in Tokyo from 23 to 27 August 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 kindly acting as host to
    the meeting. In reply the group was welcomed by Dr M. Hashimoto,
    Director-General of the Air Quality Bureau, Environment Agency, Japan.
    The Task Group reviewed and revised the second draft criteria document
    and made an evaluation of the health risks from exposure to oxides of
    nitrogen.

        The first and second drafts of the criteria document were prepared
    by Dr G. Freeman, Director, Department of Medical Sciences, Stanford
    Research Institute, Menlo Park, CA, USA. The comments on which the
    second draft was based were received from the national focal points
    for the WHO Environmental Health Criteria Programme in Bulgaria,
    Canada, Czechoslovakia, Federal Republic of Germany, India, Japan, New
    Zealand, Poland, Sweden, the USA and the USSR; and from the Food and
    Agriculture Organization of the United Nations (FAO), Rome, and the
    World Meteorological Organization (WMO), Geneva. 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 phase of the preparation of this document,
    of Professor C. M. Shy, School of Public Health, University of North
    Carolina, NC, USA, Dr D. E. Gardner, Chief, Biomedical Research
    Branch, Health Effects Research Laboratory, Environmental Protection
    Agency, Research Triangle Park, NC, USA, and Dr R. G. Derwent,
    Environmental and Medical Sciences Division, Atomic Energy Research
    Establishment, Harwell, England.

        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 (North Atlantic Treaty
    Organization, 1973; US Department of Health, Education, and Welfare,
    1976; US Environmental Protection Agency, 1971a) and in the reviews on
    oxides of nitrogen by Cooper & Tabershaw (1966), Morrow (1975), and
    Stern, ed. (1968). 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, Geneva, World
    Health Organization, 1976).

    The following conversion factors have been used in this document.a
    

    nitric     1 ppm = 1230 g/m3   carbon     1 ppm = 1150 g/m3
    oxide                           monoxide

    nitrogen   1 ppm = 1880 g/m3   ozone      1 ppm = 2000 g/m3
    dioxide

    nitrous    1 ppm = 1800 g/m3   sulfur     1 ppm = 2600 g/m3
    oxide                           dioxide




















                 

    a 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

    1.1  Summary

    1.1.1  Chemistry and analytical methods

        In the context of this criteria document, the term oxides of
    nitrogen is understood to include nitric oxide (NO) and nitrogen
    dioxide (NO2). Other oxides of nitrogen which exist in the
    atmosphere are not known to have any biological significance and have
    not been referred to in this document. At the point of discharge from
    man-made sources, the predominant oxide of nitrogen is nitric oxide
    which is readily converted to nitrogen dioxide by chemical reactions
    in the atmosphere.

        Nitric oxide and nitrogen dioxide can be measured separately or
    collectively by manual or automated techniques. However, whereas a
    certain analytical method can be quite reliable for one compound
    ("chemiluminescence" for nitric oxide: "Saltzman method" for nitrogen
    dioxide), difficulties may arise in the simultaneous monitoring of
    both oxides. Gas-phase titration, permeation tubes, and gravimetric
    standards have been used for the accurate calibration of these
    analytical procedures.

    1.1.2  Sources of oxides of nitrogen

        On a global scale, quantities of nitric oxide and nitrogen dioxide
    produced naturally by bacterial and volcanic action and by lightning
    by far outweigh those generated by man's activities. However, as they
    are distributed over the entire earth's surface, the resulting
    background atmospheric concentrations are very small.

        The major source of man-made emissions of oxides of nitrogen into
    the atmosphere is the combustion of fossil fuels in stationary sources
    (heating, power generation) and in motor vehicles (internal combustion
    engines). Other contributions to the atmosphere come from specific
    non-combustion industrial processes, such as the manufacture of nitric
    acid and explosives. Indoor sources include smoking, gas-fired
    appliances, and oil stoves. Differences in the nitrogen dioxide
    emission of various countries are mainly due to differences in fossil
    fuel consumption.

        Worldwide emissions of oxides of nitrogen in 1970 were estimated
    at approximately 53 million tonnes.

    1.1.3  Environmental levels and exposures

        The natural background concentration of nitrogen dioxide over land
    areas is usually in the range of 0.4-9.4 g/m3 (0.0002-0.005 ppm).
    This concentration is 1-2 orders of magnitude lower than the
    concentrations normally found in urban areas. Annual mean nitrogen
    dioxide concentrations in urban areas throughout the world are
    typically in the range of 20 90 g/m3 (0.01-0.05 ppm), although it
    is exceedingly difficult to generalize.

        Data for shorter averaging periods show considerable variations
    depending on meteorological and seasonal conditions and on the
    proximity and nature of local sources of pollution. Generally, the
    highest monthly means of nitrogen dioxide levels in large urban areas
    are about 60-110 g/m3 (0.03 0.06 ppm), the highest daily means
    136-400 g/m3 (0.07-0.22 ppm), and the highest hourly values
    240-850 g/m3 (0.13-0.45 ppm).

        In contrast with typical primary air pollutants, nitrogen dioxide
    concentrations do not show consistent seasonal behaviour throughout
    all urban areas of the world and are not necessarily highest during
    the months of maximum photochemical activity.

        Exposure from indoor sources such as home appliances and smoking
    should not be underestimated. In the immediate proximity of domestic
    gas-fired appliances, nitrogen dioxide concentrations of up to
    2000 g/m3 (1.1 ppm) have been measured. Tobacco smoke has been
    reported to contain nitric oxide levels of about 98-135 mg/m3
    (80-110 ppm) and nitrogen dioxide levels of about 150-226 mg/m3
    (80-120 ppm), but these levels may fluctuate considerably with the
    conditions of combustion.

    1.1.4  Effects on experimental animals

        Reversible and irreversible adverse effects may be caused by
    exposure to nitrogen dioxide, depending upon the concentration,
    length, and mode of exposure, the species of animal tested, and the
    presence of infectious agents.

        Morphological changes reported in a number of animal species
    including the mouse, rat, rabbit, guineapig, and monkey, appeared to
    be most prominent in the terminal bronchiolar and alveolar duct
    epithelia. Exposure to about 470-1900 g/m3 (0.25-1.0 ppm) resulted
    in numerous pathophysiological changes including bronchitis,
    bronchopneumonia, atelectasis, protein leakage into the alveolar
    space, changes in collagen, elastin, and mast cells of the lungs,
    reduction or loss of cilia and adenomatous changes.

        At concentrations of 3800-47 000 g/m3 (2.0-25 ppm) these
    effects became more pronounced. The more sensitive ciliated
    bronchiolar and type 1 alveolar lining cells were injured first and
    were replaced by the proliferation of more resistant nonciliated
    cells, and type 2 cells, respectively. Prolonged exposure resulted in
    a reduction in diameter of small airways by exudate, hypertrophy of
    the respiratory epithelium, and swelling of the basement membrane.

        In studies on the effect of nitrogen dioxide on lung function,
    increased respiratory rates were reported in rats exposed to
    concentrations as low as 1500 g/m3 (0.8 ppm). Reductions in both
    diffusion capacity and peak expiratory flow rates were demonstrated in
    beagles exposed to a combination of nitrogen dioxide at
    1210 g/m3 (0.64 ppm) and nitric oxide at 310 g/m3 (0.25 ppm).
    Biochemical changes included alterations in the action of several
    pulmonary enzymes, in the lipid content of the lungs, in the stability
    of pulmonary surfactant, and a decrease in the lung glutathione
    levels. As the nitrogen dioxide concentration increased to
    11-75 mg/m3 (6-40 ppm), the effects became more pronounced.

        A number of extrapulmonary effects have been reported at nitrogen
    dioxide concentrations of 560-3700 g/m3 (0.3 to 2.0 ppm).
    Examination of blood from exposed animals showed changes in the number
    of circulating erythrocytes, in enzyme activity, and in antibody
    titres. Within the range of these concentrations, effects were also
    noted on the conditioned reflexes of the central nervous system
    (600 g/m3, 0.32 ppm) and on the endocrine and reproductive systems
    (2400 g/m3, 1.3 ppm) of rats.

        With increasing levels of exposure, a variety of other effects
    were demonstrated. These included decrease in growth rate
    (5000 g/m3, 2.7 ppm) and loss in physical performance
    (9400 g/m3, 5.0 ppm).

        Exposure to nitrogen dioxide increased the susceptibility of
    experimental animals to both bacterial and viral respiratory
    infections; this response was clearly dose-related. Results indicated
    that concentration had a much greater influence on the toxicity of
    nitrogen dioxide than length of exposure, i.e. equal
    concentration-time products at different exposure times were not
    equally hazardous.

        When mice were exposed to 940 g/m3 (0.5 ppm) for 90 days and
    then artificially infected, a significant increase in the mortality
    rate was observed. A similar response was noted in squirrel
    monkeys-exposed to much higher concentrations of 9400-19 000 g/m3

    (5-10 ppm) for 1 or 2 months. When mortality rates due to respiratory
    infection were compared after continuous and intermittent exposure to
    nitrogen dioxide, there was a significant increase in both treatments
    with increasing length of exposure. However, for each given length of
    exposure, there was no statistical difference between the continuous
    and the intermittent exposure groups.

        Nitrogen dioxide interferes with the lung's ability to remove
    inhaled deposited particles efficiently by altering the phagocytic,
    enzymatic, and functional processes of the alveolar macrophages and of
    the ciliated epithelial cells.

    1.1.5  Effects on man

    1.1.5.1  Controlled exposures

        Studies on exposure to nitrogen dioxide in man have been conducted
    to determine the lowest levels at which odour can be detected and at
    which dark adaptation is altered. For odour perception the lowest
    nitrogen dioxide level was approximately 200 g/m3 (0.11 ppm). The
    lowest level for impairment of dark adaptation was reported to be
    140 g/m3 (0.074 ppm).

        Exposure to nitrogen dioxide levels of 1300-3800 g/m3
    (0.7-2.0 ppm) for 10 min gave rise to an increase in inspiratory and
    expiratory flow resistance. In another study, inhalation of nitrogen
    dioxide concentrations of 3000-3800 g/m3 (1.6-2.0 ppm) for 15 min
    caused a significant increase in total airway resistance, which became
    more pronounced at concentrations above 3800 g/m3 (2.0 ppm). A
    number of authors have reported that exposure to 7500-9400 g/m3
    (4-5 ppm) produced an increase in airway resistance and a decrease in
    the arterial partial pressure of oxygen and carbon monoxide diffusion
    capacity. However, prolongation of the exposure time to 60 min did not
    enhance the effect further. A recent report showed that in 13 out of
    20 asthmatic subjects, the reaction to the inhalation challenge with a
    bronchoconstrictor (carbachol) increased significantly after exposure
    to a nitrogen dioxide level of 190 g/m3 (0.1 ppm) for 1h. Similar
    results were reported in a study in which healthy subjects were
    exposed to a combination of nitrogen dioxide at 100 g/m3
    (0.05 ppm), ozone at 50 g/m3 (0.025 ppm), and sulfur dioxide at
    260 g/m3 (0.10 ppm) for a period of 2h.

    1.1.5.2  Accidental and industrial exposures

        Exposure to high concentrations of oxides of nitrogen has been
    reported in various occupations.

        Farmers who were exposed to silo gases from the fermentation of
    harvested crops were acutely affected by oxides of nitrogen, some of
    them fatally. It has been estimated that exposure to nitrogen dioxide
    levels of 560-940 mg/m3 (300-500 ppm) may result in fatal pulmonary
    oedema or asphyxia and that levels of 47-140 mg/m3 (25-75 ppm) can
    cause bronchitis or pneumonia.

        Miners who used explosives repeatedly in their work were reported
    to develop chronic respiratory diseases. Analysis of the products of
    explosion showed the presence of oxides of nitrogen at concentrations
    of 88-167 ppm.

        A study on surviving victims who had been exposed to the fumes of
    burning nitrocellulose did not reveal any differences in survival
    between the exposed groups and the unexposed controls over the
    following 30 years. Unfortunately quantitative exposure data for the
    various groups were not available in this study.

    There are very few studies on the acute or chronic effects of
    low-level industrial exposures.

    1.1.5.3  Community exposures

        Several studies have been reported in which an attempt has been
    made to relate pulmonary function to nitrogen dioxide exposure.
    However, the results of all these studies have either failed to
    demonstrate a significant difference in lung function between the
    groups exposed to different levels of nitrogen dioxide, or have been
    confounded by the fact that relatively high concentrations of other
    pollutants were present.

        This also applies to studies conducted to correlate the frequency
    of acute respiratory disease and chronic respiratory illness with
    concentrations of nitrogen dioxide.

        For example, a study to evaluate the effects of nitrogen dioxide
    on the incidence of acute respiratory disease in children and their
    parents living near a large point source of this pollutant
    demonstrated an excess rate of respiratory illness in comparison with
    a control group. However, the probable contribution of other
    pollutants such as sulfuric acid aerosols, nitric acid fumes, and
    suspended nitrates, made it difficult to attribute this excess to the
    presence of nitrogen dioxide.

        Similarly, because of relatively high exposure to other air
    pollutants, it has not been possible to associate observed increases
    in the frequency of chronic respiratory illness with a measured level
    of nitrogen dioxide. It has been noted, however, that these
    epidemiological studies seem to confirm the results of controlled
    studies on man and experimental animal studies.

    1.1.6  Evaluation of health risks

        As it has not yet been shown that the concentrations in ambient
    air of oxides of nitrogen, other than nitrogen dioxide, have any
    significant biological activity, a guideline for the protection of
    public health has been developed only for nitrogen dioxide.

        Compared with experimental toxicological studies, there are very
    few epidemiological studies on the effects of either occupational or
    community exposures which can provide sufficient information for the
    assessment of health risks due to exposure to nitrogen dioxide. Thus,
    a health protection guideline has been developed based on data from
    controlled human studies and animal experiments. As previously stated,
    available epidemiological data tend to support these results.

        A nitrogen dioxide concentration of 940 g/m3 (0.5 ppm) has been
    selected as an estimate of the lowest level at which adverse health
    effects due to short-term exposure to nitrogen dioxide can be expected
    to occur. Although the Task Group was aware that one study in man
    showed effects at a lower concentration, it was of the opinion that
    this required confirmation.

        By adopting a minimum safety factor of 3-5, the Task Group agreed
    that a maximum one hour exposure of 190-320 g/m3 (0.10-0.17 ppm)
    should be consistent with the protection of public health and that
    this exposure should not be exceeded more than once per month.

        A caution has been added that it might be prudent to lower this
    exposure limit in view of biological evidence of the interaction of
    nitrogen dioxide with other air pollutants present and also in view of
    the fact that some populations are highly sensitive to this substance.

    Owing to lack of information on the effects of long-term exposure to
    nitrogen dioxide in man, only a short-term exposure limit has been
    suggested.

    1.2  Recommendations for Further Research

        In discussing the health risks of nitrogen dioxide exposure, the
    Task Group concentrated on the biological activity of nitrogen dioxide
    alone, rather than in conjunction with other compounds with which it
    is commonly associated in the ambient atmosphere. However, the Task
    Group was particularly concerned with the potential for enhanced
    biological effects in ambient situations in which peak concentrations
    of nitrogen dioxide and photochemical oxidants occur together. The

    Task Group also expressed concern over atmospheric oxidation products
    of nitrogen dioxide, such as nitrous and nitric acid and various
    nitrate compounds. Taking into consideration the biological data on
    the combined effects of nitrogen dioxide and oxidants, and of the
    nitrates in the ambient air, the Task Group made the following
    recommendations for research on the health effects of these
    substances:

    a)  Controlled studies on man and experimental studies on animals
        should be conducted to compare the reaction of sensitive
        biological systems at typical peak concentrations of nitrogen
        dioxide and ozone alone, and in combination. In animals, the
        infectious disease model appears to be particularly appropriate
        for these studies. In man, the effects should be studied of
        nitrogen dioxide and oxidants, alone or in combination, on airway
        resistance before and after administration of bronchoconstrictors.

    b)  Similar experimental animal and controlled human studies should be
        conducted to evaluate the biological effects of nitric acid and
        nitrates at concentrations found in ambient air.

    c)  The possibility of delayed effects from exposure to nitrogen
        dioxide and its oxidation products should be considered. These
        possibilities may be pursued by means of epidemiological studies
        and recently developed experimental techniques to assess
        carcinogenicity and mutagenicity.

    d)  While the Task Group is aware that epidemiological studies alone
        cannot provide a quantitative basis for evaluating the health
        risks of exposure to nitrogen dioxide, the importance of
        epidemiological studies of occupational and community groups
        should not be minimized. There is a particular need for long-term
        follow-up studies which may identify chronic or delayed and often
        subtle effects in cohorts of exposed populations.

    e)  Studies of highly sensitive subjects should be given careful
        consideration. Asthmatic subjects and persons with
        cardio-pulmonary disease should be studied with respect to
        functional and symptomatic changes associated with variations in
        the average hourly concentrations of nitrogen dioxide, nitrates,
        and related compounds. Controlled exposure of asthmatic subjects
        and other highly sensitive persons to nitrogen dioxide and
        nitrates at concentrations found in the ambient air may be
        undertaken, with their consent and with due consideration for the
        protection of subjects so exposed. It would be highly desirable to
        study animal models of human asthma and hypersensitivity.

    2. CHEMISTRY AND ANALYTICAL METHODS

    2.1   Chemical and Physical Properties

        Oxides of nitrogen are usually classified in terms of the
    oxidation state of nitrogen (Table 1).


    Table 1.  Oxides of nitrogen
                                                                     

    Name                          Chemical formula
                                                                     

    nitrous oxide                 N2O
    nitric oxide                  NO
    dinitrogen trioxide           N2O3
    nitrogen dioxide              NO2
    dinitrogen tetroxide          N2O4
    dinitrogen pentoxide          N2O5
                                                                     

        Nitrous oxide (dinitrogen oxide) is the most prevalent oxide of
    nitrogen in the atmosphere. This compound is generated by anaerobic
    processes in the soil and in the surface layers of the oceans and is
    present in the atmosphere in concentrations of about 450 g/m3
    (0.25 ppm) (Robinson & Robbins, 1972). Although this species may play
    an important role in stratospheric chemistry, it is of little
    importance in the lower atmosphere and has no direct significance for
    human health.

        Nitric oxide (nitrogen oxide) and nitrogen dioxide, the most
    abundant man-made oxides of nitrogen in urban areas, are derived from
    air used in high temperature combustion processes. Both nitric oxide
    and nitrogen dioxide are found in combustion gases but nitric oxide
    predominates because its formation is favoured by high temperatures.
    The formation of nitric oxide can be described by the following
    reactions (Spedding, 1974):

                O + N2 <=> NO + N(1)
                O2 + N <=> NO + O(2)

    Atomic oxygen needed in reaction (1) is produced in the flame by 2
    parallel reactions:

                CO + OH <=> CO2 + H(3)
                H + O2 <=> OH + O(4)

        The amount of nitric oxide formed depends on the temperature of
    the flame, the concentrations of nitrogen and oxygen, and the
    residence time of gases in different zones of temperature, pressure,
    and concentration. Temperature is the most significant variable in the
    production of nitric oxide under normal combustion conditions
    (McKinnon, 1974). The production of nitric oxide per unit mass of fuel
    burned decreases with decreasing mean combustion temperature of
    different fuels, i.e.: coal, oil, natural gas. Because the internal
    combustion engine operates at a high temperature, motor vehicles are
    an important source of nitric oxide.

        Some physical properties of nitric oxide and nitrogen dioxide are
    given in Table 2. Nitric oxide is a colourless, odourless gas that is
    slightly soluble in water. Although the boiling point of nitrogen
    dioxide is 21.2C, it only exists in the gaseous form at normal air
    temperatures because of its low partial pressure.


    Table 2. Physical properties of nitric oxide and nitrogen
    dioxide a
                                                                       

    Oxides of    Molecular   Melting      Boiling     Solubility
    nitrogen     weight      point        point       in water
                             C           C          ml/litre
                                                                       

    nitric
    oxide        30.01       -- 163.6     -- 151.8    73.40
    nitrogen
    dioxide      46.01       -- 11.20         21.2    --
                                                                       

    a From: West (1976)


        Nitrogen dioxide is in equilibrium with its dimer, dinitrogen
    tetroxide (2NO2 <=> N2O4) but at atmospheric concentrations
    the fraction of nitrogen dioxide present in dimer form is negligible.

        Dinitrogen trioxide can be formed from nitric oxide and/or
    nitrogen dioxide. However, at the low concentrations of nitric oxide
    and nitrogen dioxide found even in very heavily polluted air, the
    chemical equilibrium data predict negligible concentrations of
    dinitrogen trioxide which thus has no significance as an air pollutant
    (Leighton, 1961).

        Dinitrogen pentoxide is thought to be an important reactive
    intermediate in photochemical air polution, formed mainly by the
    oxidation of nitrogen dioxide with ozone (Demerjian et al., 1974).
    However, there are no specific analytical methods for the measurement
    of this species in ambient air and there is no evidence that it has
    any significance for human health.

    2.2  Atmospheric Chemistry

        The atmospheric chemistry of the oxides of nitrogen is very
    complex, particularly when other air pollutants such as hydrocarbons
    are present. For this reason, only a simplified account can be
    presented here.

        Nitric oxide is fairly reactive and is readily oxidized in the
    atmosphere to nitrogen dioxide. The conversion takes place by means of
    several reactions depending on the concentrations of nitric oxide. At
    high concentrations, as much as 10% of nitric oxide can be oxidized by
    the reaction:

                2NO + O2 -> 2NO2(5)

        The rate of this reaction decreases with dilution and rapidly
    becomes insignificant. At low concentrations, an important reaction
    leading to nitrogen dioxide formation is:

                NO + O3 -> NO2 + O2(6)

        Nitrogen dioxide absorbs strongly in the ultraviolet region
    between 300 and 400 m and is decomposed by sunlight yielding nitric
    oxide and ozone (Leighton, 1961). Thus, in daylight, reaction (6)
    proceeds in the opposite direction and eventually an equilibrium

                NO + O3 <=> NO2 + O2(7)

    is established.

        The position of equilibrium (7) is a function of the rate of
    reaction (6) and the rate of light absorption by nitrogen dioxide
    which varies with the time of the day, latitude, and other atmospheric
    variables (Calvert, 1976). Generally, however, in unpolluted and rural
    areas, the daytime concentrations of nitric oxide are only a small
    fraction of the concentration of nitrogen dioxide.

        The air is more polluted in urban areas than in rural areas and
    the concentration of oxides of nitrogen is markedly higher. Thus, it
    is possible, particularly at night time, that reaction (6) proceeds to
    completion and that all ozone is removed leaving substantial
    concentrations of both nitric oxide and nitrogen dioxide in the
    atmosphere. During the day, the equilibrium (7) shifts in favour of
    ozone formation.

        Thus, in polluted air, the position of equilibrium (7) and the
    resulting nitric oxide, nitrogen dioxide, and ozone concentrations
    depend on a large number of meteorological and other factors, and
    particularly on the simultaneous presence of hydrocarbon pollutants.

