Hydrogen Sulfide

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

    Published under the joint sponsorship of the United Nations
    Environment Programme, the International Labour Organisation, and the
    World Health Organization

    World Health Organization

    Geneva, 1981

    ISBN 92 4 154079 6

    (c) World Health Organization 1981

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         1.1. Summary
              1.1.1. Properties and analytical methods
              1.1.2. Sources of hydrogen sulfide
              1.1.3. Environmental levels and exposures
              1.1.4. Effects on experimental animals
              1.1.5. Effects on man
             General toxicological considerations
             Occupational exposure
             Exposure of the general population
              1.1.6. Evaluation of health risks
         1.2. Recommendations for further studies


         2.1. Chemical and physical properties
         2.2. Atmospheric chemistry
         2.3. Sampling and analytical methods
              2.3.1. The methylene blue method
              2.3.2. Gas chromatography with flame photometric detection
              2.3.3. Automatic monitors in stationary field settings
              2.3.4. Direct reading portable detection systems
              2.3.5. Manual collection and analysis of air samples in
                     occupational settings


         3.1. Natural sources
         3.2. Sources associated with human activity


         4.1. Concentrations in outdoor air
         4.2. Concentrations in work places



         6.1. General toxicological considerations
         6.2. Occupational exposure
         6.3. General population exposure


         7.1. Exposure levels
         7.2. Experimental animal studies
         7.3. Effects of occupational exposure
         7.4. Effects of general population exposure
         7.5. Guidelines for the protection of public health





    Dr M. Argirova, Institute of Hygiene & Occupational Health, Sofia,

    Dr G. C. N. Jayasuriya, National Science Council, Colombo, Sri Lanka

    Dr H. Kappus, Department of Pharmacology, Medical Institute for
        Environmental Hygiene, Düsseldorf, Federal Republic of Germany

    Professor M. Katz, York University, Faculty of Sciences, Department of
        Chemistry, Downsview, Ontario, Canada

    Mr. K. Rolfe, Department of Health, Environmental Laboratory,
        Auckland, New Zealand  (Rapporteur)

    Dr H. Savolainen, Institute of Occupational Health, Helsinki, Finland

    Professor R. P. Smith, Department of Pharmacology and Toxicology,
        Dartmouth Medical School, Hannover, NH, USA  (Chairman)

    Dr S. Tarkowski, Institute of Occupational Medicine, Industrial
        Toxicology Branch, Lodz, Poland

    Mr E. Tolivia, Department of Health and Welfare, Mexico City, Mexico

    Dr V. V. Vashkova, Sysin Institute of General and Communal Hygiene,
        Moscow, USSR

     Temporary Advisers

    Dr T. H. Milby, Environmental Health Associates, Inc., Berkeley, CA,

    Dr R. C. Spear, Department of Biomedical and Environmental Health
        Sciences, School of Public Health, University of California,
        Berkeley, CA, USA

     Representatives of other Organizations

    Dr D. Djordjevic, International Labour Office, Geneva, Switzerland

    Dr S. Salem Milad, International Register of Potentially Toxic
        Chemicals, United Nations Environment Programme, Geneva,

    Mrs M. Th. van der Venne, Commission of the European Communities,
        Directorate General of Employment and Social Affairs, Health and
        Safety Directorate, Luxembourg


    Dr A. David, Office of Occupational Health, Division of
        Noncommunicable Diseases, World Health Organization, Geneva,

    Dr Y. Hasegawa, Environmental Health Criteria and Standards, Division
        of Environmental Health, World Health Organization, Geneva,
        Switzerland  (Secretary)

    Dr H. W. de Koning, Environmental Health Technology and Support,
        Division of Environmental Health, World Health Organization,
        Geneva, Switzerland

    Mr G. Ozolins, Environmental Health Criteria and Standards, Division
        of Environmental Health, World Health Organization, Geneva,


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


        A WHO Task Group on Environmental Health Criteria for Hydrogen
    Sulfide met in Geneva from 24 to 28 March 1980. Mr G. Ozolins,
    Associate Manager, Environmental Health Criteria and Standards, opened
    the meeting on behalf of the Director-General. The Task Group reviewed
    and revised the second draft of the criteria document and made an
    evaluation of the health risks from exposure to hydrogen sulfide.