        The pattern of nitrogen dioxide concentrations in urban air is
    therefore quite different from that of primary pollutants such as
    nitric oxide, carbon monoxide, and sulfur dioxide. It is much more
    similar to that of typical secondary pollutants such as ozone and
    photochemical aerosol species. The differences in behaviour between
    nitrogen dioxide and other primary pollutants show in the relationship
    between peak values and long-term mean values, and in diurnal and
    seasonal variability. These aspects are illustrated in section 4.

        The main atmospheric sink for oxides of nitrogen appears to
    involve its oxidation to nitric acid. This is an example of a general
    reaction in atmospheric chemistry where pollutants are oxidized to
    species which are more readily removed from the atmospheric
    circulation. This is particularly true for the oxides of nitrogen
    since nitric acid is much more soluble in water and much more readily
    adsorbed on the surface of suspended particulate matter.

        In view of the possible effects on human health of nitrate
    particles, this atmospheric conversion may have an added significance.
    The mechanism of the conversion most probably involves hydroxyl
    radicals as shown by the equation (8)

                OH + NO2 <=> HNO3(8)

    2.3  Analytical Methods

    2.3.1  Sampling

        Although nitric oxide and nitrogen dioxide are chemically reactive
    they behave quite predictably in glass and teflon sampling moulds.
    Residence time in the sampling manifold requires specific
    consideration, when sampling air containing nitric oxide, nitrogen
    dioxide, and ozone during daylight. Since the equilibrium (7) is
    disturbed inside the dark sampling manifold, nitrogen dioxide
    concentrations may be overestimated when sampling takes much over
    10 s (Butcher & Ruff, 1971).

        Collectors based on solid adsorbents that have been developed for
    the selective sampling of nitric oxide and nitrogen dioxide have great
    potential because of their stability and simplicity and because a wide
    selection of methods can be used for subsequent analysis. Nitrogen
    dioxide can be quantitatively absorbed on columns packed with an inert
    material coated with triethanolamine without affecting the nitric
    oxide concentrations (Levaggi et al., 1972). The nitric oxide is
    subsequently adsorbed on a second column treated with cobalt (II)
    oxide.

    2.3.2  Evaluation of analytical methods

        Detailed description of the various methods has been omitted since
    they are discussed elsewhere (US Department of Health, Education and
    Welfare, 1965; US Environmental Protection Agency, 1971a; World Health
    Organization, 1976). Instead, a critical evaluation is given of the
    important methods used for measurements in the ambient air and in the
    health-effects studies discussed in subsequent sections.

        Nitric oxide and nitrogen dioxide may be measured separately or
    collectively by manual or automated techniques.

        The colorimetric manual methods are based on a specific reaction
    in which nitrite ions and diazotizing reagents produce a deeply
    coloured azo-dye (Mulik et al., 1974; Saltzman, 1954; US Environmental
    Protection Agency, 1971b). These methods can be automated to give mean
    concentrations over averaging periods of 15-60 min (Japanese Standards
    Association, 1974; Lyshkow, 1965: Saltzman, 1960).

        The chemiluminescence techniques are automatic and specific and
    have revolutionized the measurement of oxides of nitrogen. The method
    is based on the measurement of red light produced by the reaction
    O3 + NO -> O2 + NO2 + light. The major features are selective
    response to nitric oxide, sensitivity into the 1 g/m3 range,
    linearity over a factor of 105 in concentration, and rapid (<1 s)
    time response (Fontijn et al., 1970; Stedman et al., 1972). High cost,
    complexity, and the requirement of some form of data logging system if
    long-term mean values are required for averaging periods from 1 day to
    1 year are some drawbacks of these techniques. They are ideally suited
    for the measurement of peak concentrations over averaging periods of
    from 15 s to 1 h which are largely inaccessible with manual methods.

        Besides the two basic methods there are a large number of other
    methods for the measurement of nitric oxide and nitrogen dioxide. Gas
    chromatography, long-path infrared spectroscopy, and electrochemistry
    have been used but either they are generally cumbersome or they have
    comparatively low detection limits for use in atmospheric measurements
    (World Health Organization, 1976).

    2.3.2.1  Manual methods

        The most widely used manual methods for nitrogen dioxide are the
    Saltzman method and the Jacobs-Hochheiser method; detailed analytical
    procedures are described elsewhere (World Health Organization, 1976).
    Nitric oxide can also be measured by these methods if oxidized to
    nitrogen dioxide prior to analysis. Although various solid and liquid
    oxidizing agents have been proposed for the conversion of nitric oxide
    to nitrogen dioxide, they are not very reliable and tend to
    underestimate ambient nitric oxide concentrations (World Health
    Organization, 1976).

        Colorimetric procedures are complicated because of the time
    required for the colour to develop. This colour is not permanent and
    the Saitzman procedure, in particular, is not therefore recommended
    where the samples cannot be analysed after a short time delay. The
    Jacobs-Hochheiser method is a useful modification of the diazotization
    method, applicable to 24 h samples, which can be analysed up to 1
    month after collection (Jacobs & Hochheiser, 1958). However, this
    method has several deficiencies including a variable collection
    efficiency for nitrogen dioxide (Hauser & Shy, 1972).

        Various modifications of the Jacobs-Hochheiser method are being
    evaluated. One of these, the arsenite method (Christie et al., 1970)
    suffers from serious interference by nitric oxide (Merryman et al.,
    1973). The TGS-ANSAa method (Mulik et al., 1974) appears to
    eliminate all the deficiencies of the Jacobs-Hochheiser method.

        The Saltzman procedure has been used extensively in Europe, Japan,
    and the USA and has been tested by many workers. The method requires
    simple and inexpensive apparatus and its detection limit is adequate
    for most pollution studies. However, in view of the problems
    associated with the oxidation of nitric oxide to nitrogen dioxide, it
    is only suitable for measuring nitrogen dioxide. If nitrogen dioxide
    has to be monitored over periods of more than 2 h, or if the presence
    of relatively high concentrations of other oxidizing or reducing
    agents is suspected, a series of short-term samples (15-30 min each)
    should be collected and analysed as soon as possible. This may require
    some form of automatic sampling scheme.

    2.3.2.2  Automatic methods

        Continuous analysers based on the Saltzman procedure have been
    used extensively, but this should not be encouraged, since they are
    quite complicated, requiring excessive operator attention (World
    Health Organization, 1976). In addition, certain commercial versions
    suffer from interference by ozone (Baumgardner et al., 1975).
    Furthermore, since none of the proposed oxidizing agents for the
    nitric oxide-nitrogen dioxide conversion is wholly satisfactory under
    field conditions, continuous analysers based on the Saitzman procedure
    are suitable only for the measurement of nitrogen dioxide
    concentrations.

                 
    a TGS -- absorbing solution consisting of 20 g triethanolamine +
      0.5 g guaicol + 0.25 g sodium metabisulfite/litre of distilled water.

      ANSA -- reagent consisting of 0.1% 8-anilino-1-naphthalenesulfonic
      acid in absolute methanol.

        Chemiluminescence methods are ideally suited to the measurement of
    nitric oxide concentrations and are accurate and reproducible over a
    wide range of concentrations. There are no important sources of
    interference.

        Most commercial chemiluminescence analysers for nitric oxide are
    also equipped with some form of converter which reduces the nitrogen
    dioxide to nitric oxide before reaction with ozone to yield a combined
    measurement of nitric oxide and nitrogen dioxide. Considerable
    problems may occur in the mechanics of the subtraction of the nitric
    oxide signal from the combined nitric oxide-nitrogen dioxide signal
    when the nitric oxide concentrations are much higher than the nitrogen
    dioxide concentrations.

        Although the chemiluminescence determination of nitric oxide is
    interference-free, this is not always the case with the measurement of
    nitrogen dioxide. The conversion of atmospheric ammonia to nitric
    oxide can be eliminated by the appropriate choice of converter
    material and operating temperature. Ammonia derived from animal waste
    products may interfere with the determination of nitrogen dioxide
    exposures in the animal experiments described later. Certain nitrogen
    species such as nitric acid and peroxyacetylnitrate (PAN) decompose in
    most commercial thermal converters (Winer et al., 1974). This
    contributes minor interference during photochemical air pollution
    episodes.

    2.3.3  Calibration procedures

        There are 3 independent procedures for calibrating methods for
    measuring oxides of nitrogen. One technique involves the use of
    permeation tubes for nitrogen dioxide (Lindqvist & Lanting, 1972).
    Another technique is based on the gas-phase titration of nitric oxide
    with ozone which provides simultaneous calibration for nitric oxide,
    nitrogen dioxide, and ozone using the reaction:

                NO + O3 -> NO2 + O2(6)

        Finally, dynamic dilution can be used to prepare flowing mixtures
    of nitric oxide and nitrogen dioxide in air for calibration purposes
    (Japanese Standards Association, 1976).

        There has been much discussion in the literature concerning the
    "Saltzman factor" i.e. the conversion factor for sodium nitrite to
    nitrogen dioxide. This problem arises when the method is calibrated
    against standard solutions of nitrite ions, and can be obviated by the
    calibration methods discussed above. However, it must be borne in mind
    when interpreting literature data where this form of calibration has
    been used and a value assumed for the "Saltzman factor". This factor
    is usually about 0.72 (Forweg, 1975), but may vary with experimental
    conditions and concentration.

    3.  SOURCES OF OXIDES OF NITROGEN

    3.1  Natural Sources

        Nitric oxide and nitrogen dioxide present in the air are produced
    by natural processes including lightning, volcanic eruptions, and
    bacterial action in the soil, as well as by man-made activities. It
    has been estimated that the annual, natural global emissions of these
    oxides of nitrogen are of the order of 1100 million tonnes (Robinson &
    Robbins, 1972). This by far surpasses emissions of oxides of nitrogen
    generated by man-made activities which were estimated in 1970 to be
    approximately 53 million tonnes. However, since natural emissions are
    distributed over the entire globe, the resulting air concentrations
    are practically negligible.

    3.2  Man-made Sources

        The major source of man-made emissions of oxides of nitrogen is
    the combustion of fossil fuels. The predominant oxide of nitrogen
    emitted by combustion processes is nitric oxide; nitrogen dioxide is
    produced in much smaller amounts. The observed percentage of nitric
    oxide in the total emission of oxides of nitrogen is 90-95% by volume
    although it depends on a number of factors and varies substantially
    from one source to another.

        The distribution of emissions from different sources, in selected
    countries is shown in Table 3. Because emissions of oxides of nitrogen
    are extremely variable, these estimates provide only a general guide
    on the nature and magnitude of the more important sources. Generally,
    the differences between the various countries illustrated in Table 3
    can be readily accounted for by differences in fuel use. For example,
    in the Netherlands a significant fraction of the electricity
    generating plants use natural gas whereas this use of natural gas is
    negligible in the UK. This greater reliance on natural gas in the
    Netherlands may account for the much smaller relative contribution
    from stationary sources. Table 3 also indicates that transportation
    sources are relatively more significant in Japan and the USA than in
    the Netherlands or the UK.

        Projection of emission data into the future must be treated with
    some caution. It is evident, however, that in the absence of any
    abatement strategies, oxides of nitrogen emissions in most urban areas
    will increase steadily within the next decade (Organization for
    Economic Cooperation and Development, 1973). Prior to the energy
    crisis in the 1970s, oxides of nitrogen emissions had been expected to
    about double between 1968 and 1980. Such projections may require
    reevaluation in the light of changing patterns of fuel use.

    Table 3. Emissions of oxides of nitrogen from various sources in
    selected countries expressed as 106 tonnes per year
                                                                   

                               Japana  Netherlandsb  UKc    USAd
    Source
                               1972    1972          1970   1970
                                                                   

    Transportation             0.96    0.13          0.46   11.7
    Fuel combustion in     )
      stationary sources   )           0.19          1.98   10.0
    Non-combustion         )   1.44
      industrial processes )           --            --      0.2
    Miscellaneous              --      --            --      0.9

      Total                    2.40    0.32          2.43   22.8
                                                                   

    From:  a Central Council for Environmental Pollution Control
              (1977).
           b National Air Pollution Council, the Netherlands (1976).
           c Derwent & Stewart (1973).
           d US Environmental Protection Agency (1973).

    3.2.1  Stationary sources

        As shown in Table 3, stationary combustion sources in Japan, the
    Netherlands and the UK account for 60, 59, and 82% of total emissions,
    respectively. In the USA this figure is about 44%. These emissions
    include a substantial contribution from power generating plants. In
    the UK and the USA, these large power plants contribute 52% and 21%,
    respectively, of the total emissions (Derwent & Stewart, 1973; Mason
    et al., unpublished dataa).

        The combustion of fuel in the home makes only a minor contribution
    to total emissions of oxides of nitrogen. In the UK and the USA
    domestic fuel use accounts for only 5% and 6% of total emissions
    respectively.

        Fuel combustion by the commercial and industrial sectors provides
    a substantial source of oxides of nitrogen emissions in certain urban
    areas, particularly through space heating during the winter season.

    a Paper presented at the Sixty-second Annual Meeting, Air Pollution
    Control Association, June 1969, Paper No. 96-101, p. 19.

    3.2.2  Mobile sources

        Transportation sources include personal motor vehicles, buses,
    trucks, railroad vehicles, aircraft, and ships on inland waterways. Of
    these many categories, petrol-powered motor vehicles provide by far
    the largest contribution to total emissions. As a whole,
    transportation sources make a substantial contribution to the total
    emissions of the countries listed in Table 3. For Japan, the
    Netherlands, UK, and USA, transportation sources account for 40, 41,
    18, and 51% respectively, of total emissions.

    3.2.3  Non-combustion sources

        Although the total emissions from industrial processes (other than
    from fuel combustion) are relatively small, certain processes are
    significant local sources of oxides of nitrogen. Examples of these
    non-combustion sources include the manufacture of nitric acid,
    electroplating, and processes involving concentrated nitric acid such
    as the manufacture of explosives and the manufacture of sulfuric acid
    by the chamber process. The manufacture of nitric acid is usually the
    most significant of these non-combustion sources (Bagg, 1971).

        Bacterial degradation of silage material can be a significant
    source of oxides of nitrogen and has led to certain occupational
    hazards which are mentioned in section 6.

    3.2.4  Other sources

        Exposure to oxides of nitrogen from home appliances such as gas
    stoves and from tobacco smoking should not be underestimated. Exposure
    levels due to these sources are discussed in section 4.

    4.  ENVIRONMENTAL LEVELS AND EXPOSURES

    4.1  Background Concentrations

    Available data indicate that natural background levels of nitrogen
    dioxide over land areas range from 0.4 to 9.4 g/m3
    (0.0002-0.005 ppm) and those of nitric oxide from 0 to 7.4 g/m3
    (0-0.006 ppm) (Robinson & Robbins, 1972). In Panama, for example,
    Lodge & Pate (1966) found average nitrogen dioxide values ranging from
    1.7 g/m3 (0.0009 ppm) during the dry season to 6.8 g/m3
    (0.0036 ppm) in the rainy season. The natural background level of
    nitrogen dioxide in remote areas of Western Europe ranged from below
    2.0 to 4.2 g/m3 (below 0.0011 to 0.0022 ppm) (Georgii & Weber,
    1962). These concentrations are 1-2 orders of magnitude lower than
    those typically found in urban areas.

    4.2  Urban Concentrations

        Urban concentrations of nitric oxide, nitrogen dioxide, and oxides
    of nitrogen have been measured in a number of countries in recent
    years. The results have usually been reported as 1 h, 24 h, or annual
    averages. Annual average nitric oxide levels in large cities have been
    reported to range from 49 to 95 g/m3 (0.040-0.077 ppm) (Environment
    Agency, 1974: US Environmental Protection Agency, 1971a). An
    indication of long term average concentrations of nitrogen dioxide can
    be obtained from Table 4 where the annual mean concentrations are
    shown for selected urban areas. It is important to recognize that
    these concentrations did not necessarily represent the maximum
    exposure levels in these cities, since nitrogen dioxide concentrations
    vary greatly within a given urban area. It should also be remembered
    that different measurement methods were used in different countries
    and that these might have changed during the years tabulated.

    Thus, additional information and interpretation are required before
    comparisons between cities or determination of trends can be made.

    Table 4.  Annual mean concentrations of nitrogen dioxide in selected
    cities (g/m3)a
                                                                       

                          Washington,   Frankfurt/
    Year     Rotterdam    DC            Main           Tokyo
                                                                       

    1962                  56            19
    1963                  56            23
    1964                  75            28
    1965                  56            30
    1966     35                         47
    1967     43           75            34
    1968     43                         41
    1969     43                         63             77
    1970     45           94            82             73
    1971                  75            80             58
                                                                       

    aFrom: Commissie Bodem, Water en Lucht, 1970; Environment Agency,
      1976; Jost & Rudolph, 1975; US Environmental Protection Agency
      1962-1971.

        Tables 5 and 6 illustrate the observed short-term mean
    concentrations of nitrogen dioxide. In addition to annual means, Table
    5 presents the maximum 1-h, 24-h, and monthly mean concentrations of
    nitrogen dioxide recorded at selected sites in 5 cities in Japan
    (Environment Agency, 1974). Data on maximum 24-h concentrations and
    annual means for 5 US cities are given in Table 6 (US Environmental
    Protection Agency, 1976a). The maximum 24-h mean concentrations of
    nitrogen dioxide were generally within the range of 100-400 g/m3
    (0.054).22 ppm) and the maximum 1-h concentrations over 800 g/m3
    (0.43 ppm). The maximum 24-h mean value refers to the day of the year
    with the highest mean concentration. The maximum 1-h value refers to
    the highest 1-h value in the year and the maximum monthly to the
    highest monthly mean in the year.

    Table 5.  Nitrogen dioxide concentrations (g/m3) recorded in
    selected cities in Japan during 1973 using the Saltzman methoda
                                                                      

    Cityb      Annual   Maximum      Maximum     Maximum
               mean     1 -h value   24-h mean   monthly mean
    Sendai      46        240          134           60
    Tokyo       86        840          426          105
    Kawasaki    90        440          200          113
    Osaka       86        640          228          115
    Matsue      10         60           22           14
                                                                       

    a From: Environment Agency, 1974.
    b Densely populated, except Matsue.


    Table 6.  Annual mean and maximum 24-h nitrogen dioxide
    concentrations recorded in selected cities in the USA during
    1974 using the chemiluminescence methoda
                                                                     

    City               Annual mean    Maximum 24-h mean
                                                                     

    San Jose               67               285
    Philadelphia           73               166
    Washington, DC         68               130
    New York               80               243
    Chicago                47               114
                                                                     

    a From: US Environmental Protection Agency (1976a)

        Since most air pollutant concentrations are approximately
    log-normally distributed, a fairly consistent relationship can be
    established between annual averages and the averages calculated for
    shorter averaging periods (Larsen, 1969). For nitrogen dioxide, the
    maximum 24-h mean is about 2 5 times higher than the annual mean at a
    given site. The relationship of the maximum 1-h value to the annual
    mean is not as consistent. Available data show that the hourly maximum
    value is approximately 5-10 times the annual mean. This relationship
    does not hold for averaging periods of less than 1 h or for unusual
    situations. The model also appears to overpredict maximum monthly mean
    nitrogen dioxide concentrations. Examples of the seasonal variation in
    nitric oxide and nitrogen dioxide concentrations for selected sites in

    FIGURE 1


    FIGURE 2

    FIGURE 3

    the USA and Japan are shown in Fig. 1 and 2, respectively. These
    variations are caused by meteorological factors and to a lesser degree
    by seasonal changes in emission rates. Ambient temperature, wind
    speed, and inversion height are important factors affecting the
    dilution of air pollutants. In addition, variations in nitrogen
    dioxide production by the atmospheric chemical reactions discussed in
    section 2 play a substantial role in the seasonal changes observed. In
    the cities cited, mean winter nitrogen dioxide values were 2-3 times
    higher than summer concentrations. Considering the complexity of the
    factors involved, this observation is probably not universal and other
    sites may show the opposite trends.

        The frequency of occurrence of nitrogen dioxide hourly maximum
    concentrations in an urban area in the USA is shown in Table 7
    (California Air Resources Board, 1975). During the period cited, the
    hourly maximum concentration of nitrogen dioxide exceeded
    200 g/m3 (0.11 ppm) on more than 50% of the days. On one day a 1-h
    value of 839 g/m3 (0.46 ppm) was observed which is similar to the
    high 1-h value noted in Table 5 for Tokyo.


    Table 7. Distribution of hourly maximum concentrations of nitrogen
    dioxide during July-September 1975a
                                                                     

                   Number of days with hourly maximum
             in nitrogen dioxide concentration range (g/m3)
                                                                    

    Site         200-400    400-600    600-800     > 800
                                                                     

    Los Angeles    43         5          2           1
    Azusa          51         2          0           0
    Burbank        52         9          1           0
                                                                     

    a From: California Air Resources Board, 1975.

        An example of urban, diurnal, seasonal variations in nitric oxide
    and nitrogen dioxide concentrations is given in Fig. 3 with reference
    to Delft in the Netherlands (Guicherit, 1975).

        Features of interest include 2 peak concentrations of both nitric
    oxide and nitrogen dioxide found in the morning and evening which can
    be ascribed to the influence of automotive sources and occur typically
    on clear days. A time shift in the nitrogen dioxide peak is shown
    during spring and summer indicating increased photochemical conversion
    of nitric oxide into nitrogen dioxide.

        Stationary sources involving fuel combustion for space heating can
    also produce early morning peaks.

    4.3  Indoor Exposure

        Exposure to oxides of nitrogen in the home due to the use of
    gas-fired appliances is usually underestimated. The recent expansion
    in the use of natural gas may have increased this exposure.
    Measurements conducted by Schwarzbach (1975) concerning nitrogen
    dioxide formation by gas-fired domestic appliances such as space
    heaters, boilers, and cookers showed concentrations of up to
    2000 g/m3 (1.1 ppm) at breathing height in the immediate vicinity
    of cookers.

        The concentrations of nitrogen dioxide measured in a normally
    ventilated room using an oil-fired stove ranged from 380 to
    1700 g/m3 (0.2-0.9 ppm) depending on the type of stove and from 750
    to 940 g/m3 (0.4-0.5 ppm) when a gas-fired stove was used (Watanabe
    et al., 1966). Occupational exposure is discussed in section 6.2.

    4.4  Smoking

        Special mention must be made of the intense, deliberate exposure
    of man to oxides of nitrogen in tobacco smoke. Bokhoven & Niessen
    (1961) reported that tobacco smoke contained nitric oxide and nitrogen
    dioxide levels of 98-135 mg/m3 (80-110 ppm) and 150-226 mg/m3
    (80-120 ppm)b, respectively. This is equivalent to a nitric oxide
    intake of 160-500 g per cigarette (Horton et al., 1974).

        Haagen-Smit et al. (1959) made no distinction between nitric oxide
    and nitrogen dioxide in reporting oxides of nitrogen levels of
    145-655 ppm in tobacco smoke.

    5. EFFECTS ON EXPERIMENTAL ANIMALS

        A considerable amount of toxicological data is available relating
    exposure to nitrogen dioxide with a variety of respiratory effects.
    The purpose of this section is to review and summarize selected animal
    studies which are most relevant for the evaluation of the health
    hazards resulting from exposure to nitrogen dioxide.