        The first and second drafts were prepared jointly by Dr T. H.
    Milby of the Environmental Health Associates, Inc., Berkeley, CA, USA,
    and Dr R. C. Spear of the Department of Biomedical and Environmental
    Health Sciences, University of California, Berkeley, 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 Australia, Belgium, Czechoslovakia, Finland, Federal
    Republic of Germany, Mexico, New Zealand, Poland, USA and USSR, and
    from the International Labour Organisation, Geneva, the International
    Centre for Industry and Environment, France, and the International
    Petroleum Industry Environmental Conservation Association, London.
    Comments were also received from Professor M. Katz (Canada) and
    Professor R. Lilis (USA). Some comments were received after the second
    draft had been prepared and were reviewed by the Task Group during its
    meeting. These comments were from the national focal points for the
    WHO Environmental Health Criteria Programme in Japan and the United
    Kingdom and from the Commission of the European Communities,
    Luxembourg, and the International Union of Pure and Applied Chemistry,

        The collaboration of these national institutions, international
    organizations and individual experts is gratefully acknowledged.
    Without their assistance this document could not have been completed.

        This document is based primarily on original publications listed
    in the reference section. However, several recent publications broadly
    reviewing health aspects of hydrogen sulfide have also been used,
    including those of the National Research Council, USA (1979) and NIOSH

        Details of the WHO Environmental Health Criteria Programme,
    including some of the terms frequently used in the documents, can be
    found in the introduction to the publication "Environmental Health
    Criteria 1 - Mercury", published by the World Health Organization,
    Geneva, 1976, now also available as a reprint.

        The following conversion factors have been used in this document:
        hydrogen sulfide: 1 ppm = 1.5 mg/m3, 1 mg/m3 = 0.7 ppm.

                             *           *

        Financial support for the publication of this criteria document
    was kindly provided by the Department of Health and Human Services
    through a contract from the National Institute of Environmental Health
    Sciences, Research Triangle Park, North Carolina, USA - a WHO
    Collaborating Centre for Environmental Health Sciences.


    1.1  Summary

    1.1.1  Properties and analytical methods

        Hydrogen sulfide is a colourless gas with a characteristic odour
    that is soluble in various liquids including water, alcohol, ether,
    and solutions of amines, alkali carbonates, and bicarbonates. It can
    undergo a number of oxidation reactions to yield principal products
    consisting of sulfur dioxide, sulfuric acid, or elemental sulfur.
    Reaction rates and oxidation products depend on the nature of the
    oxidizing agent.

        The methylene blue colorimetric method has acceptable specificity,
    accuracy, and sensitivity for hydrogen sulfide determinations, and is
    generally recognized as a standard analytical procedure. It has been
    used successfully, in automatic continuous monitoring, but
    sophisticated maintenance facilities and highly trained technicians
    are required for this method. Gas chromatography coupled with flame
    photometric detection is an alternative method for hydrogen sulfide
    determination, either as a laboratory method or for continuous
    monitoring in stationary field settings.

        Most of the direct-reading methods of hydrogen sulfide
    determination in the occupational environment are susceptible to
    various forms of interference. However, methods employing chemical
    detector tubes appear to be useful in occupational settings, where
    hazardous levels of hydrogen sulfide can occur. Under these
    conditions, reliability and accuracy compensate for a certain lack of

    1.1.2  Sources of hydrogen sulfide

        Hydrogen sulfide is one of the principal compounds involved in the
    natural cycle of sulfur in the environment. It occurs in volcanic
    gases and is produced by bacterial action during the decay of both
    plant and animal protein. It can also be produced by bacteria through
    the direct reduction of sulfate. Significant concentrations of
    hydrogen sulfide occur in some natural gas fields and in geothermally
    active areas.