        Very few studies have been reported on the effects of nitric oxide
    on experimental animals and, even in the most recent studies, the
    concentrations used have been much higher than ambient air levels
    (Greenbaum et al., 1967; Oda et al., 1975; Wagner, 1977, unpublished
    dataa). Thus, the following discussion has been almost entirely
    limited to studies on the effects of exposure to nitrogen dioxide.

    5.1  Local Effects on the Respiratory System

    5.1.1  Morphological changes

        There are several reports that describe alterations in the
    morphological integrity of the lung after exposure to nitrogen dioxide
    concentrations of 1900 g/m3 (1.0 ppm) and below. Salamberidze
    (1969) did not find any pathological or histological changes in rats
    exposed for 90 days to a nitrogen dioxide level of 100g/m3
    (0.05 ppm). However, electron microscopic studies by Buell (1970)
    revealed damage to insoluble collagen fibres isolated from the lungs
    of rabbits exposed to a nitrogen dioxide level of 470 g/m3
    (0.25 ppm) for 4 h/day, 5 days/week, for 24-36 days.

        Jakimcuk & Celikanov (1968) reported that continuous exposure of 
    rats to a nitrogen dioxide concentration of 600 g/m3 (0.32 ppm) for
    90 days resulted in morphological changes such as peribronchitis,
    bronchitis, and light pneumosclerosis. Similar studies with a nitrogen
    dioxide concentration of 150 g/m3 (0.08 ppm) did not produce
    significant changes.

                 

    a Report on research work performed under the USA/Federal Republic of
      Germany Cooperative Program in Natural Resources, Environmental
      Pollution and Urban Development. Institut for Wasser-, Boden-, und
      Lufthygiene des Bundesgesund-heitsamtes, 1977.

    b According to G. Freeman, the values given for nitrogen dioxide are
      too high. He considers levels of 19-95 mg/m3 (10-50 ppm) to be more
      probable (Personal communication, 1977).

        Inhalation of nitrogen dioxide concentrations of 1900 g/m3
    (1.0 ppm) for 1 h or 940 g/m3 (0.5 ppm) for 4 h led to significant
    morphological changes in the mast cells of the lung in rats (Thomas et
    al., 1967). In exposed animals, the cells were ruptured and there was
    evidence of loss of cytoplasmic granules. These changes, which were
    observed in the pleura, bronchi, and surrounding tissue with more
    marked effects around the mediastinum, were reversible in 24 h.

        Sherwin & Carlson (1973) found a relative increase in protein
    content in the lung lavage fluid of guineapigs continuously exposed to
    a nitrogen dioxide level of 750 g/m3 (0.4 ppm) for 1 week in
    comparison with that of control animals. While the meaning of the
    elevated protein levels is not yet clear, the authors believe that
    both protein leakage from the capillary bed and the increased rate of
    cell turnover within the exposed lung were responsible.

        Blair et al. (1969) exposed mice to a nitrogen dioxide
    concentration of 940 g/m3 (0.5 ppm) for 6, 18 and 24 h daily and
    studied the sequential alterations in lung morphology. After 3-12
    months, the alveoli were expanded in all exposed mice. The authors
    stated that the overall lesions appeared to be consistent with the
    development of early focal emphysema. Inflammation of the bronchioles,
    surface erosion of the epithelium, and blockage of the
    bronchiolar-alveolar junction were also observed.

        Continuous exposure of mice to nitrogen dioxide concentrations of
    940-1500 g/m3 (0.5-0.8 ppm) for 1 month produced numerous
    structural changes. These effects included proliferation of the
    epithelial cells in the mucous membrane; degeneration and ablation of
    mucous membranes: oedematous changes in alveolar epithelial cells:
    shortening of cilia; and influx of monocytes (Hattori et al., 1972;
    Nakajima et al., 1969). Chen et al. (1972) studied recovery processes
    after nitrogen dioxide exposure. Immediately following exposure to
    nitrogen dioxide levels of 1900-2800 g/m3 (1.0-1.5 ppm) for 1
    month, the histological changes in exposed mice were identical to
    those reported above. However, when the animals were allowed to
    recover in clean air for 1-3 months there was a pronounced
    infiltration of lymphocytes around the brochioles which was not found
    in mice killed either during or immediately following exposure to
    nitrogen dioxide. The authors suggested that this response resembled
    those of an autoimmune disease.

        Freeman et al., (1966) found slight bronchiolar, epithelial
    hypertrophy and the development of a moderate degree of tachypnea in
    rats continuously exposed for 33 months (approximately natural
    life-time) to a nitrogen dioxide level of 1500 g/m3 (0.8 ppm). The
    authors repeated these long-term exposure studies at a concentration
    of 3800 g/m3 (2.0 ppm). (Freeman et al., 1968a, 1968b, 1969;

    Freeman, 1970; Stephens et al., 1971a, 1971b, 1972). Exposure of rats
    to this concentration of nitrogen dioxide resulted in a number of
    microscopic and ultrastructural changes in the terminal bronchioles,
    alveolar ducts, and alveoli. The lungs were about 10% heavier than
    normal and the animals continued to exhibit tachypnea. There was
    homogeneous and uniform hypertrophy of the bronchiolar epithelium,
    loss of bronchiolar cilia, depression of natural cellular exfoliation,
    and blebbing of bronchiolar cells. Intra-cytoplasmic, crystalloid
    inclusion bodies appeared later. Electron microscopy revealed
    thickening of lung collagen fibrils and of the alveolar basement
    membranes.

        Cell renewal rates were also studied in rats exposed to nitrogen
    dioxide (Evans et al., 1972, 1973a, 1973b, 1975), by measuring the
    uptake of tritiated thymidine by actively dividing alveolar cells.
    Continuous exposure to 3800 g/m3 (2.0 ppm) caused a marked increase
    in number of type 2 alveolar cells. The labelling index reached a
    maximum at 48 h and by the seventh day had returned to its normal
    baseline level.

        Monkeys  (Macaca speciosa) exposed continuously for 14 months to
    a nitrogen dioxide concentration of 3800 g/m3 (2.0 ppm) developed
    hypertrophy of the bronchiolar epithelium. Mixing an aerosol of sodium
    chloride at 330 g/m3 with the nitrogen dioxide did not appear to
    alter the response (Furiosi et al., 1973). Several species of
    laboratory animals were also exposed to nitrogen dioxide levels of
    19 mg/m3 (10.0 ppm) or more in order to evaluate effects which could
    possibly lead to chronic obstructive pulmonary disease. At nitrogen
    dioxide levels of 19-47 mg/m3 (10-25 ppm) for 26 and 13 weeks
    respectively, rats developed large, air-filled lungs that did not
    collapse under atmospheric pressure. The lungs became grossly
    emphysematous and the thoracic cage enlarged with dorsal kyphosis
    (Freeman & Haydon 1964; Freeman et al., 1968a, 1968b, 1969).
    Connective tissue changes involving both collagen and elastic tissue
    were observed. Animals began to die of respiratory failure after 16
    months.

        For further information and for detailed descriptions of
    morphological effects at these high concentrations the following
    publications are suggested: Freeman & Haydon (1964); Kleinerman &
    Cowdrey (1968); Kleinerman & Wright (1961); Parkinson & Stephens
    (1973).

    5.1.2  Functional changes

        Both short-term and long-term exposure to concentrations of
    nitrogen dioxide exceeding 1500 g/m3 (0.8 ppm) have been reported
    to cause changes in pulmonary function.

        Rats exposed for 990 days to a nitrogen dioxide concentration of
    1500 g/m3 (0.8 ppm) maintained elevated respiratory rates
    throughout their life (Freeman et al., 1966; Haydon et al., 1965).

        Beagles exposed daily to a mixture of nitrogen dioxide and nitric
    oxide at approximate levels of 1210 g/m3 (0.64 ppm) and 310 g/m3
    (0.25 ppm) respectively, for 61 months demonstrated reductions in both
    diffusion capacity and peak expiratory flow rates (Lewis et al.,
    1974). However, exposure to mixtures of nitrogen dioxide at
    940-1900 g/m3 (0.5-1.0 ppm) and nitric oxide at 250 g/m3
    (0.2 ppm) for 16 h per day, for 72 weeks, did not result in any
    changes in carbon monoxide diffusion capacity, compliance, or total
    expiratory resistance to airflow (Vaughan et al., 1969).

        Neither transpulmonary resistance nor compliance was affected in
    rats exposed to 3800 g/m3 (2.0 ppm) throughout their lifetimes
    (approx. 2 years) although tachypnoea was consistently present
    (Freeman et al., 1968c). Nonhuman primates also breathed more rapidly
    when exposed for over 7 years to nitrogen dioxide levels of
    3800 g/m3 (2.0 ppm) (Freeman & Juhos, 1976, Freeman et al., 1969)
    or to 9400 g/m3 (5.0 ppm) for 2 months (Henry et al., 1970). In the
    latter study, tidal volumes were also significantly reduced.

        Guineapigs exposed to 9400 g/m3 (5 ppm) for 7 h per day, 5
    days per week for 5 months showed no changes in expiratory flow
    resistance (Balchum et al., 1965). Murphy et al., (1964) exposed
    guineapigs to 9800 g/m3 (5.2 ppm) for 4 h and recorded increased
    respiratory rate and decreased tidal volumes. Pulmonary function
    returned to normal when the animals were returned to clean air.

        Rats exposed to a nitrogen dioxide level of 5500 g/m3
    (2.9 ppm) for 24 h each day, 5 days per week for 9 months showed a
    significant decrease (13%) in lung compliance compared with controls
    (Arner & Rhoades, 1973). However, Wagner and co-workers (1965) were
    unable to detect any significant effects in rabbits exposed to a
    nitrogen dioxide concentration of 9400 g/m3 (5 ppm) for 6 h daily
    over a period of 18 months.

        Davidson et al. (1967) exposed rabbits for 24 h/day for 3 months
    to nitrogen dioxide levels of 15-22.6 mg/m3 (8-12 ppm) and observed
    reversible increases in non-elastic resistance and in functional
    residual capacity as well as a diminution in compliance.

        The dose-effect relationship was studied in the lungs of rats and
    cats exposed to nitrogen dioxide concentrations of 940-38 000 g/m3
    (0.5-20 ppm) (Zorn, 1975). A tendency towards an increase in
    respiratory rates and a decrease in arterial oxygen pressure was shown
    at concentrations as low as 1900 g/m3 (1.0 ppm). A single 2-h

    exposure to nitrogen dioxide levels of 19, 28, 66, and 94 mg/m3 
    (10, 15, 35, and 50 ppm) affected the pulmonary function in squirrel
    monkeys. At concentrations of 19-28 mg/m3 (10-15 ppm) the tidal
    volume decreased with little change in respiratory rate (Henry et
    al., 1969).

    5.1.3  Biochemical effects

        Nakajima & Kusumoto (1968) reported an initial reduction in the
    quantity of reduced glutathione in the lung and liver of mice exposed
    to a nitrogen dioxide concentration of 1500 g/m3 (0.8 ppm)
    continuously for 5 days. On the fifth day, the level of glutathione
    approached normal and was no longer significantly different from that
    of the controls. However, with continuous exposure over 6 months the
    animals tended to lose weight and the glutathione level fell once more
    (Nakajima et al., 1969, Nakajima, 1973).

        Chow et al. (1974) observed a rise in glutathione peroxidase
    (1.11.1.9) activity in the lungs of rats exposed to nitrogen dioxide
    concentrations of 1900 and 4300 g/m3 (1.0 and 2.3 ppm) for 4 days.
    At about 12 mg/m3 (6.2 ppm) there was a significant increase in the
    activities of glutathione reductase (NAD(P)H) (1.6.4.2) and
    glucose-6-phosphate dehydrogenase (1.1.1.49) in comparison with the
    controls. The authors believe that alterations in such enzyme systems
    are sensitive and specific bioindicators of tissue damage.

        Fukase et al. (1976) reported that exposure to nitrogen dioxide at
    about 11 mg/m3 (6 ppm) for 4 h every day for 30 days caused an
    increase in glutathione reductase (NAD(P)H) (1.6.4.2) and
    glucose-6-phosphate dehydrogenase (1.1.1.49) activities. Exposure to a
    nitrogen dioxide level of 28 mg/m3 (15 ppm) for 7 days resulted in a
    significant increase in glutathione levels. Exposure to 53 mg/m3
    (28 ppm) for 7 days resulted in significant increases in glutathione
    levels and in the activities of glutathione reductase (NAD(P)H)
    (i.6.4.2), glucose-6-phosphate dehydrogenase (1.1.1.49) and
    glutathione peroxidase (1.11.1.9).

        Biochemical evidence indicating that on exposure to nitrogen
    dioxide there is a proliferation of type 2 alveolar cells to replace
    the injured type I cells in lung tissue has been submitted by Sherwin
    et al. (1972). In these investigations, guineapigs were exposed to a
    concentration of 3800 g/m3 (2 ppm) continuously for 1-3 weeks; a
    significant increase occurred in the lactate dehydrogenase
    (cytochrome) (1.1.2.3) index of the lower lobes.

        Oxygen consumption and lactate dehydrogenase (cytochrome)
    (1.1.2.3), and aldolase (4.1.2.13) activity levels were all elevated
    in lung, liver, kidney, and spleen tissue following short-term and
    long-term exposure of guineapigs to nitrogen dioxide (Buckley &
    Balchum 1965, 1967a, 1967b). In the short-term treatment the
    guineapigs exposed to 75 mg/m3 (40 ppm) for a total of 4 h were
    killed 2 h after treatment. The long-term treatment included exposure
    to 28 mg/m3 (15 ppm) continuously for 10 weeks. The mechanisms
    involved in these changes have not yet been indentified but they may
    reflect an acute response to stress.

        By applying the disc electrophoresis method, Sherwin & Carlson
    (1973) demonstrated higher protein levels in the lavage fluid of
    guineapigs exposed for 1 week to a nitrogen dioxide concentration of
    750 g/m3 (0.4 ppm).

        Thomas et al. (1968) found that short-term exposure (4 h) to
    1900 g/m3 (1.0 ppm) resulted in the lipoperoxidation of lung lipids
    in rats. Rats fed on a vitamin E-deficient diet and then exposed to
    nitrogen dioxide had more peroxidation of surfactant and tissue lipids
    than did rats on a vitamin E-supplemented diet (Roehm et al., 1971).
    Anti-oxidants appeared to serve as protection against peroxidation and
    free radical formation (Menzel et al., 1972).

        Arner & Rhoades (1973) reported a significant decrease (8.7%) in
    the lung lipid content of rats exposed to a nitrogen dioxide
    concentration of 5500 g/m3 (2.9 ppm) for 9 months and also a marked
    decrease in the percentage of total saturated phospholipid fatty
    acids. This reduction in saturation was primarily due to a decrease in
    the percentage of hexadecanoic (palmitic) acid. There were also
    significant changes in the surface properties of the lung washings
    from animals exposed to nitrogen dioxide indicating an increase in
    surface tension and a decrease in stability of the pulmonary
    surfactant. Continuous exposure for 14 days to a nitrogen dioxide
    concentration of 9400 g/m3 (5.0 ppm) markedly decreased the
    lecithin turnover rate in rat lungs (Thomas & Rhoades, 1970). The
    authors suggested that the pulmonary phospholipid synthesis might be
    altered by nitrogen dioxide exposure.

    5.2  Other Effects

        Although the primary target for nitrogen dioxide exposure is the
    lung, data are accumulating that indicate that it may effectively
    alter a wide range of other systems.

    5.2.1  Effects on growth and body weight

        Numerous investigators have measured the growth rate of animals
    during exposure to nitrogen dioxide and have produced conflicting
    results. Body weights of rats and hamsters were reported to be
    significantly lower than those of the controls in studies by: Kaut el
    al. (1966) with a nitrogen dioxide exposure of 5000 g/m3 (2.7 ppm)
    for 8 weeks; Freeman & Haydon (1964) with a concentration of
    24 mg/m3 (12.5 ppm) for 213 days; and Kleinerman & Rynbrandt (1976)
    with a concentration of 38 mg/m3 (20 ppm) for 24 h. Other
    investigators using guineapigs, hamsters, mice, rabbits, dogs, and
    rats failed to find any such effects (Nakajima et al. (1969) with
    nitrogen dioxide exposures of 1300 1500 g/m3 (0.7 0.8 ppm) for 1
    month; Salamberidze (1969) with a concentration of 100 g/m3
    (0.05 ppm) for 90 days; and Wagner et al. (1965) with a concentration
    of 1900-47 000 g/m3 (1.0-25 ppm) for 18 months).

    5.2.2  Immunological effects

        Nitrogen dioxide exposure seems to alter the immunological
    reaction of experimental animals. Continuous exposure to
    9400 g/m3 (5.0 ppm) for 3-5 months appeared to depress the squirrel
    monkey's ability to form protective serum neutralizing antibodies.
    However, when monkeys were exposed for 16 months to a nitrogen dioxide
    concentration of 1900 g/m3 (1.0 ppm), the exposed animals
    consistently showed higher serum neutralizing antibody titres than the
    control (Ehrlich & Fenters, 1973; Fenters et al., 1971). The same
    workers (Ehrlich et al., 1975) exposed mice to 940 g/m3 (0.5 ppm)
    continuously while superimposing a 1-h peak of 3800 g/m3 (2.0 ppm)
    each day over a period of 3 months in order to determine if this
    exposure produced changes in the circulating immunoglobulins.
    Non-vaccinated mice showed a marked decrease in levels of IgA and an
    increase in serum IgM, IgG1, and IgG2. These investigators also
    measured haemagglutination-inhibition (HI) and serum neutralization
    (SN) antibody formation and found that nitrogen dioxide depressed the
    SN antibody formation but did not alter the HI titres.

        Contrary to these findings, Antweiler et al. (1975) did not find
    any alteration in the ability of the guineapig to produce antibodies,
    even after 33 days of exposure to 10 mg/m3 (5.3 ppm).

        A month of continuous exposure to a nitrogen dioxide level of
    1700 g/m3 (0.9 ppm) reduced the ability of the mouse spleen to
    produce primary antibodies (Nakamura et al., 1971).

        Balchum et al. (1965) reported a circulating substance in the
    serum of guineapigs exposed to nitrogen dioxide which had properties
    similar to a lung antibody. This substance reacted  in vitro with
    proteins extracted from the lung tissue of control animals. The titres
    of this reactive substance increased with the intensity and duration
    of exposure to nitrogen dioxide. Exposure concentrations were 9400 and
    28 000 g/m3 (5 and 15 ppm) for periods of up to 1 year.

    5.2.3  Haematological effects

        Salamberidze (1969) studied the effects of continuous exposure
    (24 h/day) of rats to a nitrogen dioxide concentration of 100 g/m3
    (0.05 ppm) for periods of up to 90 days. The author did not find any
    effects on haemoglobin or erythrocytes.

        Polycythemia, with reduced mean corpuscular volume but a normal
    mean corpuscular haemoglobin concentration was found in rats and
    monkeys  (Macaca speciosa) following continuous exposure for 3 months
    to 3800 g/m3 (2.0 ppm) (Furiosi et al., 1973). Mitina (1962)
    reported leucocytosis in rabbits exposed to nitrogen dioxide
    concentrations of 2400-5700 g/m3 (1.3-3.0 ppm) for 2 h per day for
    15 and 17 weeks. This effect was followed by a phagocytic depression
    of circulating leucocytes. The leucocytic response was accelerated by
    the presence of sulfur dioxide.

        Carson et al. (1962) exposed dogs to nitrogen dioxide
    concentrations of 73-310 mg/m3 (39-164 ppm) for periods ranging from
    5 min to 1 h. They did not find any changes in haematocrit or blood
    platelet counts 4, 24, 48, or 72 h after exposure. Wagner et al.
    (1965) did not find any haematological effect when dogs were exposed
    for 18 months to nitrogen dioxide concentrations of 1900 or
    9400 g/m3 (1 or 5 ppm).

        The addition of nitrogen dioxide at concentrations of
    940-1500 g/m3 (0.5-0.8 ppm) to carbon monoxide at 58 mg/m3
    (50 ppm) failed to affect the carboxyhaemoglobin concentrations in the
    blood of mice in studies reported by Nakajima & Kusumoto (1970).

        The chemical action of nitrogen dioxide on the circulating
    erythrocyte  in vivo is poorly understood. Although
    methaemoglobinaemia could possibly result from exposure to low levels
    of nitrogen dioxide, evidence confirming this is not available. Wagner
    (1977, unpublished data a) could not find a detectable increase in
    the concentration of methaemoglobin in rats exposed to 9400 g/m3
    (5 ppm) for as long as 10 months. However, exposure to 19 mg/m3
    (10 ppm) for 1 h induced a significant increase in the concentration
    of methaemoglobin in arterial blood. Nakajima & Kusumoto (1968) did
    not find any increase in the concentration of methaemoglobin in the
    blood of mice continuously exposed to a nitrogen dioxide concentration
    of 1500 g/m3 (0.8 ppm) for 5 days.

    5.2.4  Miscellaneous biochemical effects

        Continuous exposure of rats to a nitrogen dioxide concentration of
    100 g/m3 (0.05 ppm) for 90 days did not produce any effects on the
    activities of cholinesterase (3.1.1.8), catalase (1.11.1.6), and
    SH-groups in blood (Salamberidze, 1969).

        Veninga & Lemstra (1975) reported that a single 2-h exposure to
    560-7500 g/m3 (0.3-4.0 ppm) produced elevated levels of ascorbic
    acid in the liver of mice.

        Kosmider & Misiewicz (1973) exposed guineapigs to a nitrogen
    dioxide concentration of 1900 g/m3 (1.0 ppm), continuously, for a
    total of 180 days. The authors observed increased aminotransferase
    activity in the blood serum and heart homogenates but decreased levels
    in the brain and in the liver. No significant alterations were evident
    in the basic alkaline phosphatase (3.1.3.1) and magnesium-activated
    phosphatase activities in the blood serum of dogs exposed to nitrogen
    dioxide levels of either 1900 or 9400 g/m3 (1 or 5 ppm) for 18
    months (Wagner et al., 1965).

        However, Wagner (1972) found an elevation in serum cholesterol in
    rats after continuous exposure to 9400 g/m3 (5 ppm) for
    1 year.

        Drozdz and co-workers (1973, 1974, 1975) measured the effects of 6
    months continuous exposure to a nitrogen dioxide concentration of
    2000 g/m3 (1.1 ppm) on the guineapig. They studied alterations in
    the activity of various enzymes in both the blood and liver as well as
    in the central nervous system. The experiments demonstrated that
    prolonged exposure to nitrogen dioxide led to disturbances in the
    levels of glucose, lactic acid, total lipids, seromucoids, hexoses,
    hexamines, and sialic acid. Kaut et al. (1966) reported a decrease in
    the albumin/globulin (A/G) quotient as well as in the vitamin C
    content of the suprarenal glands, in rats exposed to a nitrogen
    dioxide concentration of 5000 g/m3 (2.7 ppm) for 2-8 weeks, 6 h per
    day, 5 days per week.

        Continuous exposure to high nitrogen dioxide concentrations of
    28-216 mg/m3 (15 115 ppm) produced numerous systemic biochemical
    changes. Nitrogen dioxide has been reported to produce increases in:
    serum protease inhibitor activity; plasma corticosterone levels;
    oxygen consumption in the spleen and kidney; lactate dehydrogenase
    (cytochrome) (1.1.2.3) activity in liver and kidney; and aldolase
    (4.1.2.13) activity in the liver, kidney, spleen, and serum (Buckley &

                 

    a See footnote, p. 32.

    Balchum, 1965; Kleinerman & Rynbrandt, 1976; Tusl, 1975). Svorcova &
    Kaut (1971) reported an elevation in urinary nitrites and nitrates in
    rabbits immediately after exposure for 15 min to a concentration of
    45 mg/m3 (23.9 ppm). It is possible that this indicates that
    nitrogen dioxide is rapidly converted to nitrite and nitrate ions and
    that these ions are excreted in the urine shortly afterwards.

    5.2.5  Effects on reproduction

        Salamberidze & Cereteli (1971) studied changes in the female rat's
    reproductive and endocrine systems resulting from exposure to
    2360 g/m3 (1.3 ppm), 12 h per day for 3 months. There was a
    prolongation of the estrus cycle associated with an increased
    interestrual period and a decrease in the number of monthly cycles.
    The litter size and the fetal weights decreased but the capacity for
    pregnancy was not affected. These effects may reflect the effect of
    nitrogen dioxide exposure on endocrine or reproductive function.

    5.2.6  Effects on the central nervous system

        Salamberidze (1969) did not find any effects on the central
    nervous system in rats exposed to a nitrogen dioxide concentration of
    100 g/m3 (0.05 ppm) for 90 days.

        At a slightly higher concentration, Jakimcuk & Celikanov (1968)
    reported a significant delay in the conditioned reflexes of the
    central nervous system in rats after 90 days of exposure to a nitrogen
    dioxide concentration of 600 g/m3 (0.32 ppm).

    5.2.7  Behavioural changes

        Murphy et al. (1964) reported that the voluntary running activity
    of male mice was depressed when the concentration of nitrogen dioxide
    reached 14 mg/m3 (7.7 ppm) and the animals were exposed for 6 h. A
    similar loss in activity was reported by Tusl et al. (1973). They
    measured the influence of nitrogen dioxide on the performance of rats
    during physical exertion as measured by swimming. In rats exposed to
    9400 g/m3 (5.0 ppm), a decrease of 25% in performance occurred in
    the fifth to sixth week of the experiment. Animals exposed to
    1900 g/m3 (1.0 ppm) also showed a tendency towards decreased
    performance, although this was not statistically significant.

    5.2.8  Carcinogenicity, mutagenicity, and teratogenicity

        In order to study the possible formation of nitrosamines by the
    reaction of nitrogen dioxide with tissue amines, mice were exposed to
    a nitrogen dioxide concentration of 75 mg/m3 (40 ppm) for periods up
    to 1 years. Although proliferative alterations at the terminal
    bronchioles were always present, no carcinomas were found in the lungs
    of these animals (Henschler & Ross, 1966).

        Kaut (1970) analysed lung tissue to detect nitro- and
    nitroso-compounds, especially nitrosamines in white rats exposed to
    mixtures of oxides of nitrogen ranging from 5 to 250 ppm for 3 h. The
    compounds were found  in vitro in tissues exposed to high
    concentrations of oxides of nitrogen, but not  in vivo.

        Rats were exposed for periods ranging from 2 months to a lifetime
    (2 years) to automotive exhaust gas containing carbon monoxide, oxides
    of nitrogen, carbon dioxide, and aldehydes at concentrations of
    58 mg/m3 (50 ppm), 23 ppm, 6700 mg/m3 (3700 ppm), and 2.0 ppm,
    respectively. According to the author, spontaneous tumours and
    abscesses were more frequent in the group exposed to the gas than in
    the control group but none occurred in the lung tissue (Stupfel et al.
    1973).

        These studies cannot be considered to provide any evidence of the
    carcinogenic effect of oxides of nitrogen.

        No evidence is available on the mutagenicity and teratogenicity of
    oxides of nitrogen  per se, but nitrous acid has been reported to be
    mutagenic in some laboratory tests.

    5.3  Interaction of Nitrogen Dioxide and Infectious Agents

        The influence of nitrogen dioxide on susceptibility to respiratory
    infection and its adverse effect on the pulmonary defence system of
    the host has been clearly demonstrated in several species of animals.

        Ehrlich (1966) and Henry et al. (1969) showed that exposure of
    mice, hamsters, and squirrel monkeys to nitrogen dioxide made them
    more vulnerable to respiratory infection with  Klebsiella
     pneumoniae, the mouse being the most sensitive. With a 2-h exposure,
    the minimum concentration of nitrogen dioxide required to produce a
    significant rise in the mortality rate was 6600 g/m3 (3.5 ppm). No
    effect was observed at 4700 g/m3 (2.5 ppm). However, when mice were
    exposed continuously for 1 year to 940 g/m3 (0.5 ppm), a
    statistically significant increase in mortality rate occurred after 90
    days (Blair et al., 1969; Ehrlich & Henry, 1968).

        Ito et al. (1971) exposed female mice to nitrogen dioxide
    concentrations of 940-1900 g/m3 (0.5-1.0 ppm) continuously for 39
    days and studied the influence of the nitrogen dioxide on infection
    with influenza virus histopathologicaily. Advanced interstitial
    pneumonia and adenomatous proliferation in the epithelium of the
    peripheral bronchi were noted. Intermittent exposure to 19 mg/m3
    (10.0 ppm) for 2 h daily, for 5 days, also significantly increased the
    susceptibility of mice to influenza virus infection as demonstrated by
    increased mortality.

        Motomiya et al. (1972) studied the interaction of nitrogen dioxide
    and the influenza virus. They reported a high incidence of adenomatous
    proliferation of peripheral, bronchial, epithelial cells in mice
    exposed for 3 months to nitrogen dioxide levels of 560-940 g/m3
    (0.3-0.5 ppm) followed by infection with the influenza virus. The
    effect was more serious than that seen in infected mice kept in clean
    air. Continuous exposure for an additional 3 months did not enhance
    the effect.

        Continuous exposure of squirrel monkeys to nitrogen dioxide levels
    of 9400 g/m3 and 19 mg/m3 (5 and 10 ppm) for 1 or 2 months
    increased their susceptibility to both bacterial and viral infections.
    All the animals exposed to 19 mg/m3 (10 ppm) died within 2-3 days of
    infection with the influenza virus. At 9400 g/m3 (5 ppm) 1 of the 3
    experimental monkeys died. All control monkeys had symptoms of vital
    infection but no deaths occurred. When the nitrogen dioxide exposed
    monkeys were challenged with Klebsiella pneumoniae, 2 out of 7 monkeys
    exposed to 9400 g/m3 (5 ppm) for 2 months died and the remainder
    had the infectious agent in the lungs at autopsy. At 19 mg/m3
    (10 ppm) for 1 month 1 of 4 monkeys died and 2 had the infectious
    agent in the lungs at autopsy (Henry et al., 1969, 1970). Both
    squirrel monkeys and hamsters showed a reduction in resistance to
     Klebsiella pneumoniae after a single 2-h exposure to nitrogen
    dioxide concentrations of 66-75 mg/m3 (35-40 ppm) (Ehrlich 1966).

        Gardner et al. (1977) and Coffin et al. (1976) studied the
    time-dose-response for nitrogen dioxide --  Streptococcus pyogenes
    interaction. Mice were exposed to nitrogen dioxide concentrations
    ranging from 940 g/m3-53 mg/m3 (0.5 28 ppm) for various periods
    of time ranging from 10 min to 12 months before treatment with the
    bacterial aerosol. When comparisons were made, it was evident that the
    mortality rate increased with increasing concentrations of nitrogen
    dioxide. Different relationships between concentration and time
    produced significantly different mortality responses. The data suggest
    that concentration has a greater influence on the effect of nitrogen
    dioxide than length of exposure.

        Studies were also conducted to compare continuous with
    intermittent exposure to nitrogen dioxide at concentrations of 2800
    and 6600 g/m3 (1.5 and 3.5 ppm). There was a significant increase
    in mortality rate for each of the experimental groups with increasing
    duration of exposure. When the data were adjusted for the total
    difference in concentration x time, the mortality rate was essentially
    the same for both groups.

        There are numerous studies that show that nitrogen dioxide is
    injurious to specific pulmonary defence systems in the lung. It
    interferes with the efficiency of clearing inhaled particles including
    bacteria and viruses from the airway and with the phagocytosis and
    digestion of such particles by the alveolar macrophage. As this is the
    major defence against infection by inhalation, any alteration in this
    system would increase the risk of infection (Green & Kass, 1964; Kass
    et al. 1966).

        Aranyi & Ehrlich (1973, unpublished dataa) isolated alveolar
    macrophages from mice continuously exposed to a nitrogen dioxide
    concentration of 4700 g/m3 (2.5 ppm) for 3 h daily, 5 days per
    week, for one month. Scanning electron microscopy revealed changes in
    the surface of these cells and a reduction in the ability of the cells
    to phagocytize  Escherichia coli, in vitro.

        Goldstein et al. (1973) measured the effect of nitrogen dioxide on
    antibacterial activity in the mouse. Pulmonary bactericidal activity
    decreased progressively with exposure to increasing concentrations of
    nitrogen dioxide. This defect was present in animals exposed for 4 h
    to concentrations of nitrogen dioxide of 13 mg/m3 (7 ppm) or more.
    With exposure for 17 h, this bactericidal dysfunction occurred at
    levels as low as 4300 g/m3 (2.3 ppm). These reports are consistent
    with the earlier report of Gardner et al. (1969) who obtained
    macrophages from the lungs of rabbits after  in vivo phagocytosis and
    found a pronounced inhibition of phagocytic activity (50%) after a 3-h
    exposure to nitrogen dioxide at 19 mg/m3 (10 ppm). A 2-h exposure to
    15 mg/m3 (8.0 ppm) increased the proportion of polymorphonuclear
    leucocytes in the lavage fluid. This condition persisted for more than
    72 h after the cessation of exposure.

        Acton & Myrvik (1972) found that rabbits exposed to nitrogen
    dioxide concentrations of 28 mg/m3 (15.0 ppm) for a short period of
    time (3 h) had alveolar cells with a lower capacity to develop
    virus-induced resistance and to phagocytize BCG vaccine. Valand et al.
    (1970) demonstrated that macrophages washed from the lungs of rabbits
    exposed for 3 h to a nitrogen dioxide concentration of 47 mg/m3
    (25 ppm) and injected with parainfluenza-3 virus failed to develop
    resistance to rabbit pox virus. The alveolar macrophages obtained from
    these exposed animals failed to produce interferon.

        Buckley & Loosli (1969) exposed mice for 6 weeks to a nitrogen
    dioxide concentration of 71 mg/m3 (38.0 ppm) and were unable to
    detect any alteration in the rate of clearance of an aerosol of
    staphylococci.

                 

    a Illinois Institute of Technology Research Institute Report No.
      L6070-2, EPA Contract No. 68-02-0761.

        The second mechanism of host defence is the mechanical or physical
    removal of inhaled and deposited particles by means of the mucociliary
    escalator.

    Giordano & Morrow (1972) determined that exposure to a nitrogen
    dioxide concentration of 11 mg/m3 (6 ppm) continuously for 6 weeks
    depressed this mucociliary transport.

    5.4  Summary Table

        Experimental animal studies which provide useful quantitative
    information for the establishment of guidelines for the protection of
    public health with respect to nitrogen dioxide exposure are summarized
    in Table 8.


        Table 8  Experimental animal studies

    I. Local effects on the respiratory system
                                                                                                                                                

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

    1900        1.0         4        24       glutathione peroxidase                              rat        5 (11)     Chow et al. (1974)
                                              (1.11.1.9) activity significantly
                                              increased in lung

    1900        1.0         1        4        peroxidation of lung lipids                         rat        6 (10)     Thomas et al. (1968)

    1500        0.8         990      24       elevated respiratory rates            9/9 (0/12)    rat        9 (12)     Freeman et el. (1966)
                                              throughout their lives

    1500        0.8         990      24       minimal bronchiolar epithelial        9/9 (0/12)    rat        9 (12)     Freeman et al. (1966)
                                              hypertrophy

    1500        0.8         5        24       significant decrease in lung                        mouse      10 (10)    Nakajima & Kusumoto
                                              reduced glutathione                                                       (1968)

    940-1500    0.5-0.8     30       24       degeneration and desquamation         10/10 (0/5)   mouse      10 (5)     Nakajima et al (1969)
                                              of mucous membrane;
                                                                                                                                                

    I. Local effects on the respiratory system cont'd
                                                                                                                                                

    Nitrogen dioxide
    concentration           Length of
                            exposure
                                               Effects                               Responsea     Species    Number     Reference

    (g/m3)     (ppm)       (number  (h/day)                                                                    of
                            of                                                                                  animals
                            days)
                                                                                                                                                

    940-1500    0.5-0.8     30       24       shortening and reduction of the       6/10 (0/5)    mouse      10 (5)     Hattori et al. (1972)
                                              cilia of ciliated epithelial cells;
                                              oedematous change of alveolar
                                              epithelial cells
                                              proliferation of epithelial cells
                                              of the peripheral bronchus
                                              (adenomatous changes)

    1200 plus   0.64 plus   61       16       reduction in pulmonary                6/11 (3/8)    dog        11 (18)    Lewis et al. (1974)
    310 nitric  0.25        months            diffusion capacity
    oxide       nitric
                oxide

    1200 plus   0.64 plus   61       16       decreased peak expiratory flow        5/11 (1/8)    dog        11 (18)    Lewis et al. (1974)
    310 nitric  0.25        months            rate
    oxide       nitric
                oxide

    940 plus    0.5 plus    18       16       no changes in carbon                  0/12 (0/20)   dog        12 (20)    Vaughan et al.(1969)
    250 nitric  0.2         months            monoxide diffusion capacity,
    oxide       nitric                        compliance, or total expiratory
                oxide                         resistance

    940         0.5         90-360   6,18,24  evidence of focal emphysema           12/12 (0/4)   mouse      12 (4)     Blair et al. (1969)
                                                                                                                                                

    I. Local effects on the respiratory system cont'd
                                                                                                                                                

    Nitrogen dioxide
    concentration           Length of
                            exposure

                                                 Effects                               Responsea     Species    Number     Reference

    (g/m3)     (ppm)       (number  (h/day)                                                                 of
                            of                                                                               animals
                            days)
                                                                                                                                                

    940         0.5         1        4        reduction in mitochondria of          6/6 (0/6)     rat        6 (6)      Thomas et al. (1967)
                                              alveolar cells and degradation
                                              of mast cells

    750         0.4         7        24       significant increase in protein                     guineapig  9 (9)      Sherwin & Carlson (1973)
                                              content of lung lavage fluid

    600         0.32        90       24       morphological changes such as                       rat        16 (15)    Jakimcuk & Celikanov
                                              bronchitis, peribronchitis, and                                           (1969)
                                              light pneumosclerosis; no effect
                                              observed at 150 g/m3
                                              (0.08 ppm)

    470         0.25        24-36    4        structural changes in lung            2/3 (0/1)     rabbit     3 (1)      Buell (1970)
                                              collagen fibres (electron
                                              microscope)

    100         0.05        90       24       no pathological or histological       0/10 (0/10)   rat        10 (10)    Salamberidze (1969)
                                              effects
                                                                                                                                                

    II.  Other effects
                                                                                                                                                

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

    1900        1.0         16 months  24    higher serum neutralizing antibody titres than      squirrel   5           Ehrlich & Fenters (1973)
                                             control                                             monkey

    1900        1.0         180        24    decreased aminotransferase activity in brain        guineapig  30 (50)     Kosmider & Misiewicz
                                             and liver, increased activity in blood serum                               (1973)
                                             and heart

    1700        0.9         30         24    reduction in antibody production in spleen          mouse      9 (9)       Nakamura et al. (1971)

    1300-1500   0.7-0.8     1 month    24    no change in growth rate                            mouse      20 (20)     Nakajima et al. (1969)

    940 with    0.5 with    3 months   24    changes in circulating immunoglobulins;             mouse      112-160     Ehrlich et al. (1975)
    1 h peak    1 h peak                     depression in serum neutralizing antibody titres               (112-160)
    (3800)      (2) daily
    daily

    940-1500 +  0.5-0.8 +   1-1.5      24    no change in blood carboxyhaemoglobin               mouse      94 (49)     Nakajima & Kusumoto
    58 000      50          months                                                                                      (1970)
    carbon      carbon
    monoxide    monoxide

    600         0.32        90         24    significant changes in conditioned reflexes of      rat        15 (15)     Jakimcuk & Celikanov
                                             the central nervous system                                                 (1968)
                                                                                                                                                

    II.  Other effects cont'd
                                                                                                                                                

    Nitrogen dioxide
    concentration           Length of
                            exposure
                                                 Effects                               Responsea     Species    Number     Reference

    (g/m3)     (ppm)       (number  (h/day)                                                                 of
                            of                                                                               animals
                            days)
                                                                                                                                                

    560-7500    0.3-4.0     1          2     increase in ascorbic acid levels in liver           mouse      20 (114)    Veninga & Lemstra
                                                                                                                        (1975)

    100         0.05        90         24    no effects on weight gain, central nervous          rat        10 (10)     Salamberidze (1969)
                                             system, activities of cholinesterase (3.1.1.8),
                                             catalase (1.11.1.6) and SH-groups in blood,
                                             haemoglobin, or erythrocytes
                                                                                                                                                

    a Number of animals showing effects/total number of animals; numbers in brackets refer to control groups.

    Table 8. Experimental animal studies--continued

    III.  Interaction with infectious agents
                                                                                                                                                

    Nitrogen dioxide
    concentration         Length of
                          exposure
                                                 Effects                              Responsea     Species    Number     Reference

    (g/m3)   (ppm)       (number  (h/day)                                                                 of
                          of                                                                               animals
                          days)
                                                                                                                                                

    940-1900  0.5-1.0     39       24        higher incidence of                  7/12 (1/12)   mouse      12 (12)    Ito et al. (1971)
                                             adenomatous proliferation of
                                             bronchial and bronchiolar
                                             epithelium than unexposed
                                             challenged group
    940       0.5         12       24        increased susceptibility to                        mouse      4 (4)      Blair et al. (1969)
                          months             infection; first statistical                                             Ehrlich & Henry (1968)
                                             significance evident at 90 days;
                                             reduced pulmonary clearance of
                                             inhaled microbes

    560-940   0.3-0.5     3        24        more severe adenomatous                            mouse      12 (8)     Motomiya et al. (1972)
                          months             proliferation of the peripheral
                                             bronchial cells than unexposed
                                             challenged group

    560-940   0.3-0.5     6        24        no further enhancement by an                       mouse      12 (8)     Motomiya et al. (1972)
                          months             additional 3 months exposure
                                                                                                                                                

    a Number of animals showing effects/total number of animals; numbers in brackets refer to control groups.
    

    6.  EFFECTS ON MAN

    6.1  Controlled Exposures

        The effects of nitrogen dioxide on both healthy subjects and
    patients have been studied (with their consent) under controlled
    conditions. Although the studies are few in number and only concern
    short-term exposure, much useful information has been obtained for
    assessing health effects in man.

        Henschler et al. (1960) studied normal, healthy males, aged 20-35
    years to obtain data concerning the threshold of odour perception for
    nitrogen dioxide. When the concentrations reached 230 g/m3
    (0.12 ppm), 3 out of 9 subjects perceived the odour immediately and 8
    out of 13 could detect concentrations of 410 g/m3 (0.22 ppm). At a
    higher concentration of 790 g/m3 (0.42 ppm), 8 out of 8 subjects
    immediately recognized the odour. When the nitrogen dioxide
    concentration was increased very gradually from 0 to 51 mg/m3
    (27 ppm), the volunteers failed to detect the odour. Awareness of the
    odour increased when the humidity was increased from 60% to 80%.
    Similar studies on odour perception were carried out by Feldman (1974)
    and galamberidze (1967) and in these experiments the nitrogen dioxide
    olfactory thresholds were found to be 200 and 230 g/m3 (0.11 and
    0.12 ppm), respectively.

        Threshold values for the impairment of dark adaptation by nitrogen
    dioxide have also been reported. Salamberidze (1967) determined that
    the threshold after 5 and 25 min of nasal inhalation of nitrogen
    dioxide was 140 g/m3 (0.074 ppm).

        Studies with human volunteers were also made to determine subtle
    changes in respiratory function during nitrogen dioxide exposure.
    Volunteer patients with moderate degrees of chronic respiratory
    diseases were studied as well as healthy individuals.

        Airway resistance increased significantly compared with
    pre-exposure resistance following 5-min exposure of 15 healthy
    individuals to nitrogen dioxide levels of 5600-75000 g/m3
    (3-40 ppm) (Nakamura, 1964). Suzuki & Ishikawa (1965) exposed 10
    healthy subjects to nitrogen dioxide concentrations ranging from
    1300-3800 g/m3 (0.7-2.0 ppm) for 10 min. The inspiratory and
    expiratory flow resistance rose to about 150 and 110% of control
    values, respectively, 10 min after the exposure.
    


        Abe (1967) reported studies in which 5 healthy males were
    exposed to nitrogen dioxide levels of 7500-9400 g/m3 (4-5 ppm)
    for 10 min. Inhalation of nitrogen dioxide caused an increase in
    both expiratory and inspiratory flow resistance reaching a maximum
    30 min after the end of the exposure. Values for effective
    compliance obtained 30 min after cessation of exposure showed a 40%
    decrease compared with controls.

        Orehek et al. (1976) measured the bronchomotor sensitivity of
    asthmatic patients to a bronchoconstrictor agent (carbachol) before
    and after exposure to nitrogen dioxide. These authors attempted to
    establish dose-response curves for the specific airway resistance
    (SRaw) of 20 asthmatics who were exposed for 1 h to 190 and
    380 g/m3 (0.1 and 0.2 ppm). The degree of enhancement of
    bronchial sensitivity by the nitrogen dioxide was variable among the
    individuals tested. Nitrogen dioxide at 190 g/m3 (0.1 ppm)
    induced a significant increase in initial SRaw and enhanced the
    bronchoconstrictor effect in 13 subjects. In 7 subjects this level
    of nitrogen dioxide did not modify either of these effects. Several
    possible reasons were advanced by the authors to explain why some
    asthmatic subjects responded and others did not. It seems clear that
    sensitivity to nitrogen dioxide can vary among individuals. Yokoyama
    (1968, 1970, 1972) also reported considerable individual variation
    in response among volunteers exposed from 10 to 120 min to several
    concentrations of nitrogen dioxide.

        Nieding and his associates conducted a series of studies on the
    effects of nitrogen dioxide on pulmonary function in man.

        In 1970, they reported pulmonary function studies on 13 healthy
    subjects and 88 patients with chronic bronchitis who were exposed
    for 15 min to nitrogen dioxide levels of 940 9400 g/m3
    (0.5-5.0 ppm). Inhalation of concentrations below 2800 g/m3
    (1.5 ppm) had no significant effect. Concentrations between 3000 and
    9400 g/m3 (1.6 and 5.0 ppm) caused a significant increase in
    airway resistance in the patients with chronic bronchitis. The
    patients also reacted with a significant decrease in arterial oxygen
    pressure and an increase in the alveolar-arterial oxygen pressure
    gradient when they inhaled levels of 7500-9400 g/m3 (4-5 ppm). No
    effect was seen at 3800 g/m3 (2 ppm).

        In these studies, exposure of healthy individuals to
    9400 g/m3 (5.0 ppm) caused a significant decrease in arterial
    oxygen pressure while the end expiratory oxygen partial pressure
    remained unchanged. Increased end expiratory arterial oxygen
    pressure difference was accompanied by a significant increase in
    systolic pressure in the pulmonary artery (Nieding et al., 1970,
    unpublished dataa).

        Nieding et al., (1971) observed an elevation in airway
    resistance in 15 patients with chronic bronchitis following exposure
    to nitrogen dioxide concentrations of 3000-3800 g/m3
    (1.6-2.0 ppm) for 15 min. At concentrations above 3800 g/m3
    (2.0 ppm) the increase became more pronounced. Below a concentration
    of 2800 g/m3 (1.5 ppm), no significant changes were observed.

        Inhalation of 9400 g/m3 (5.0 ppm) for 15 min caused a
    significant decrease in the carbon monoxide diffusing capacity of 16
    healthy volunteers (Nieding et al., 1973a). When the alveolar
    partial pressures of oxygen before, during, and after inhalation of
    nitrogen dioxide were compared, the mean values for the 14 chronic
    bronchitis patients tested were not statistically different.
    However, the arterial oxygen pressure decreased from an average of
    102 x 102 to 95 x 102 Pa during nitrogen dioxide exposure. There
    was a corresponding significant increase in alveolo-arterial oxygen
    pressure gradients from an average of 34 x 102 to 43 x 102 Pa.
    Continued exposure for 60 min did not result rain any further
    significant disturbances in respiratory gas exchange (Nieding et
    al., 1973a).

        Nieding et al., (1977) exposed 11 healthy male subjects, aged
    24-38 years, to a nitrogen dioxide level of 9400 g/m3 (5.0 ppm)
    for 2 h a day. Changes in pulmonary function were compared with 1 h
    pre- and post-control periods without nitrogen dioxide and with an
    untreated control series. Under test conditions, including
    intermittent light exercise, a significant increase in airway
    resistance and decrease in the difference between the alveolar and
    arterial oxygen pressures was observed. The effect of a nitrogen
    dioxide concentration of 9400 g/m3 (5.0 ppm) was not further
    enhanced by combination with ozone at a concentration of 200 g/m3
    (0.1 ppm) or by combination with the same concentration of ozone and
    sulfur dioxide at 13 000 g/m3 (5.0 ppm), respectively. However,
    the recovery time was delayed in the last two experiments. Exposure
    to a combination of nitrogen dioxide at 100 g/m3 (0.05 ppm),
    ozone at 50 g/m3 (0.025 ppm), and sulfur dioxide at 260 g/m3
    (0.10 ppm) for 2 h showed no effect on airway resistance or on the
    difference between the alveolar and arterial oxygen pressures.
    However, in these studies there was a dose-dependent increase in the
    sensibility of the bronchial tree to acetylcholine as compared with
    the control.

        Nieding et al., (1973b) also investigated the acute effects of
    nitric oxide on lung function in man and found that although nitric
    oxide had an adverse effect on the human lung function, it was
    markedly less toxic than nitrogen dioxide.

                 

    a Paper presented at the Second International Clean Air Congress of
      the International Union of Air Pollution Prevention Associations,
      Washington DC, 6-11 December, 1970.

    6.2  Accidental and Industrial Exposures

        In certain occupations, workers are intermittently exposed to
    high concentrations of oxides of nitrogen, particularly nitric oxide
    and nitrogen dioxide. These exposures occur in work that involves
    welding; in the industrial use of nitric acid compounds as in the
    production of sulfuric, picric, and chromic acids; in the
    manufacture of toluene, metallic nitrates and nitrocellulose
    (gunpowder); in the production of nitroglycerine and dynamite; and
    in mining and working in tunnels carrying motor vehicle traffic.
    Camiel & Berkan (1944) described a spectrum of pathological effects
    in the lung resulting from occupational exposure to nitrogen oxides;
    the effects varied from a mild inflammatory response in the mucosa
    of the tracheobronchial tree at low concentrations of oxides of
    nitrogen to bronchiolitis, bronchopneumonia, and acute pulmonary
    oedema at high exposures. Milne (1969) described a biphasic reaction
    to oxides of nitrogen in an industrial chemist engaged in
    manufacture of silver nitrate by mixing fuming nitric acid with
    silver. In reviewing the literature, Milne found that the biphasic
    response was quite typical of many reported cases of high industrial
    exposures to oxides of nitrogen. The biphasic reaction observed in
    acute industrial exposure to oxides of nitrogen provoked cough,
    dyspnoea, and a sense of strangulation immediately or shortly after
    exposure, apparent recovery over a latent period of 2-3 weeks, and
    finally the sudden onset of severe respiratory distress which
    terminated fatally or from which the worker apparently fully
    recovered. The early manifestations of the biphasic response were
    usually caused by acute bronchitis or pulmonary oedema, while the
    second and delayed phase was invariably due to bronchiolitis fibrosa
    obliterans. Lowry & Schuman (1956) described 4 cases of
    bronchiolitis fibrosa obliterans in farmers who had entered
    fresh-filled silos, in which high concentrations of nitrogen dioxide
    had built up. In each case, the farmer experienced cough and
    dyspnoea shortly after entering the silo. After several days,
    symptoms largely disappeared but were followed in 2 or 3 weeks by
    cough, malaise, weakness, dyspnoea, and fever. Chest roentogenograms
    showed multiple discrete nodules scattered in both lungs. Two of the
    patients died while the other 2 responded dramatically to high doses
    of steroids. The authors reported that nitrogen dioxide
    concentrations of 380-7500 mg/m3 (200-4000 ppm) were measured in
    freshly filled experimental silos. Grayson (1956), reporting on 2
    additional cases of nitrogen dioxide poisoning from sige gas,
    estimated that exposure to 560-940 mg/m3 (300-500 ppm) is likely
    to result in fatal pulmonary oedema or asphyxia, 280-380 mg/m3
    (150-200 ppm) is associated with bronchiolitis fibrosa obliterans,
    94-190 mg/m3 (50-100 ppm) with reversible bronchiolitis and focal
    pneumonitis, and 47-140 mg/m3 (25 75 ppm) with bronchitis or
    bronchopneumonia with complete recovery.

        Muller (1969) reported the occurrence of prolonged cough,
    dyspnoea, and chronic bronchitis after acute exposure to oxides of
    nitrogen formed by underground blasting in mines. Kennedy (1972)
    examined 100 miners with a history of exposure to oxides of nitrogen
    fumes from underground shotfiring: he found that a new type of shell
    containing 50% ammonium nitrate and 34% magnesium nitrate had been
    introduced into British collieries in 1959 and was associated with
    an apparent marked increase in work absences due to chest illnesses.
    Analysis of the products of explosion revealed oxides of nitrogen
    levels of 88-167 ppm; conventional power shots produced oxides of
    nitrogen concentrations of 50 ppm or more. Of the 100 miners, 84 had
    prolonged exposure to fumes from underground shotfiring, and most
    had residual volumes 150% higher than expected. Unfortunately,
    Kennedy's data are biased by the fact that the coal miners were
    referred to him because of the presence of respiratory disability
    and therefore were not representative of miners as a population:
    data concerning the smoking habits of the affected miners or the
    prevalence of impaired ventilatory status in miners not exposed to
    products of explosion were not obtained.

        Unfortunately, there have been few follow-up studies of persons
    exposed intermittently or chronically to elevated concentrations of
    nitrogen dioxide. Gregory et al. (1969) performed a study on the
    mortality of survivors from the Cleveland Clinic fire of May, 1929.
    At that time, nitrocellulose was the basic material for X-ray film.
    Apparently the film storage area of the Cleveland Clinic was badly
    ventilated and the flammable gas given off by the film ignited
    resulting in the rapid formation of nitric oxide, nitrogen dioxide,
    carbon monoxide, and hydrogen cyanide. Within 2 hours, 97 persons
    died, and within the next 30 days another 26 died. The overall
    survival during the next 30 years, of clinic employees, firemen and
    policemen at the scene, and rescue workers was evaluated by Gregory
    et al. (1969) and compared with that of an unexposed group
    consisting of persons who were comparable in job and economic status
    but were definitely not at the scene of the disaster. None of the
    exposed groups showed any difference in survival suggesting no
    residual excess mortality due to acute exposure to the mixture of
    gases, which included an estimated nitric oxide concentration of
    63 000 mg/m3 (51 500 ppm). Clearly, the study suffers from lack of
    data on exposure of the various "exposed" groups, as well as from
    lack of more refined follow-up data on the survivors.

        Mogi et al. (1968) and Yamazaki et al. (1969) evaluated
    pulmonary function in 475 railway workers in Japan employed in
    tunnels and repair sheds in which diesel exhaust fumes were
    concentrated. Although the best respiratory function was among
    employees who worked where the pollution level was lowest, no
    consistent gradient of pulmonary function was associated with areas
    of low, medium, and high concentrations of nitrogen dioxide.

        Giguz (1968) found an 11-24% higher incidence of acute
    respiratory disease in 140 adolescents in the USSR engaged in
    vocational training in a nitrogen fertilizer manufacturing plant,
    than in 85 adolescents taking vocational training involving little
    or no contact with chemicals. The author states that average
    concentrations of ammonia and oxides of nitrogen in the fertilizer
    plant did not exceed the maximum permissible levels of the USSR but
    monitoring data are not presented in the report. The possibility of
    other causal or contributory factors was not discussed.

        Thus, the literature on industrial exposure to oxides of
    nitrogen provides little useful data on the chronic or acute effects
    of low level exposures. Follow-up studies of special occupational
    groups should be conducted in order to provide better data for
    occupational health standards for oxides of nitrogen.

    6.3  Community Exposures

        In comparison with the large number of epidemiological studies
    of populations exposed to sulfur oxides and particulate matter,
    there have been few investigations in which nitrogen dioxide was
    considered as the primary environmental factor in community
    exposure.

        Prior to 1973, methods for measuring nitrogen dioxide in ambient
    air had been subject to a number of analytical and instrumental
    difficulties (Hauser & Shy, 1972), and few reliable data on
    community exposures were obtained until recent years. In addition,
    in the general community environment, nitrogen dioxide results from
    the high temperature combustion of fossil fuels and is nearly always
    found in combination with other fossil fuel combustion products such
    as sulfur dioxide, particulates, and hydrocarbons. Thus, there have
    been few opportunities to study populations in which observed health
    effects could be attributed mainly to nitrogen dioxide exposure.
    Epidemiological data concerning the health effects of nitric oxide
    are not available.

    6.3.1  Effects on pulmonary function

        Several epidemiological surveys of lung function in relation to
    community exposure to nitrogen dioxide have been reported. Shy et
    al. 1970a) observed slightly lower ventilatory function, adjusted
    for age, sex, and height, in 306, 7-and 8-year-old school children
    living in close proximity to a large industrial source of nitrogen
    dioxide. The results were of borderline significance, and elevated
    concentrations of sulfate and nitrate particulates were also
    reported. Speizer & Ferris (1973a) performed pulmonary function
    tests on 267 central city and suburban Boston policemen with

    different levels of exposure to automobile exhaust. In spite of
    differences in concentrations of nitrogen dioxide and sulfur
    dioxide, no differences were found in the results of any of the
    tests. Test results were standardized for age, height, and cigarette
    smoking habits. In a preliminary report, Speizer & Ferris (1976)
    suggested that the combination of smoking and automobile exhaust
    exposure accounted for a significant decline in pulmonary diffusing
    capacity in a follow-up study of the Boston policemen 3 years after
    their first survey. Cohen et al. (1972) compared a variety of
    pulmonary function tests in 136 nonsmoking Seventh Day Adventists
    living in Los Angeles, where concentrations of nitrogen dioxide and
    oxidants were relatively high, with 207 members of the same
    religious affiliation living in San Diego, California, where levels
    were lower. No group differences in lung function were detected.

        Kagawa & Toyama (1975) and Kagawa et al. (1976) studied the
    weekly variation in pulmonary function of 20, normal, 11-year-old
    school children in Tokyo in relation to variations in temperature
    and ambient concentrations of ozone, nitric oxide, nitrogen dioxide,
    hydrocarbons, sulfur dioxide and particulate matter. Students were
    tested from June 1972 to October 1973. Oxides of nitrogen were
    determined by the Saltzman method. Temperature was the factor most
    closely correlated with variations in specific airway conductance
    (negative correlation) and maximum expiratory flow rate (Vmax) at
    25% and 50% forced vital capacity (FVC) (positive correlation).
    Significant negative correlations were observed in sensitive
    children between ozone and specific airway conductance, and between
    nitrogen dioxide, nitric oxide, sulfur dioxide and particulate
    matter and Vmax at 25% or 50% FVC. During the high temperature
    season (April-October), nitrogen dioxide, sulfur dioxide, and
    particulate matter were significantly negatively correlated with
    both Vmax at 25% or 50% FVC and specific airway conductance. In one
    subject it was observed that Vmax at 50% FVC decreased steeply at
    nitrogen dioxide concentrations of 75 g/m3 (0.04 ppm) and above.
    However, the observed effect was not associated with nitrogen
    dioxide alone but with combined exposure to nitrogen dioxide, sulfur
    dioxide, particulate matter, and ozone. The range of hourly nitrogen
    dioxide concentrations at the time of the lung function test
    (1:00 pm), which was used for correlation during the period of study
    in the high temperature season was approximately 40-360 g/m3
    (0.02-0.19 ppm).

    6.3.2  Effects on the incidence of acute respiratory disease

        Experimental animal studies described in section 5 established a
    causal relationship between nitrogen dioxide exposure and impaired
    resistance to respiratory infections. The mechanisms for this effect
    have been studied and include nitrogen dioxide-induced impairment of
    pulmonary clearance, antibody formation, interferon production, and
    bactericidal activity in lung tissue. Several epidemiological
    studies conducted in the USA and the USSR suggest that nitrogen
    dioxide exposure may also impair resistance to respiratory
    infections in human populations, although the studies by no means
    incriminate nitrogen dioxide as the only responsible pollutant.

        Lindberg (1960) reported a 17-fold excess in upper respiratory
    disease frequency in 1375 children living near a large
    superphosphate manufacturing plant in the USSR compared with 678
    children living 10 km from the plant. Concentrations of oxides of
    nitrogen 34 times higher than the maximum allowable concentration of
    2900 g/m3 (1.5 ppm) were found in the ambient air 500 m away from
    the plant together with sulfuric acid aerosol and fluorine. Excess
    respiratory disease could have been due to the mixture of pollutants
    or to other unmeasured substances emitted by the plant. In a similar
    study, Poljak (1968) reported that a population residing within 1 km
    of the chemical works in Sehelkovo, USSR, but not employed in the
    industry made 44% more visits to the health clinics for respiratory,
    visual, nervous system, and skin disorders than a population living
    more than 3 km away. A nitrogen dioxide concentration of
    1600 g/m3 (0.85 ppm) combined with high concentrations of sulfur
    dioxide and sulfuric acid were reported 1000 m from the plant and
    this combination of pollutants may well have accounted for the
    observed respiratory effects.

        Shy et al. (1970b) and Pearlman et al. (1971) evaluated the
    frequency of acute respiratory disease in children and their parents
    living near a large point source of nitrogen dioxide in Chattanooga,
    Tennessee. Three populations -- one close to the source with a high
    nitrogen dioxide exposure, and 2 populations with low nitrogen
    dioxide exposure -- were included in the investigations. The
    incidence of acute respiratory disease was observed prospectively at
    2-week intervals during the 1968-69 school year. After adjusting for
    group differences in family size and composition, the incidence of
    acute respiratory disease in the high exposure population was found
    to be 19% higher than in the 2 comparison groups (Shy et al.,
    1970b). Similarly in a retrospective study (Pearlman et al., 1971),
    the frequency of lower respiratory disease was found to be greater
    in 6- and 7-year-old children and in infants born between 1966 and
    1969 in the area of high nitrogen dioxide exposure. Response was

    validated by physician and hospital records. Pollutant
    concentrations were monitored in the neighborhoods of each study
    population. However the original atmospheric measurements for
    nitrogen dioxide were based on the Jacobs-Hochheiser method, which
    has since been criticised for variable collection efficiency at
    different nitrogen dioxide concentrations (Hauser & Shy, 1972).
    Exposures of the population were therefore re-evaluated, using data
    obtained by the Saltzman method at an 11-station Chattanooga air
    monitoring network operated by the US Army and the US Environmental
    Protection Agency for a period of 14 months immediately preceding
    the health studies (US Environmental Protection Agency, 1976b). The
    nitrogen dioxide data and data on other pollutants obtained during
    the study and presented in Table 9 show that the largest group
    differences in pollutant exposures were in the concentrations of
    nitrogen dioxide and suspended nitrates. However, it was known that
    the point source of nitrogen dioxide (a trinitrotoluene
    manufacturing plant) had experienced problems with sulfuric acid
    emissions into the atmosphere prior to the study, and these
    emissions, along with nitric acid fumes, nitrates and nitrogen
    dioxide, may have contributed to the observed excess in acute
    respiratory disease. As in all complex low level exposures, it is
    not possible to implicate one pollutant as the responsible agent for
    the excess disease reported in the Chattanooga studies.

        Table 9.  Mean concentrations of pollutants (g/m3) in the Chattanooga studiesa
                                                                                       

                        High nitrogen dioxide exposure area     Comparison populations
    Pollutant
                        School 1   School 2   School 3          Group A   Group B
                                                                                       

    nitrogen dioxide      282        150       150                 113       56
    sulfur dioxide       < 26       < 26      < 26                < 26     < 26
    suspended sulfates   13.2       11.4      10.0                 9.8     10.0
    suspended nitrates    7.2        6.3       3.8                 2.6      1.6
    total suspended
    particulates           96         83        63                  72       62
                                                                                       

    a Adapted from: Shy et al., 1970a; US Environmental Protection Agency 1976b.

    
        Investigators from the US Environmental Protection Agency
    (1976b) compared the incidence of acute respiratory disease among
    housewives cooking with either gas or electric stoves. Because of
    high flame temperatures, gas stoves fix atmospheric nitrogen
    resulting in peak 1/2-1 h nitrogen dioxide concentrations of
    940 g/m3 (0.5 ppm). Electric stoves do not operate at a
    temperature sufficiently high to form nitrogen dioxide. Housewives
    cooking with gas stoves did not show any evidence of increased
    respiratory disease and the results suggest that short-term
    intermittent exposures of 940 g/m3 or more do not appear to
    impair respiratory defence mechanisms in adult women.

        Petr & Schmidt (1967) reported excess acute respiratory disease
    among children living near a large chemical complex, compared with
    children living in relatively clean towns. However, the authors did
    not provide data on concentrations of nitric oxide and nitrogen
    dioxide individually, and did not measure other pollutants such as
    sulfuric acid, sulfates, and total particulate matter which may have
    been produced by the chemical factories.

        The epidemiological studies of acute respiratory disease in
    populations exposed for long periods to elevated nitrogen dioxide
    concentrations provide evidence that supports the animal data on
    nitrogen dioxide-induced impairment of resistance to respiratory
    infection. Failure to provide data on other pollutants present at
    the same time or on peak daily concentrations of nitrogen dioxide is
    a serious shortcoming in most of these (and other) epidemiological
    studies. It is difficult to determine whether a given concentration
    of nitrogen dioxide was responsible for the observed health effects,
    or whether one of the other pollutants, alone or in combination with
    nitrogen dioxide, was the causal agent. The health effects may have
    been due to prolonged exposure (associated with yearly averaging
    times) or to repeated small insults to the respiratory system caused
    by daily peak exposures of one or more hours duration. Judging by
    experimental animal data cited in section 5, intermittent peak
    exposures superimposed on longer periods of low-level exposure may
    play a dominant role in the development of impaired resistance to
    acute respiratory infection.

    6.3.3  Effects on the prevalence of chronic respiratory disease

        The experimental basis for the effect of nitrogen dioxide on the
    parenchymal tissue of the lung is well established and has been
    discussed in section 5. Few epidemiological studies concerning the
    relationship between chronic respiratory disease prevalence and
    population exposure to nitrogen dioxide have provided a consistent
    pattern of results.

        Fujita and associates (1969) observed an increased prevalence of
    chronic bronchitis among 7800 post office employees in the Tokyo,
    Tsurumi, and Kawasaki areas surveyed on 2 occasions in 1962 and
    1967. The authors attributed the doubling of bronchitis prevalence
    found in 1967, compared with 1962, to increasing atmospheric
    concentrations of sulfur dioxide and nitrogen dioxide. However, the
    authors' report provided inadequate documentation of air pollution
    concentrations, and the data which were presented suggested
    relatively high levels of sulfur dioxide (290 g/m3, 0.11 ppm) and
    low concentrations of nitrogen dioxide (75 g/m3, 0.04 ppm) in
    1966.

        An Expert Committee on Air Quality Criteria for Oxides of
    Nitrogen and Photochemical Oxidants, Japan (1972) reported the
    preliminary results of a 6-city survey of chronic bronchitis
    prevalence among 400 housewives living in each of the cities.
    Although the prevalence of disease by city was correlated with
    levels of nitrogen dioxide and nitric oxide, relatively high average
    concentrations of total suspended particulates (350-500 g/m3) and
    of sulfur dioxide (100-130 g/m3, 0.04-0.05 ppm) were measured in
    the more polluted cities where the highest prevalence of chronic
    bronchitis was observed. On the other hand, nitrogen dioxide
    concentrations were relatively low in the more polluted cities,
    ranging from average values of 38 to 140 g/m3 (0.02-0.077 ppm).
    It appears, therefore, that the differences in bronchitis prevalence
    were more likely to be associated with exposures to a
    particulate/sulfur oxides complex than to nitrogen dioxide.

        As a follow-on to the Chattanooga studies reported earlier,
    Chapman et al. (1973) evaluated the prevalence of chronic bronchitis
    in 3500 adults who were parents of high school children residing in
    the 3 study areas of Chattanooga reported in Table 9. Although
    cigarette smoking and alveolar carbon monoxide concentrations
    obtained from end-expiratory breath samples showed significant
    correlations with the prevalence and severity of chronic respiratory
    disease, the study did not demonstrate an association between
    exposure to nitrogen dioxide and disease prevalence. Similarly,
    Cohen et al. (1972) failed to find a difference in the prevalence of
    chronic respiratory disease on comparing nonsmoking Seventh Day
    Adventists residing in Los Angeles and in San Diego, California.
    Mean nitrogen dioxide concentrations from 1963 to 1967 were
    94 g/m3 (0.05 ppm) in Los Angeles and 38 g/m3 (0.02 ppm) in
    San Diego.

        Speizer & Ferris (1973b) studied chronic bronchitis prevalence
    in 128 Boston policemen who patrolled congested business and
    shopping areas of central Boston with 140 suburban policemen who
    travelled in patrol cars in less congested suburban Boston
    communities. A slight but not statistically significant excess in
    chronic respiratory disease was found in nonsmokers and current
    smokers, but not in ex-smokers, who spent more time in heavy
    traffic. In central Boston, the mean nitrogen dioxide concentration
    was 100 g/m3 (0.055 ppm) and the sulfur dioxide concentration was
    91 g/m3 (0.035 ppm). Suburban concentrations were 75 g/m3
    (0.04 ppm) and 26 g/m3 (0.01 ppm) for nitrogen dioxide and sulfur
    dioxide respectively (Burgess et al., 1973).

        Shimizu (1974) reported an increase in the prevalence of chronic
    bronchitis on comparing the results of 2 surveys of all residents
    over 40 years of age in selected districts of the Osaka Prefecture,
    Japan. The first survey was conducted from 1962 to 1966, and the
    second survey from 1970 to 1972. During the years between the 2
    surveys, concentrations of sulfur oxides decreased mainly due to a
    marked decrease in the sulfur content of fuel oil. On the basis of
    air dispersion models and data on fuel consumption by stationary and
    mobile sources of oxides of nitrogen the concentration of oxides of
    nitrogen was calculated theoretically for each square kilometre of
    the Osaka Prefecture, and oxides of nitrogen concentrations were
    estimated to have increased. However, no actual measurements of
    nitrogen dioxide were available in this study.

        Based on surveys of chronic respiratory disease in 5 cities of
    the Chiba Prefecture, Japan, Yoshida et el. (1976) reported
    significant positive correlations between the prevalence of
    persistent cough and phlegm in adults, aged 40 and over, living
    within 2 kilometres of an air monitoring station, and the
    concentrations of sulfur dioxide and nitrogen dioxide. The authors
    expressed this association in the form of a multiple regression
    equation:

                      Y = 1.98 X1 + 1.14 X2 - 1.63

    where Y = age, sex, and smoking adjusted prevalence of persistent
    cough and phlegm (%), X1 = mean annual sulfur dioxide
    concentration (pphm) and X2 = mean annual nitrogen dioxide
    concentration (pphm). In order to attain 3% of chronic brochitis
    prevalence rate, which is supposed to be a "natural" prevalence rate
    under the Japanese sulfur dioxide ambient air standard (0.04 ppm or
    100 g/m3 daily mean, 0.018 ppm or 47 g/m3 annual mean), it was
    calculated that the annual nitrogen dioxide concentration should be
    below 17 g/m3 (0.009 ppm).

        The epidemiological studies of chronic respiratory disease
    prevalence described earlier do not establish an association between
    disease prevalence and population exposures to nitrogen dioxide per
    se. In general, large differences in group exposure to nitrogen
    dioxide did not exist in these studies, thus diminishing the
    likelihood of finding a pollutant-related change. The Japanese
    studies are noteworthy in suggesting an increase in chronic
    respiratory disease prevalence over time periods when nitrogen
    dioxide exposures were estimated to increase. These results suggest
    the need for more longitudinal studies of populations exposed to
    changing concentrations of air pollutants. Such opportunities exist
    in rapidly developing urban areas, where the standard of living and
    related industrial and transportation activity are likely to change
    over a relatively short time.

    6.4  Summary Tables

        Table 10 is a summary of controlled human studies which provide
    a quantitative basis for evaluating health risks from exposure to
    nitrogen dioxide. Table 11 recapitulates epidemiological studies
    which tend to support evidence from animal experiments and
    controlled human studies, though they do not furnish quantitative
    information in establishing guidelines for health protection.




        Table 10. Controlled human studies

    I.  Sensory effects
                                                                                                                                                

    Nitrogen dioxide
    concentration
                       Length of       Effects                              Responsea        Subjects               Reference
    (g/m3)    (ppm)   exposure
                                                                                                                                                

    790        0.42                    odour perceived immediately after    8/8              8 healthy subjects     Henschler et al. 1960
                                       beginning of the exposure
    410        0.22                    odour perceived immediately after    8/13             13 healthy subjects    Henschler el al. 1960
                                       beginning of the exposure

    230        0.12                    odour perceived immediately after    3/9              9 healthy subjects     Henschler et al. 1960
                                       beginning of the exposure

    230        0.12                    odour perceived immediately after    "most" of the    14 healthy subjects    Salamberidze (1967)
                                       beginning of the exposure            subjects

    200        0.11                    odour perceived immediately after    26/28            28 healthy subjects    Feldman (1974)
                                       beginning of the exposure

    0-51 000   0-27    54 min          no odour perception, when raising    0/6              6 healthy subjects     Henschler et al. (1960)
                                       the concentration slowly within 54
                                       min from 0 to 27 ppm; increase of
                                       relative humidity enhanced odour
                                       perception

    140        0.074   5 and 25 min    decreased dark adaptation in all     4/4              4 healthy subjects     Salamberidze (1967)
                                       subjects (nasal breathing)
                                                                                                                                                

    a Response = number of subjects showing effects/total number of subjects

    Table 10. Controlled human studies--continued

    II.  Effects on lung function
                                                                                                                                                

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

    9400            5             1           2         significant increase of Rawa              11 healthy subjects     Nieding et al. (1976)
                                                        decrease of AaDO2c under
                                                        intermittent light exercise

    9400            5             1           15 min    PAO2e before, during and after            14 chronic bronchitis   Nieding et al. (1973a)
                                                        exposure unchanged, but PaO2t             patients
                                                        significantly decreased; AaDO2
                                                                                                  increased
    9400            5             1           15 min    DLcod significantly decreased             16 healthy subjects     Nieding et al. (1973a)

    7500-9400       4-5           1           10 min    decrease in lung compliance with          5 healthy subjects      Abe (1967)
                                                        corresponding increases in expiratory
                                                        and inspiratory flow resistance

    3000-3800       1.6-2.0       1           15 min    Raw-increase                              15 patients with        Nieding et al. (1971)
                                                                                                  chronic bronchitis

    1300-3800       0.7-2.0       1           10 min    increase in inspiratory and expiratory    10 healthy subjects     Suzuki & Ishikawa
                                                        flow resistance                                                   (1965)

    190             0.1           1           1         a slight but significant increase in      20 asthmatics           Orehek et al. (1976)
                                                        initial SRawb and enhancement of
                                                        bronchoconstrictor effect of
                                                        carbachol in 13 subjects
                                                                                                                                                

    Table 10. Controlled human studies--continued

    II.  Effects on lung function cont'd.
                                                                                                                                                

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

    100 in          0.05 in       1           2         no effect on Raw and AaDO2;               11 healthy subjects     Nieding et al. (1976)
    combination     combination                         sensitivity of the bronchial tree to
    with 50 ozone   with 0.025                          acetylcholine increased compared
    and 260         ozone and                           with that before exposure to the
    sulfur dioxide  0.10 sulfur                         pollutants
                    dioxide
                                                                                                                                                

    aRaw   = Airway resistance
    bSRaw  = Specific airway resistance, i.e. the product of airway    dDLco = Diffusing capacity of the lung for carbon monoxide
            resistance and thoracic gas volume                        eAO2   = Alveolar partial presures of oxygen
    cAaDO2 = Alveolar to arterial oxygen pressure difference          fPaO2 = Arterial partial pressures of oxygen

    Table 11.  Epidemiological studies of community exposure

    I.  Pulmonary function
                                                                                                                                                

                                                           Averaging
    Concentrations and population                          time        Effect and/or response                             Reference
                                                                                                                                                

    Pulmonary function tests on twenty normal              1 h         association with decrease in specific airway       Kagawa et al. (1976)
    11-year-old school children were made once or                      conductance and Vmax at 50% FVC during the high
    twice a week for 17 months. The concentrations                     temperature season; in one subject, Vmax at
    of nitrogen dioxide in the higher temperature season               50% FVC decreased steeply at nitrogen dioxide
    at the time of measurement ranged from approximately               levels of approximately 75 g/m3 (0.04 ppm);
    40-360 g/m3 (0.02-0.19 ppm).                                      the observed effect is not associated with
                                                                       nitrogen dioxide alone, but with combined
                                                                       exposure to nitrogen dioxide, sulfur dioxide
                                                                       particulates and ozone.
                                                                                                                                                

    Table 11.  Epidemiological studies of community exposure cont'd.

    II.  Acute respiratory disease
                                                                                                                                                

           Concentrations
                                                               Averaging
    Exposed                      Control                       time        Effect and/or response                         Reference
                                                                                                                                                

    150-282 g/m3 (0.08-0.15     56-113 g/m3 (0.03-0.06       1 year      increased incidence of acute respiratory       Shy et al. (1970a,
    ppm) nitrogen dioxide with   ppm) nitrogen dioxide with                disease in school children and parents         1970b)
    4-7 g/m3 nitrates, 10-13    2-3 g/m3 nitrates, 10 g/m3              in Chattanooga.
    g/m3 sulfates, <26 g/m3    sulfates, <26 g/m3 (<0.01
    (<0.01ppm) sulfur dioxide    ppm) sulfur dioxide
    63-96 g/m3 particulates;    62-72 g/m3 particulates                  increased incidence of lower respiratory       Pearlman et al.
    exposure to sulfuric acid                                              disease in Chattanooga infants and school      (1971)
    and nitric acid fumes also                                             children.
    present but not measured.

    >940 g/m3 (>50 ppm)         <940 g/m3 (<0.50 ppm)        1 h         no evidence of increased acute respiratory     US Environmental
    nitrogen dioxide -1 h       nitrogen dioxide                          disease in housewives cooking with gas stoves  Protection Agency
    peak indoor concentration                                              compared with those using electric stoves.     (1976b)
                                                                                                                                                

    Table 11.  Epidemiological studies of community exposure -- continued

    III.  Chronic respiratory disease
                                                                                                                                                

                  Concentrations
                                                               Averaging
    Exposed                      Control                       time        Effect                                         Reference
                                                                                                                                                

    100 g/m3 (0.055 ppm)        75 g/m3 (0.04 ppm)           1 year      no significant increase in chronic             Speizer & Ferris
    nitrogen dioxide with 91     nitrogen dioxide with 26                  respiratory symptoms among central city        (1973b)
    g/m3 (0.035 ppm) sulfur     g/mg (0.01 ppm) sulfur                   traffic police officers in Boston.
    dioxide                      dioxide

    94 g/m3 (0.05 ppm)          43 g/m3 (0.023 ppm)          1 year      no effect on prevalance of chronic             Cohen et al. (1972)
    nitrogen dioxide with 26     nitrogen dioxide with 26                  respiratory symptoms or on lung functions
    g/m3 (0.01 ppm) sulfur      g/m3 (0.01 ppm) sulfur                   of nonsmoking subjects living in Southern
    dioxide, 120 g/m3           dioxide, 78 g/m3 (0.074                  California.
    particulates, 280 g/m3      ppm) particulates, 150 g/m3
    (0.14 ppm) oxidants          (0.074 ppm) oxidants
    (mean of daily               (mean of daily
    1-h maxima)                  1-h maxima)
                                                                                                                                                

    


    7.  EVALUATION OF HEALTH RISKS FROM EXPOSURE TO
        OXIDES OF NITROGEN

        It is well established that respiratory disease is an important
    cause of disability and death. There is also considerable evidence
    that some of these diseases are associated with the inhalation of
    polluted air. Most of these associations have been established with
    regard to the presence in the ambient air of sulfur dioxide,
    particulate matter, and/or smoke (World Health Organization, 1972).
    Oxides of nitrogen as well as some other pollutants were not
    considered in the studies on sulfur oxides and suspended
    particulates although it is likely that they were present. It is
    possible that nitrogen dioxide could play a role in causing
    respiratory disease but, to date, only a limited number of
    epidemiological investigations have been carried out with regard to
    the effects on human health of this pollutant. There is, however, a
    considerable amount of data derived from experimental animal studies
    and controlled studies on human volunteers showing a high biological
    activity of nitrogen dioxide even at low concentrations. These data
    are useful as bases for assessing the toxic effects of nitrogen
    dioxide and for establishing guidelines for exposure limits for the
    protection of public health.

        At present, there is no evidence that nitric oxide
    concentrations typically observed in the ambient air have a
    significant biological effect. The Task Group did not, therefore,
    develop guidelines for nitric oxide exposure limits for the
    protection of public health.

    7.1   Exposure Levels

        Exposure of human populations to nitrogen dioxide varies widely
    both with respect to time and place. In rural areas, far from
    man-made sources, nitrogen dioxide concentrations have been
    estimated at 5 g/m3 (0.0025 ppm), while in most major cities
    annual means of 20-90 g/m3 (0.01-0.05 ppm) have been recorded. In
    most of these cities the maximum 24-h means range from 130 to
    400 g/m3 (0.07-0.21 ppm). Peak concentrations may be
    substantially higher. In some of the larger urban areas, maximum 1-h
    concentrations in excess of 800 g/m3 (0.43 ppm) have been
    measured. The available data indicate that in most situations the
    maximum 24-h mean in a given year is 2-5 times higher than the
    annual mean for that location. The 1-h maximum values are about 5-10
    times higher than the recorded annual means. These peak
    concentrations generally occur on clear days twice daily, in the
    morning and evening hours.

        Normally, nitrogen dioxide is accompanied by many other air
    pollutants such as particulate matter, carbon monoxide, sulfur
    dioxide, ozone etc. and this is of special concern from an
    epidemiological point of view, since the effects on human health may
    well be additive or even synergistic.

        Another aspect which must be considered is that a certain
    portion of the population is also exposed, though intermittently, to
    extremely high concentrations of nitrogen dioxide in their working
    or home environment or due to the inhalation of tobacco smoke.
    Cigarette smoke may contain nitrogen dioxide concentrations as high
    as 226 mg/m3 (120 ppm). Directly above gas stoves, nitrogen
    dioxide concentrations may reach levels as high as 2000 g/m3
    (1.1 ppm).

        Current trends indicate that emissions of oxides of nitrogen
    will continue to increase, primarily because of increased use of
    fossil fuels both in stationary sources and transportation.

    7.2  Experimental Animal Studies

        The toxic effects of nitrogen dioxide have been studied
    extensively in a wide variety of experimental animal models.
    Analysis of the available data clearly indicates that a number of
    factors can influence the host's response to this air pollutant. The
    species of animals tested, the duration, concentration, and mode of
    exposure, and the pre-existence of disease can modify the expected
    response to nitrogen dioxide. A summary of selected studies is given
    in Table 8.

        The primary target of nitrogen dioxide is the respiratory
    system. A variety of effects have been measured which can be related
    to the concentration and time of the nitrogen dioxide exposure.
    Effects measured include changes in pulmonary function,
    morphological changes, depression of host defence mechanisms, oedema
    and, at high concentrations, death. In addition a number of systemic
    or extra-pulmonary responses have also been observed, i.e. decrease
    in growth rate; alteration in immunological response; polycythemia
    and leucocytosis: changes in reproductive function; delay in
    conditioned reflexes of the central nervous system; and depression
    in physical activity. The lowest concentration at which adverse
    effects on pulmonary function were found was 1500 g/m3 (0.8 ppm).
    At this level the respiratory rates of rats remained elevated
    throughout life.

        Continuous exposure of mice, rats, or rabbits to concentrations
    of 470-1900 g/m3 (0.25-1.0 ppm) produced a number of
    morphological changes in the respiratory system. Structural changes
    in lung collagen, alveolar distension, shortening of ciliated
    epithelial cells, and adenomatous proliferation of bronchial and
    bronchiolar epithelium have been observed after exposures of 30 days
    or less. With long-term exposure of various animal species (rat,
    rabbit, monkey, guineapig) to higher concentrations
    (3800-47 000 g/m3, 2-25 ppm), the above effects became more
    pronounced and changes in respiration rate, tidal volume,
    immunological and biochemical parameters became noticeable.

        Exposure to nitrogen dioxide has been shown to increase the
    susceptibility of the host to respiratory infections. This effect is
    clearly dose-related and has been shown with short-term, continuous,
    and intermittent exposures. When mice were exposed for 90 days to a
    nitrogen dioxide concentration as low as 940 g/m3 (0.5 ppm) and
    then immediately given a laboratory-induced infection, a significant
    increase in mortality rate was observed. Nitrogen dioxide can also
    enhance the risk of respiratory infection in other animal species
    (hamster, squirrel monkey) although considerably higher
    concentrations are required (e.g. 1-2 months treatment with
    9400 g/m3-19 mg/m3 (5-10 ppm).

    7.3  Controlled Studies in Man

        Controlled human studies which can be used for evaluating health
    effects are limited to short-term exposures. It can be seen from
    Table 10 that functional changes of the lung in healthy human
    subjects such as an increase in air way resistance begin after 10
    min of inhalation of nitrogen dioxide concentrations of 1300 g/m3
    (0.7 ppm) or more. Recently it was shown that the reaction to
    inhalation challenge with a bronchoconstrictor (carbachol) in
    asthmatic subjects increased after exposure to a nitrogen dioxide
    concentration of 190 g/m3 (0.1 ppm) for 1 h. A similar reaction
    was shown in healthy subjects exposed to a combination of nitrogen
    dioxide at 100 g/m3 (0.05 ppm), ozone at 50 g/m3 (0.025 ppm),
    and sulfur dioxide at 260 g/m3 (0.10 ppm) for 2 h. These
    reactions might be of importance especially in subjects with
    respiratory disease when gaseous pollutants act in combination with
    inhaled particles such as pollen, spores, suspended particulate
    matter, or dust. The olfactory threshold for nitrogen dioxide and
    the level at which changes in dark adaptation occurred were both
    about 200 g/m3 (0.11 ppm).

    7.4  Effects of Accidental and Industrial Exposures

        Inadvertent and accidental exposure of human subjects to high
    concentrations of nitrogen dioxide has occurred among welders,
    farmers working in freshly filled silos, miners using explosive
    chemicals and in other occupations. Exposure levels have not been
    precisely documented in these situations, but severe acute and
    delayed effects were experienced in the form of pneumonia,
    bronchiolitis and sometimes pulmonary oedema. From these studies, it
    has been estimated that short-term exposures of 1 h or less to
    nitrogen dioxide concentrations of 47-140 mg/m3 (25-75 ppm) can
    cause pneumonia and bronchitis, while exposure to 560-940 mg/m3
    (300-500 ppm) may cause fatal, pulmonary oedema or asphyxia.

        In general, acute or chronic effects of low level industrial
    exposure to nitrogen dioxide have not been systematically evaluated.

    7.5  Effects of Community Exposures

        In comparison with the large body of data on populations exposed
    to sulfur oxides and particulate matter, there are few
    epidemiological studies in which nitrogen dioxide has been
    considered to be the primary environmental factor in community
    exposures. The epidemiological studies described in section 6.3
    demonstrate increased risk of acute respiratory disease and
    diminished lung function particularly among school children exposed
    to community air containing nitrogen dioxide, sulfur oxides,
    particulate matter, and, in some cases, photo-chemical oxidants. It
    is difficult to determine whether a given level of nitrogen dioxide
    was responsible for the observed health effects, or whether one of
    the other pollutants, alone or in combination with nitrogen dioxide
    was the causal agent. Furthermore, the health effects may have been
    due to chronic exposures or to repeated small insults to the
    respiratory system associated with exposures to daily peak
    concentrations. Judged by experimental animal data, intermittent
    peak values superimposed on longer periods of low level exposure may
    play a dominant role in the development of impaired resistance to
    acute respiratory infections.

        The Task Group concluded, therefore, that the results of
    reported epidemiological studies cannot themselves provide a
    quantitative basis for evaluating health risks from exposure to
    nitrogen dioxide. In particular, the Task Group agreed that a
    specific concentration of nitrogen dioxide for a given averaging
    time could not be conclusively associated with the health effects
    observed in various epidemiological studies. The significance of the
    reported studies is that they support evidence from animal
    experiments and controlled human studies of increased risk of acute
    respiratory infections and altered lung function.

    7.6  Evaluation of Health Risks

        As has already been mentioned, there are not enough
    epidemiological data related to occupational or community exposures
    to serve as a basis for developing reliable air quality guides for
    nitrogen dioxide and for quantitative risk evaluation. However, the
    existing data do not contradict the findings that pulmonary effects
    are related to nitrogen dioxide exposure.

        Thus, in an attempt to develop recommendations for guidelines on
    exposure limits consistent with the protection of human health, the
    Task Group had to rely mainly upon data from animal experiments and
    controlled human studies. The Group considered as adverse effects
    not only the morphological and other changes caused by higher
    nitrogen dioxide concentrations but also the effects on the
    respiratory system induced by lower concentrations. These changes
    include increased airway resistance, increased sensitivity to
    bronchoconstrictors, and enhanced susceptibility to respiratory
    infections. Although some of these effects were reversible, the Task
    Group's opinion was that such effects should be prevented. The Task
    Group estimated that adverse effects on the respiratory system of
    test animals might arise with short-term as well as long-term
    exposure to nitrogen dioxide at concentrations beginning from
    approximately 940 g/m3 (0.5 ppm). Adverse effects in man have
    occurred at approximately the same concentrations of nitrogen
    dioxide. Under controlled conditions human subjects exposed to
    nitrogen dioxide concentrations of 1300-3800 g/m3 (0.7-2.0 ppm)
    for 10 min exhibited increased airway resistance. Furthermore
    exposure to a nitrogen dioxide concentration of 190 g/m3
    (0.1 ppm) for 1 h enhanced the bronchoconstrictor effect of a
    chemical aerosol (carbachol) in asthmatics.

        A WHO Expert Committee in 1972 examined available information on
    some air pollutants including nitrogen dioxide. The biological
    activity of nitrogen dioxide in animals as well as plants was
    recognized but the Expert Committee believed that there was
    insufficient information upon which to base specific air quality
    guides in the absence of conclusive epidemiological data (World
    Health Organization, 1972).

        The present Group felt it appropriate and prudent not to wait
    for more conclusive epidemiological evidence but to use available
    controlled study data from animals and human subjects in an attempt
    to develop guidelines for exposure limits consistent with the
    protection of public health. Such an approach seemed even more
    reasonable since results of epidemiological studies tended to
    support these data.

        The Task Group selected the nitrogen dioxide level of
    940 g/m3 (0.5 ppm) as an estimate of the lowest observed
    effect-level for short-term exposures, because, at this
    concentration, effects had been shown in many controlled studies on
    animals and man. The Task Group was aware that one controlled human
    study showed an adverse effect at a lower concentration of
    190 g/m3 (0.1 ppm) in asthmatics. This study needs confirmation
    and the Task Group agreed that at present the lowest adverse effect
    level for highly sensitive human subjects is not known and needs to
    be assessed. In view of the uncertainty concerning the lowest
    adverse effect level and the high biological activity of nitrogen
    dioxide, the Task Group concluded that a considerable safety factor
    was required. The difference between the approximate lowest observed
    effect level of 940 g/m3 (0.5 ppm) for 1 h and background
    concentrations of about 5 g/m3 (0.0025 ppm) would allow no more
    than a maximum safety factor of 200. The maximum safety factor is,
    in fact, reduced to a value of 20, since maximum hourly nitrogen
    dioxide concentrations in small towns and villages remote from
    pollution sources may reach 50 g/m3 (0.025 ppm). In larger
    cities, maximum hourly values may reach 470 g/m3 (0.25 ppm) or
    more. At approximately these concentrations some effects have been
    shown in a few controlled studies on man and animals. The Group
    considered this highly unsatisfactory, particularly in view of the
    fact that there is reason to believe that, if effective measures are
    not taken, concentrations of oxides of nitrogen in urban communities
    will rise due to increased use of fossil fuels.

        Any safety factor must be arbitrary but, obviously, it should be
    sufficient to protect populations living in large urban communities.
    Taking into consideration all available information, the Task Group
    decided to propose a minimum safety factor of 3-5 for short-term
    exposure to nitrogen dioxide, and agreed that an exposure limit
    consistent with the protection of public health might be provided by
    a nitrogen dioxide concentration of 190 to 320 g/m3
    (0.10-0.17 ppm) for a maximum 1-h exposure. This 1-h exposure should
    not be exceeded more than once per month.

        Evidence on the interaction of nitrogen dioxide with other
    co-existing biologically active air pollutants may well suggest the
    need for larger safety factors and therefore lower maximum
    permissible exposure levels. Even now, there may be a need to
    increase the safety factor in order to protect the highly sensitive
    portion of the population.

        In its evaluation of health risks, the Task Group believed that
    the biomedical effects of long-term exposure to nitrogen dioxide in
    man had not been ascertained to the extent that a recommendation for
    the protection of public health could be made, and therefore did not
    propose an exposure limit pertaining to long-term averaging times.

    REFERENCES

    ABE, M. (1967) Effects of mixed NO2-SO2 gas on human pulmonary
    functions.  Bull. Tokyo Med. Dent. Univ., 14: 415-433.

    ACTON, J. D. & MYRIVIK, Q. N. (1972) Nitrogen dioxide effects on
    alveolar macrophages.  Arch. environ. Health, 24: 48-52.

    ANTWEILER, H., KOMPCH, K.-H., & BROCKHAUS, A. (1975) [Investigations
    on the influence of NO2 and SO2 as well as a combination of the
    two gases on the production of precipitating antibodies in
    guinea-pigs.]  Zentralbl. Bakt. Hyg., l. Abt. Orig. B, 160: 212-224
    (in German).

    ARNER E. C. & RHOADES, R. A. (1973) Long-term nitrogen dioxide
    exposure.  Arch. environ. Health, 26: 156-160.

    BAGG, J. (1971) The formation and control of oxides of nitrogen in
    air pollution, In: Strauss, W., ed.  Air Pollution Control, Part I,
    NY, Wiley-Interscience, pp. 35-94.

    BALCHUM, O. J., BUCKLEY, R. D., SHERWIN, R., & GARDNER, M. (1965)
    Nitrogen dioxide inhalation and lung antibodies.  Arch. environ.
     Health, 10: 274-277.

    BAUMGARDNER, R. E., CLARK, T. A., HODGESON, J. A., & STEVENS, R. K.
    (1975) Determination of an ozone interference in the continuous
    Saltzman nitrogen dioxide procedure.  Anal. Chem., 47: 515-521.

    BLAIR, W. H., HENRY, M. C., & EHRLICH, R. (1969) Chronic toxicity of
    nitrogen dioxide. II. Effect on histopathology of lung tissue.
     Arch. environ. Health, 18: 186-192.

    BOKHOVEN, D. & NIESSEN, H. J. (1961) Amounts of oxides of nitrogen
    and carbon monoxide in cigarette smoke, with and without inhalation.
     Nature (Lond.), 192: 458-459.

    BUCKLEY, R. D. & BALCHUM, O. J. (1965) Acute and chronic exposures
    to nitrogen dioxide: effects on oxygen consumption and enzyme
    activity on guinea pig tissues.  Arch. environ. Health, 10:
    220-223.

    BUCKLEY, R. D. & BALCHUM, O. J. (1967a) Effects of NO2 on lactic
    dehydrogenase isozymes.  Arch. environ. Health, 14: 424-428.

    BUCKLEY, R. D. & BALCHUM, O. J. (1967b) Enzyme alterations following
    nitrogen dioxide exposure.  Arch. environ. Health, 14: 687-692.

    BUCKLEY R. D. & LOOSLI, C. G. (1969) Effects of nitrogen dioxide
    inhalation on germ-free mouse lung.  Arch. environ. Health, 18:
    588-595.

    BUELL, G. C. (1970) Biochemical parameters in inhalation
    carcinogenesis. In: Hanna, M. G., Jr., Nettesheim, P. & Gilbert, J.
    R., ed.  Inhalation carcinogenesis, Oak Ridge, Te, US Atomic Energy
    Commission, pp. 209-228.

    BURGESS, W., DIBERARDINIS, L., & SPEIZER, F. E. (1973) Exposure to
    automobile exhaust. III. An environmental assessment.
     Arch. environ. Health, 26: 325-329.

    BUTCHER, S.S. & RUFF, R. E. (1971) Effect of inlet residence time on
    analysis of atmospheric nitrogen oxides and ozone.  Anal. Chem.,
    43: 1890-1892.

    CALIFORNIA AIR RESOURCES BOARD (1975)  California air quality data.
    July, August and September 1975. Vol. VII, No. 3, Sacramento, CA,
    California Air Resources Board Technical Services Division, p. 21.

    CALVERT, J. G. (1976) Test of the theory of ozone generation in Los
    Angeles atmosphere.  Environ. Sci. Technol., 10: 248-256.

    CAMIEL, M. R. & BERKAN, H. S. (1944) Inhalation pneumonia from
    nitric fumes  Radiology, 42: 175-182.

    CARSON, T. R., ROSENHOLTZ, M. S., WILINSKI, F. T., & WEEKS, M. H.
    (1962) The responses of animals inhaling nitrogen dioxide for
    single, short-term exposures.  Am. Ind. Hyg. Assoc. J., 23:
    457-462.

    CENTRAL COUNCIL FOR ENVIRONMENTAL POLLUTION CONTROL (1977)
     [Long-term plan for environmental protection.] Tokyo, Environment
    Agency, p. 72 (in Japanese).

    CHAPMAN, R. S., SHY, C. M., FINKLEA, J. F., HOUSE, D. E., GOLDBERG,
    H. E., & HAYES, C.G. (1973) Chronic respiratory disease. In military
    inductees and parents of schoolchildren.  Arch. environ. Health,
    27: 138-142.

    CHEN, C., KUSUMOTO, S., & NAKAJIMA, T. (1972) [The recovery
    processes of histopathological changes in the respiratory organs of
    mice after NO2 exposure with special reference to chronic
    tracheitis and bronchitis.]  Proc. Osaka Pref Inst. Public Health,
    10: 43-49 (in Japanese).

    CHOW, C. K., DILLARD, C. J., & TAPPEL, A. L. (1974) Glutathione
    peroxidase system and lysozyme in rats exposed to ozone or nitrogen
    dioxide.  Environ. Res., 7: 311-319.

    CHRISTIE, A. A., LIDZEY, R. G., & RADFORD, D. W. F. (1970) Field
    methods for the determination of NO2 in air. Analyst, 95: 519-524.

    COFFIN, D. L. GARDNER, D. E,, & BLOMMER, E. J. (1976) Time/dose
    response for nitrogen dioxide exposure in an infectivity model
    system.  Environ. Health Perspect., 13: 11-15.

    COHEN, C. A., HUDSON, A. R., CLAUSEN, J. L., & KNELSON, J. H. (1972)
    Respiratory symptoms, spirometry, and oxidant air pollution in
    non-smoking adults.  Am. Rev. resp. Dis., 105: 251-261.

    COMMISSIE BODEM, WATER EN LUCHT (1970)  Annual Report 1970.
    Rotterdam, The Netherlands, pp. 23 + tables (in Dutch).

    COOPER, W. C. & TABERSHAW, I. R. (1966) Biological effects of
    nitrogen dioxide in relation to air quality standards.
     Arch. environ. Health, 12: 522-530.

    DAVIDSON, J. T., LILLINGTON, G. A., HAYDON, G. B., & WASSERMAN, K.
    (1967) Physiologic changes in the lungs of rabbits continuously
    exposed to NO2.  Am. Rev. resp. Dis., 92: 790-796.

    DEMERJIAN, K. L., KERR, J. A., & CALBERT, J. G. (1974) The mechanism
    of photochemical smog formation. In: Pitts, J. N. Jr, Metcalf, R.
    L., & Lloyd, A. C., ed. Advances in environmental science and
    technology, Vol. 4, NY, Wiley Interscience, pp. 1-262.

    DERWENT, R. G. & STEWART, H. N.M. (1973) Air pollution from the
    oxides of nitrogen in the United Kingdom.  Atmos. Environ., 7:
    385-401.

    DROZDZ, M., JOZKIEWICZ, S., LUDYGA, K., ROKICKI, M., & ROKICKI, W.
    (1973) [Effect of chronic action of NO2 on the activity of enzymes
    of carbohydrate metabolism in blood and liver homogenates of guinea
    pigs and protective action of gas ammonium].  Biuletyn Sl. San.
     Epid. woj. Katowickiego, 17: 395-403 (in Polish).

    DROZDZ, M., LUCIAK, M., KOSMIDER, S., MOLSKA-DROZDZ, T., LUDYGA, K.,
    & PASZEWICZ, J. (1974) [The enzymatic and morphological changes in
    the central nervous system of guinea pigs chronically exposed to
    nitrogen dioxide].  Biuletyn Sl. San. Epid. woj. Katowickiego, 18:
    131-141 (in Polish).

    DROZDZ, M., LUCIAK, M., KOSMIDER, S., MOLSKA-DROZDZ, T., LUDYGA, K.,
    & PASIEWICZ, J. (1975) [Effect of chronic action of nitrogen dioxide
    on metabolic disturbances and histopathological changes in the liver
    of guinea pigs].  Med. Pracy, 26: 157-168 (in Polish).

    EHRLICH, R. (1966) Effect of nitrogen dioxide on resistance to
    respiratory infection.  Bact. Rev., 30: 604-614.

    EHRLICH, R. & FENTERS, J. D. (1973) Influence of NO2 on
    experimental influenza in squirrel monkeys. In:  Proceedings of the
     Third International Clean Air Congress, Dussendorf, German), 1973,
    pp. A11-A13.

    EHRLICH, R. & HENRY, M. C. (1968) Chronic toxicity of nitrogen
    dioxide. I. Effect on resistance to bacterial pneumonia.
     Arch. environ. Health, 17: 860-865.

    EHRLICH, R., SILVERSTEIN, E., MAIGETTER, R., FENTERS, J. D., &
    GARDNER, D. (1975) Immunologic response in vaccinated mice during
    long-term exposure to nitrogen dioxide.  Environ. Res. 10: 217-223.

    ENVIRONMENT AGENCY (1974)  [Air pollution in Japan. Air monitoring
     data in fiscal year, 1973.] Tokyo, Air Pollution Control Division,
    Air Quality Bureau, pp. 118-135 (in Japanese).

    ENVIRONMENT AGENCY (1976)  [Air pollution in Japan. Air monitoring
     data in fiscal year, 1975.] Tokyo, Air Pollution Control Division,
    Air Quality Bureau, p. 22 (in Japanese).

    EVANS, M. J., STEPHENS, R. J., CABRAL, L. J., & FREEMAN, G. (1972)
    Cell renewal in the lungs of rats exposed to low levels of NO2.
     Arch. environ. Health, 24: 180-188.

    EVANS, M. J., CABRAL, L. J., STEPHENS, R. J., & FREEMAN, G. (1973a)
    Renewal of alveolar epithelium in the rat following exposure to
    NO2. Am. J. Pathol., 70: 175-198.

    EVANS, M. J., CABRAL, L. J., STEPHENS, R. J., & FREEMAN, G. (1973b)
    Cell division of alveolar macrophages in rat lung following exposure
    to NO2. Am. J. Pathol., 70: 199-208.

    EVANS, M. J., CABRAL, L. J., STEPHENS, R. J., & FREEMAN, G. (1975)
    Transformation of alveolar Type 2 cells to Type 1 cells following
    exposure to NO2.  Exp. mol. Pathol., 22: 142-150.

    EXPERT COMMITTEE ON AIR QUALITY CRITERIA FOR OXIDES OF NITROGEN AND
    PHOTO-CHEMICAL OXIDANTS, JAPAN (1972)  [Supplement of environmental
     standards for oxides of nitrogen and photochemical oxidants.]
    Sub-Council for Air Pollution Control of the Central Council for
    Environmental Pollution Control, Environment Agency, Japan p. 7 (in
    Japanese).

    FELDMAN, Y. G. (1974) [The combined action on a human body of a
    mixture of the main components of motor traffic exhaust gases
    (carbon monoxide, nitrogen dioxide, formaldehyde and hexane).]
     Gig. i. Sanit., No. 10: 7-10 (in Russian).

    FENTERS, J. D., EHRLICH, R., FINDLAY, J., SPANGLER, J., & TOLKACZ,
    V. (1971) Serologic response in squirrel monkeys exposed to nitrogen
    dioxide and influenza virus.  Am. Rev. resp. Dis., 104: 448-451.

    FONTIJN, A., SABADELL, A. J., & RONCO, R. J. (1970) Homogeneous
    chemiluminescent measurement of nitric oxide with ozone.
     Anal. Chem., 42: 575-579.

    FORWEG, W. (1975) [Measurements of emissions and immissions of
    nitrogen oxides]  VDI-Berichte, 247: 24-28 (in German).

    FREEMAN, G. (1970) Discussion: Ultrastructural early lesion induced
    in the rat lung by NO2. In: Hanna, M. G., Jr., Nettesheim, P., &
    Gilbert, J. R., ed.  Inhalation carcinogenesis. Oak Ridge,
    Tennessee, US Atomic Energy Commission, Division of Technical
    Information Extension, pp. 267-269 (AEC Symposium Series 18, Publ.
    No. NTIS Conf-691001).

    FREEMAN, G. & HAYDON, G. B. (1964) Emphysema after low-level
    exposure to NO2.  Arch. environ Health, 8: 125-128.

    FREEMAN, G. & JUHOS, L. (1976)  Trace substances and tobacco smoke
     in interaction with nitrogen oxides: biological effects.
    Environmental Health Effects Research Series, Research Triangle
    Park, NC, US Environmental Protection Agency, pp. 38
    (EPA-600/1-76-021 ).

    FREEMAN, G., FURIOSI, N.J., & HAYDN, G. B. (1966) Effects of
    continuous exposure to 0.8 ppm NO2 on respiration of rats.
     Arch. environ. Health, 13: 454-456.

    FREEMAN, G., CRANE, S.C., & STEPHENS, R. J. (1968a) Pathogenesis of
    the nitrogen dioxide-induced lesion in the rat lung: a review and
    presentation of new observations.  Am. Rev. resp. Dis., 98:
    429-443.

    FREEMAN, G., CRANE, S.C., STEPHENS, R. J., & FURIOSI, N.J. (1968b)
    Environmental factors in emphysema and a model system with NO2.
     Yale J. Biol. Med., 40: 566-575.

    FREEMAN, G., STEPHENS, R. J., CRANE, S.C., & FURIOSI, N.J. (1968c)
    Lesion of the lung in rats continuously exposed to two parts per
    million of nitrogen dioxide.  Arch. environ. Health, 17: 181-192.

    FREEMAN, G., CRANE, S.C., STEPHENS, R. J., & FURIOSI, N.J. (1969)
    The subacute nitrogen dioxide-induced lesion of the rat lung.
     Arch. environ. Health, 18: 609-612.

    FUJITA, S., TANAKA, M., KAWAME, S., YOSHIOKA, I., FURUYA, T.,
    SHIBATA, S., KOSODA, T., MAKITA, M., FUJIWARA, Y., UEDA, Y., &
    TOKUDA, K. (1969) [Studies on chronic bronchitis-epidemiological
    survey (second report).]  Commun. Med., 21: 197-203 (in Japanese).

    FUKASE, O., ISOMURA, K., & WATANABE, H. (1976) [Effects of nitrogen
    oxides on peroxidative metabilism of mouse lung.]
     J. Jpn Soc. Air Pollut., 11: 65-69 (in Japanese).

    FURIOSI, N.J., CRANE, S.C., & FREEMAN, G. (1973) Mixed sodium
    chloride aerosol and NO2 in air. Biological effects on monkeys and
    rats.  Arch. environ. Health 27: 405-408.

    GARDNER, D. E., HOLZMAN, R. S., & COFFIN, D. L. (1969) Effects of
    nitrogen dioxide on pulmonary cell population.  J. Bacteriol., 98:
    1041-1043.

    GARDNER, D. E., MILLER, F. J., BLOMMER, E. J., & COFFIN, D. L.
    (1977) Relationships between nitrogen dioxide concentration, time,
    and level of effect using an animal infectivity model. In:
     Proceedings of the International Symposium on Photochemical
     Oxidants Pollution and its Control, Raleigh, pp. 513-525.

    GEORGII, H. W. & WEBER, E. (1962) [Investigations of air chemistry
    in the alps.]  Zentr albl. Biol. Aerosol Forsch., 10: 97-105 (in
    German).

    GIGUZ, T. L. (1968) [Effect of low concentrations of ammonia and
    nitrogen oxides on adolescents undergoing vocational training in the
    chemical industry.]  Gig. i Sanit., No. 5: 100-102 (in Russian).

    GIORDANO, A. M. & MORROW, P. E. (1972) Chronic low-level nitrogen
    dioxide exposure and mucociliary clearance.  Arch. environ. Health,
    25: 443-449.

    GOLDSTEIN, E., EAGLE, M. C., & HOEPRICH, P. D. (1973) Effect of
    nitrogen dioxide on pulmonary bacterial defense mechanisms.
     Arch. environ. Health, 26: 202-204.

    GRAYSON, R. R. (1956) Silage gas poisoning: nitrogen dioxide
    pneumonia, a new disease in agricultural workers.  Ann. intern.
     Med., 45: 393-408.

    GREEN, G. M. & KASS, E. H. (1964) Factors influencing the clearance
    of bacteria by the lung.  J. clin. Invest., 43: 769-776.

    GREENBAUM, R., BAY, J., HARGREAVES, M. D., KAIN, M. L. KELMAN, G.
    R., NUNN, J. F., PRYS-ROBERTS, C., & SIEBOLD, K. (1967) Effects of
    higher oxides of nitrogen on the anaesthetized dog.  Br. J.
     Anaesth., 39: 393-404.

    GREGORY, K. L., MALINOSKI, V. F., & SHARP, C. R. (1969) Cleveland
    clinic fire: survivorship study, 1929-1965.  Arch. environ, Health,
    18: 508-515.

    GUICHERIT, R. (1976) [Photochemical smog formation in the
    Netherlands], Delft, The Netherlands, TNO Research Institute for
    Environmental Hygiene, p. 36. (in Dutch).

    HAAGEN-SMIT, A. J., BRUNELLE, M. F., & HARA, J. (1959) Nitrogen
    oxide content of smokes from different types of tobacco.  Am. Med.
     Assoc. Arch. Indust. Health, 20: 399-400.

    HATTORI, S., TATEISHI, R., HORAI, T., & NAKAJIMA, T. (1972)
    [Morphological changes in the bronchial alveolar system of mice
    following continuous exposure to NO2 and CO.]  J. Jpn Soc.
     Thorac. Disease, 10: 16-22 (in Japanese).

    HAUSER, T. R. & SHY, C. M. (1972) Position paper: NOx measurement.
     Environ. Sci. Technol., 6: 890-894.

    HAYDON, G. B., FREEMAN, G., & FURIOSI, N.J. (1965) Covert
    pathogenesis of NO2-induced emphysema in the rat.  Arch. environ.
     Health, 11: 776-783.

    HENRY, M. C., EHRLICH, R., & BLAIR, W. H. (1969) Effect of nitrogen
    dioxide on resistance of squirrel monkeys to Klebsiella pneumoniae
    infection.  Arch. environ. Health, 18: 580-587.

    HENRY, M. C., FINDLAY, J., SPANGLER, J., & EHRLICH, R. (1970)
    Chronic toxicity of NO2 in squirrel monkeys. III. Effect on
    resistance to bacterial and vital infection.  Arch. environ. Health,
    20: 566-570.

    HENSCHLER, D. & Ross, W. (1966) [On the problem of the carcinogenic
    effect of inhaled oxides of nitrogen.]  Naunyn-Schmiedebergs Arch.
     exp. Path. & Pharmak., 253: 495-507 (in German).

    HENSCHLER, D., STIER, A., BECK, H., & NEUMAN, W. (1960) [Olfactory
    threshold of some important irritant gases and effects in man at low
    concentrations.]  Arch. Gewerbepathol. Gewerbehyg. (Berlin) 17:
    547-570 (in German).

    HORTON, A.D., STOKELY, J. R, & GUERIN, M. R. (1974) Gas
    chromatographic determination of nitric oxide in cigarette smoke.
     Anal. Lett., 7: 177-185.

    ITO, K., MOTOMIYA, K., YOSHIDA, R., OTSU, H., & NAKAJIMA, T. (1971)
    [Effect of nitrogen dioxide inhalation on influenza virus infection
    in mice.]  Jpn J. Hyg., 26: 304-314 (in Japanese).

    JACOBS, M. B. & HOCHHEISER, S. (1958) Continuous sampling and
    ultramicro-determination of nitrogen dioxide in air.  Anal. Chem.,
    30: 426-428.

    JAKIMCUK, P. P. & CELIKANOV, K. N. (1968) [Materials for hygienic
    establishment of 24 hours maximal permissible concentration of
    nitrogen dioxide in the atmosphere.] In: Biological effect and
    hygienic significance of atmospheric pollutants, Vol. II, Moscow,
    pp. 164-171 (in Russian).

    JAPANESE STANDARDS ASSOCIATION (1974) [Continuous analysers for
    oxides of nitrogen in ambient air.] Japanese Industrial Standards
    JIS B7953, Tokyo, Japan (in Japanese).

    JAPANESE STANDARDS ASSOCIATION (1976) [General rule for calibration
    method of gas analysers.] Japanese Industrial Standards, JIS K 0055,
    Tokyo, Japan (in Japanese).

    JOST, D. & RUDOLF, W. (1975) [NO/NO2 concentrations in the Federal
    Republic of Germany]  Staub Reinhaltung der Luft, 35: 150-154 (in
    German).

    KAGAWA, J. & TOYAMA, T. (1975) Photochemical air pollution: its
    effects on respiratory function of elementary schoolchildren.
     Arch. environ. Health, 30: 117-122.

    KAGAWA, J., TOYAMA, T., & NAKAZA, M. (1976) Pulmonary function test
    in children exposed to air pollution. In: Finkel, A. J. & Duel, W.
    C., ed.  Clinical implications of air pollution research, Acton,
    MA, Publishing Sciences Group, pp. 305-320.

    KASS, E. H., GREEN, G. M., & GOLDSTEIN, E. (1966) Mechanisms of
    anti-bacterial action in the respiration system.  Bacteriol. Rev.,
    30: 488-496.

    KAUT, V. (1970) [The contribution of oxides of nitrogen to the
    formation of nitrosamines.]  Cesk. Hyg., 15: 213-215 (in
    Czechoslovak).

    KAUT, V., TUSL, M., SVORCOVA, S., & TOMANA, M. (1966) [Changes in
    the organs of rats after inhalation of low concentrations of
    nitrogen oxides.]  Cesk. Hyg., II: 479-484 (in Czechoslovak).

    KENNEDY, M. C. S. (1972) Nitrous fumes and coal-miners with
    emphysema.  Ann. occup. Hyg., 15: 285-300.

    KLEINERMAN, J. & COWDREY, C. R. (1968) The effects of continuous
    high level nitrogen dioxide on hamsters.  Yale J. biol. Med., 40:
    579-585.

    KLEINERMAN, J. & RYNBRANDT, D. (1976) Lung proteolytic activity and
    serum protease inhibition after NO1 exposure.  Arch. environ.
     Health, 31: 37-41.

    KLEINERMAN, J. & WRIGHT, G. W. (1961) The reparative capacity of
    animal lungs after exposure to various single and multiple doses of
    nitrite.  Am. Rev. resp. Dis., 83: 423-424.

    KOSMIDER, S. & MISEWICZ, A. (1973) [The influence of long-term
    exposure to the oxides of nitrogen on the aminotransferase activity
    studied in clinical and animal experiments.]  Z. Ges. Hyg. ihre
     Grenzgebiete (Berlin), 19: 108-110 (in German).

    LARSEN, R. I. (1969) A new mathematical model of air pollutant
    concentration averaging time and frequency.  J. Air Pollut. Control
     Assoc., 19: 24-30.

    LEIGHTON, P. A. (1961)  Photochemistry of air pollution. NY,
    Academic Press.

    LEVAGGI, D. A., SIU, W., FELDSTEIN, M., & KOTHNY, E. L. (1972)
    Quantitative separation of nitric oxide from nitrogen dioxide at
    atmospheric concentration ranges.  Environ. Sci. Technol., 6:
    250-252.

    LEWIS, T. R., MOORMAN, W. J., TANG, Y., & STARA, J. F. (1974)
    Long-term exposure to auto exhaust and other pollutant mixtures.
     Arch. environ. Health, 29: 102-106.

    LINDBERG, Z. Ja. (1960) [Effect of superphosphate production
    discharges on children's health].  Gig. i Sanit., No. 5: 89-96 (in
    Russian).

    LINDQVlST, F. & LANTING, R. W. (1972) A modified permeation device
    for the preparation of trace gas mixtures.  Atmos. Environ., 6:
    943-946.

    LODGE, J.P. Jr & PATE, J. B. (1966) Atmospheric gases and
    particulates in Panama. Science, 153: 408-410.

    LOWRY, T. & SCHUMAN, L. M. (1956) Silo-filler's disease. A syndrome
    caused by nitrogen dioxide.  J. Am. Med. Assoc., 162: 153-160.

    LYSHKOW, N. A. (1965) A rapid and sensitive colorimetric reagent for
    nitrogen dioxide in air.  J. Air Pollut, Control Assoc., 15:
    481-484.

    MACKINNON, D. J. (1974) Nitric oxide formation at high temperatures.
     J. Air Pollut. Control Assoc, 24 (3): 237-239.

    MENZEL, D. B., ROEHM, J. N., & LEE, S. D. (1972) Vitamin E: the
    biological and environmental antioxidant.  Agric. Food Chem., 20:
    481-486.

    MERRYMAN, E. L., SPICER, C. W., & LEVY, A. (1973) Evaluation of
    arsenite-modified Jacobs-Hochheiser procedure.  Environ. Sci.
     Technol., 7: 1056-1059.

    MILNE, J. E. H. (1969) Nitrogen dioxide inhalation and bronchiolitis
    obliterans. A review of the literature and report of a case.
     J. occup. Med., 11: 538-547.

    MITINA, L. S. (1962) [The combined effect of small concentrations of
    nitrogen dioxide and sulfur dioxide gases].  Gig. i Sanit., No. 10:
    3-8 (in Russian).

    MOGI, S., SHIMIZU, M., KONDO, N., YAMAZAKI, K., & JINGUJI, S. (1968)
    [Effects of diesel exhaust gas on the health of workers. Report No.
    1, Environmental Survey.]  Railw. Labor Sci., 22: 1-25 (in
    Japanese).

    MORROW, P. E. (1975) An evaluation of recent NOT toxicity data and
    an attempt to derive an ambient air standard for NOT by established
    toxicological procedure.  Environ. Res., 10: 92-112.

    MOTOMIYA, K., ITO, K., YOSHIDA, R., IDE, G., OTSU, Y., & NAKAJIMA,
    T. (1972) [Effects of NO2 gas exposure on influenza virus
    infection of mice-long-term, low-concentration experiment.)
     Rep. Environ. Res. Org. Chiba Univ., 1: 27-33 (in Japanese).

    MULLER, B. (1969) Nitrogen dioxide intoxication after a mining
    accident.  Respiration, 26: 249-261.

    MULIK, J., FUERST, R., GUYER, M., MEEKER, J., & SAWICKI, E. (1974)
    Development and optimization of twenty-four hour manual methods for
    the collection and colorimetric analysis of atmospheric NO2.
     Int. J. environ. anal. Chem., 3: 333-348.

    MURPHY, S. D., ULRICH, C. E. FRANKOWITZ, S. H., & XINTARAS, C.
    (1964) Altered function in animals inhaling low concentrations of
    ozone and nitrogen dioxide.  Am. Ind. Hyg. Assoc. J., 25: 246-253.

    NAKAJIMA, T. (1973) [Biological effects of oxides of nitrogen].
     Proc. Symp. J. Jpn Soc. Air Pollut., 8: 223-233 (in Japanese).

    NAKAJIMA, T. & KUSUMOTO, S. (1968) [Effects of nitrogen dioxide
    exposure on the contents of reduced glutathione in mouse lung].
     Proc. Osaka Pref. Inst. Publ. Health, 6: 17-21 (in Japanese).

    NAKAJIMA, T. & KUSUMOTO, S. (1970) [Studies on the carboxyhemoglobin
    level of mice continuously exposed to the mixed gas of CO and
    NO2].  Proc. Osaka Pref Inst. Publ. Health, 8: 25-28 (in
    Japanese).

    NAKAJIMA, T., KUSUMOTO, S., CHEN, C., & OKAMOTO, K. (1969) [Studies
    on the contents of reduced glutathione and histopathological studies
    on the lungs of mice exposed to nitrogen dioxide gas].  Proc. Osaka
     Pref Inst. Publ. Health, 7: 35-44 (in Japanese).

    NAKAMURA, K. (1964) [Response of pulmonary airway resistance by
    interaction of aerosols and gases in different physical and chemical
    nature.]  Jpn J. Hyg, 19: 322-333 (in Japanese).

    NAKAMURA, S., NAKAYAMA, Y., MIURA, T., & MORI, K. (1971) [Studies on
    the formation of reticuloendothelial system. VIII. Effects of NO2
    exposure on antibody formation.]  Proc. Osaka Pref. Inst. Publ.
     Health, 9: 20-22 (in Japanese).

    NATIONAL AIR POLLUTION COUNCIL, THE NETHERLANDS (1976)
     [Air Pollution Indicative medium term programme 1976-1980.] p. 20
    (in Dutch).

    NIEDING, G. VON, WAGNER, M., KREKELER, H., SMIDT, U., & MUYSERS, K.
    (1971) [Minimum concentrations of NO2 causing acute effects on the
    respiratory gas exchange and airway resistance in patients with
    chronic bronchitis].  Int. Arch. Arbeitsmed., 27: 338-348 (in
    German).

    NIEDING, G. VON, KREKELER, H., FUCHS, R., WAGNER, M., & KOPPENHAGEN,
    K. (1973a) Studies of the acute effects of NO2 on lung function:
    influence on diffusion, perfusion, and ventilation in the lungs.
     Int. Arch. Arbeitsmed., 31: 61-72.

    NIEDING, G. VON, WAGNER, H. M., & KREKELER, n. (1973b) Investigation
    of the acute effects of nitrogen monoxide on lung function in man.
    In: Proceedings of the Third International Clean Air Congress, 8-12
    October 1973, Dusseldorf, pp. A 14-A16.

    NIEDING, G. VON, WAGNER, H. M., LOLLGEN, H., & KREKELER, H. (1977)
    [Acute effects of ozone on lung function in man.]  VDI-Berichte,
    270: 123-129 (in German).

    NORTH ATLANTIC TREATY ORGANIZATION (1973)  Air quality criteria for
     oxides of nitrogen. A report by the Expert Panel on Air Quality
    Criteria, Committee on the Challenges of Modern Society, Brussels,
    pp. 8-1 to 8-28.

    ODA, H., KUSUMOTO, S., & NAKAJIMA, T. (1975) Nitrosyl-hemoglobin
    formation in the blood of animals exposed to nitric oxide.
     Arch. environ. Health, 30: 453-456.

    OREHEK, J., MASSARI, P., GAYRARD, P., GRIMAUD, C., & CHARPIN, J.
    (1976) Effect of short-term, low-level nitrogen dioxide exposure on
    bronchial sensitivity of asthmatic patients.  J. clin. Invest., 57:
    301-307.

    ORGANISATION FOR ECONOMIC COOPERATION AND DEVELOPMENT (1973) Report
    and conclusions of the Joint Ad Hoc Group on Air Pollution from Fuel
    Combustion in Stationary Sources, Paris, p. 37.

    PARKINSON, D. R. & STEPHENS, R. J. (1973) Morphological surface
    changes in the terminal bronchiolar region of NO2-exposed rat
    lung.  Environ. Res., 6: 37-51.

    PEARLMAN, M. E., FINKLEA, J. F., CREASON, J. P., SHY, C. M., YOUNG,
    M. M., & HORTON, R. J. M. (1971) Nitrogen dioxide and lower
    respiratory illness,  Pediatrics, 47: 391-398.

    PETR, B. & SCHMIDT, P. (1967) [The influence of an atmosphere
    contaminated with sulfur dioxide and nitrous gases oil the health of
    children].  Z. Gesamte Hygiene und ihre Grenzgebiete, 13: 34-38 (in
    German).

    POLJAK, V. E. (1968) [Air pollution around a chemical works and the
    effects of its discharges on sanitary living conditions].  Gig. i.
     Sanit. No 5: 107-108 (in Russian).

    ROBINSON, E. & ROBBINS, R. C. (1972) Emissions, concentrations, and
    fate of gaseous atmospheric pollutants. In: Strauss, W., ed.,
     Air pollution control Part II, NY, Wiley-Interscience, pp. 1-93.

    ROEHM, J. N., HADLEY, J. G., & MENZEL, D. B. (1971) Antioxidants
    versus lung disease.  Arch. intern. Med., 128: 88-93.

    SALAMBERIDZE, O. P. (1967) [Reflex action of a mixture of sulfur
    dioxide and nitrogen dioxide].  Gig. i. Sanit. No. 7: 9-13 (in
    Russian).

    SALAMBERIDZE, O. P. (1969) [The joint action of small concentrations
    of sulfur dioxide and nitrogen dioxide gases under conditions of a
    chronic test.]  Gig. i. Sanit., No. 4: 10-14 (in Russian).

    SALAMBERIDZE, O. P. & CERETELI, N. T. (1971) [Effect of small
    concentrations of sulfurous gas and nitrogen dioxide on the estrual
    cycle and the genital function of animals in experiments.]  Gig. i.
     Sanit., No. 8: 13-17 (in Russian).

    SALTZMAN, B. E. (1954) Colorimetric microdetermination of nitrogen
    dioxide in the atmosphere.  Anal. Chem., 26: 1949-1955.

    SALTZMAN, B. E. (1960) Modified nitrogen dioxide reagent for
    recording air analysers.  Anal. Chem. 32: 135-136.

    SCHUCK, E. A., PITTS, J. N., & WAN, J. K. S. (1966) Relationships
    between certain meteorological factors and photochemical smog.
     Air Water Pollut. Int. J. 10: 689-711.

    SCHWARZBACH, E. (1975) [On the occurrence of nitrogen oxides from
    the combustion of natural gas.]  VDI-Berichte, 247: 16-18 (in
    German).

    SHERWIN, R. P. & CARLSON, D. A. (1973) Protein content of lung
    lavage fluid of guinea pigs exposed to 0.4 ppm NO2.  Arch.
     environ. Health, 27: 90-93.

    SHERWIN, R. P., DIBBLE, J., & WEINER, J. (1972) Alveolar wall cells
    of the guinea pig: increase in response to 2 ppm NO2.
     Arch. environ. Health, 24: 43-47.

    SHY, C. M., CREASON, J.P., PEARLMAN, M. E., MCCLAIN, K. E., &
    BENSON, F. B. (1970a) The Chattanooga schoolchildren study. I.
    Methods, description of pollutant exposure, and results of
    ventilatory function testing.  J. Air Pollut. Control Assoc., 20:
    539-545.

    SHY, C. M., CREASON, J.P., PEARLMAN, M. E., MCCLAIN, K. E., &
    BENSON, F. B. (1970b) The Chattanooga schoolchildren study. II.
    Incidence of acute respiratory illness.  J. Air Pollut. Control
     Assoc., 20: 582-588.

    SHIMIZU, T. (1974) [A statistical study on relations between the
    symptom prevalence of chronic bronchitis and air pollution.]
     Jpn. J. Thorax, 12: 199-206 (in Japanese).

    SPEDDING, D. J. (1974)  Air Pollution, Oxford, Clarendon Press, pp.
    76 (Oxford Chemistry Series).

    SPEIZER, F. E. & FERRIS, B. G., Jr (1973a) Exposure to automobile
    exhaust. II. Pulmonary function measurements.  Arch. environ.
     Health, 26: 319-324.

    SPEIZER, F. E. & FERRIS, B. G., Jr (1973b) Exposure to automobile
    exhaust. I. Prevalence of respiratory symptoms and disease.
     Arch. environ. Health, 26: 313-318.

    SPEIZER, F. E., & FERRIS, B. G., Jr (1976) Change in diffusing
    capacity of the lungs as a function of smoking and exposure to
    automobile exhaust.  Am. Rev. resp. Dis., 113: 96.

    STEDMAN, D. H., DABY, E. E., STUHL, F., & NIKI, H. (1972) Analysis
    of ozone and nitric oxide by a chemiluminescent method in laboratory
    and atmospheric studies of photochemical smog.  J. Air Pollut.
     Control Assoc., 22: 260-263.

    STEPHENS, R. J., FREEMAN, G., CRANE, S.C., & FURIOSI, N.J. (1971a)
    Ultra-structural changes in the terminal bronchiole of the rat
    during continuous low-level exposure to nitrogen dioxide.
     Exp. mol. Pathol., 14: 1-19.

    STEPHENS, R. J., FREEMAN, G., & EVANS, M. J. (1971b) Ultrastructural
    changes in connective tissue in lungs of rats exposed to NO2.
     Arch. intern. Med, 127: 873-883.

    STEPHENS, R. J., FREEMAN, G., & EVANS, M. J. (I 972) Early response
    of lungs to low levels of nitrogen dioxide.  Arch. environ. Health,
    24: 160-179.

    STERN, A. C. ed. (1968)  Air Pollution, Vol. 1, Air Pollution and
     its effects. NY, Academic Press, pp. 694.

    STUPFEL, M., MAGNIER, M., ROMARY, F., TRAN, M., & MOUTET, J. (1973)
    Lifelong exposure of SPF rats to automotive exhaust gas.
     Arch. environ. Health, 26: 264-269.

    SUZUKI, T. & ISHIKAWA, K. (1965) [A study on the effects of smog on
    man. Special studies on air pollution control: Report No. 2].
    Science and Technology Agency (Japan) pp. 199-221 (in Japanese).

    SVORCOVA, S. & KAUT, V. (1971) [The arterio-venous differences in
    the nitrite and nitrate ion concentrations after nitrogen oxides
    inhalation]  Cesk. Hyg., 16: 71-76 (in Czechoslovak).

    THOMAS, T. & RHOADES, R. A. (1970) 14C-1 palmitate incorporation
    by rat lung: effect of nitrogen dioxide.  Proc. Soc. Exp. Biol.
     Med., 108: 1181-1183.

    THOMAS, H. V., MUELLER, P. K. & WRIGHT, R. (1967) Response of rat
    lung mast cells to nitrogen dioxide inhalation.  J. Air Pollut.
     Control Assoc., 17: 33-35.

    THOMAS, H. V., MUELLER, P. K., & LYMAN, R. L. (1968)
    Lipoperoxidation of lung lipids in rats exposed to nitrogen dioxide.
     Science, 159: 532-534.

    TUSL, M. (1975) [Changes in the plasma-corticosterone in rats
    following inhalation of nitrogen oxides].  VDI-Berichte, 247: 90-91
    (in German).

    TUSL, M., STOLIN, V., WAGNER, M., & AST, D. (1973) Physical exertion
    (swimming) in rats under the effect of chemical agents. In: Horvath,
    M., ed.  Adverse effects of environmental chemicals and psychotropic
    drugs: quantitative interpretations of functional tests.
    Amsterdam-London-NY, Elsevier, pp. 155-160.

    US DEPARTMENT OF HEALTH, EDUCATION AND WELFARE (1965)  Selected
     methods for the measurement of air pollutants. Interbranch Chemical
    Advisory Committee, Cincinnati, OH, pp. C-1 to C-7 (Public Health
    Service Publ. No. 999-AP-11).

    US DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE (1976)  Criteria for
     a recommended standard-occupational exposure to oxides of
     nitrogen. US Public Health Service, Center for Disease Control,
    National Institute for Occupational Safety and Health, pp. 46-74
    (HEW Publ. No. (NIOSH) 76-149.

    US ENVIRONMENTAL PROTECTION AGENCY (1962-1971) CAMP Data. Research
    Triangle Park, NC, National Aerometric Data Bank.

    US ENVIRONMENTAL PROTECTION AGENCY (1971a)  Air quality criteria for
     nitrogen oxides. Washington, DC, Environmental Protection Agency,
    Air Pollution Control Office pp. 5-1 to 5-8, 6-10 to 6-I 1, 9-1 to
    9-31 (Publ. No. AP-84).

    US ENVIRONMENTAL PROTECTION AGENCY (1971b) Part 410 -- National
    primary and secondary ambient air quality standards,  Federal
     Register, 36 (84) Part II: 8186-8201.

    US ENVIRONMENTAL PROTECTION AGENCY (1973)  The National Air
     Monitoring Program: Air quality and emission trends. Annual
     Report, Vol. 1, Research Triangle Park, NC, US Environmental
    Protection Agency, pp. 1-7 (EPA-450/1-73-001 a).

    US ENVIRONMENTAL PROTECTION AGENCY (1976a)  Monitoring and Air
     Quality Trends Report, 1974. Research Triangle Park, NC,
    Environmental Protection Agency pp. E1 to E44 (EPA-450/1-76-001).

    US ENVIRONMENTAL PROTECTION AGENCY (1976b)  Scientific and technical
     data base for criteria and hazardous pollutants. 1975 ERC/RTP
    Review. Research Triangle Park, NC, Health Effects Research
    Laboratory, EPA, pp. 192-194, 206-207 (EPA Publ. No.
    EPA-600/1-76-023).

    VALAND, S. B., ACTON, J. D., & MYRVIK, Q. N. (1970) Nitrogen dioxide
    inhibition of viral-induced resistance in alveolar monocytes.
     Arch. environ. Health, 20: 303-309.

    VAUGHAN, T. R., JENNELLE, L. F., & LEWIS, T. R. (1969) Long-term
    exposure to low levels of air pollutants. Effects on pulmonary
    function in the beagle.  Arch. environ. Health, 19: 45-50.

    VENINGA, T. & LEMSTRA, W. (1975) Extrapulmonary effects of ozone
    whether in the presence of nitrogen dioxide or not.  Int. Arch.
     Arbeitsmed., 34: 209-220.

    WAGNER, H. M. (1972) [The effects of the individual components of
    motor-vehicle exhaust gases on man and animals]  Schr. Reihe Ver.
     Wass. Boden Lufthyg., H38: 313-325 (in German).

    WAGNER, W. D., DUNCAN, B. R., WRIGHT, P. G., & STOKINGER, H. E.
    (1965) Experimental study of threshold limit of NO2.
     Arch. environ. Health, 10: 455-466.

    WATANABE, H., TOMITA, K., KANEKO, F., & NAKADOI, T. (1966) [A
    hygienic study of kerosene heaters.]  Jpn. J. publ. Health, 13: 1-7
    (in Japanese).

    WEAST, R. C. (1976)  Handbook of Chemistry and Physics. 57th
    edition, Cleveland, OH, Chemical Rubber Co., p. B-137.

    WINER, A.M., PETERS, J. W., SMITH, J.P., & PITTS, J. N., Jr (1974)
    Response of commercial chemiluminescent NO-NO2 analysers to other
    nitrogen-containing compounds.  Environ. Sci. Technol., 8:
    1118-1121.

    WORLD HEALTH ORGANIZATION (1972)  Air quality criteria and
     guides for urban air pollutants. Report of a WHO Expert Committee.
    WHO Technical Report Series No. 506, pp. 28-29.

    WORLD HEALTH ORGANIZATION (1976)  Selected methods of measuring air
     pollutants, Geneva, World Health Organization, pp. 67-79 (WHO
    Offset Publication No. 24).

    YAMAZAKI, K., MOGI, T., NISHIMOTO, Y., & KOMAZAWA, T. (1969) [Effect
    of diesel exhaust gas on health of workers: Report II. Multivariate
    analysis of pulmonary function.]  Rail. Labor Sci., 23: 23-33 (in
    Japanese).

    YOKOYAMA, E. (1968) Effects of acute controlled exposure to NO2 on
    mechanics of breathing in healthy subjects.  Bull. Inst. Public
     Health, 17: 337-346.

    YOKOYAMA, E. (1970) [Comparison of the ventilatory effects of SO2
    and NO2-exposure of human volunteers.]  Jpn. J. ind. Health,
    12: 4-8 (in Japanese).

    YOKOYAMA, E. (1972) [The respiratory effects of exposures to
    SO2-NO2 mixture on healthy subjects.]  Jpn. J. ind. Health. 14:
    449-454 (in Japanese).

    YOSHIDA, R., ADACHI, M., NITTA, Y., MURAI, M., & IWASAKI, A. (1976)
    [Epidemiological study on chronic bronchitis in Chiba Prefecture.]
     Jpn. J. public Health, 23: 435-441 (in Japanese).

    ZORN, H. (1975) [The alveolar-arterial oxygen-tension differential
    and tissue-oxygen partial pressure during exposure to NO2]
     VDI-Bericht, 247: 50-51 (in German).


    See Also:
       Toxicological Abbreviations
       Nitrogen, oxides of (EHC 188, 1997, 2nd edition)