        Hydrogen sulfide can be formed whenever elemental sulfur or
    certain sulfur-containing compounds come into contact with organic
    materials at high temperatures. In industry, it is usually produced as
    an undesirable by-product, though it is an important reagent or
    intermediate in some processes. Hydrogen sulfide occurs as a
    by-product in: the production of coke from sulfur-containing coal, the
    refining of sulfur-containing crude oils, the production of carbon
    disulfide, the manufacture of viscose rayon, and in the Kraft process
    for producing wood pulp.

    1.1.3  Environmental levels and exposures

        Though concentrations of hydrogen sulfide in urban areas may
    occasionally be as high as 0.050 mg/m3 (0.033 ppm) with averaging
    times of 30 min-1 h, they are generally (below 0.0015 mg/m3 
    (0.001 ppm). Peak concentrations as high as 0.20 mg/m3 (0.13 ppm)
    have been reported in the neighbourhood of point sources. In a
    geothermal area, 1-h mean concentrations of up to 2 mg/m3 (1.4 ppm)
    have been observed. When hydrogen sulfide was accidentally released in
    an incident in Poza Rica, Mexico, in 1950, the number of deaths that
    followed indicated that exposure levels probably exceeded 
    1500-3000 mg/m3 (1000-2000 ppm).

        It is believed that workers are not usually exposed to hydrogen
    sulfide concentrations above the occupational exposure limits of 
    10-15 mg/m3 (7-10 ppm) (8-h time-weighted average) adopted by many
    governments. There are, however, numerous reports of accidental
    exposures to concentrations that have ranged from 150 mg/m3
    (100 ppm) to as high as 18 000 mg/m3 (12 000 ppm). Such massive
    exposures to hydrogen sulfide have resulted either from leaks in
    industrial gas streams containing high levels of hydrogen sulfide or
    from the slow, insidious accumulation of hydrogen sulfide in low-lying
    areas. The second case may arise when hydrogen sulfide of biogenic
    origin is generated from such sources as sewage disposal plants and

    1.1.4  Effects on experimental animals

        In experimental animals, the effects of high doses of hydrogen
    sulfide and high doses of cyanide are very similar. Cyanide inhibits
    the enzyme cytochrome  c oxidase [EC] a, thereby
    interfering with tissue use of oxygen to the point where metabolic
    demands cannot be met. Hydrogen sulfide also exhibited an inhibitory
    action on a purified preparation of cytochrome  c oxidase.

        Results of studies on a number of animal species including canary,
    rat, guineapig, cat, dog, and goat showed that inhalation of hydrogen
    sulfide at a concentration of 150-225 mg/m3 (100-150 ppm)
    resulted in signs of local irritation of eyes and throat after many
    hours of exposure; at 300-450 mg/m3 (200-300 ppm), eye and mucous
    membrane irritation appeared after 1 h inhalation and slight general
    effects after prolonged inhalation; at 750-1050 mg/m3 (500-700 ppm),
    local irritation and slight systemic signs appeared within 1 h and


    a  The numbers within brackets following the names of enzymes are
       those assigned by the Enzyme Commission of the Joint IUPAC-IUB
       Commission on Biochemical Nomenclature.

    death after several hours; at 1350 mg/m3 (900 ppm), serious systemic
    effects appeared in less than 30 min and death within 1 h; at 
    2250 mg/m3 (1500 ppm), collapse and death occurred within 15-30 min;
    and, at 2700 mg/m8 (1800 ppm), there was immediate collapse with
    respiratory paralysis, and death. There is little information on the
    effects on experimental animals of long-term, low-level exposure to
    hydrogen sulfide gas.

    1.1.5  Effects on man  General toxicological considerations

        Hydrogen sulfide is both an irritant and an asphyxiant gas. Its
    direct irritant action on the moist tissues of the eye produces
    keratoconjunctivitis, known as "gas eye". When inhaled, hydrogen
    sulfide exerts an irritant action throughout the entire respiratory
    tract, although the deeper structures suffer the greatest damage. A
    consequence may be pulmonary oedema. At concentrations of 
    1500-3000 mg/m3 (1000-2000 ppm), hydrogen sulfide gas is rapidly
    absorbed through the lung into the blood, which initially induces
    hyperpnoea (rapid breathing). This is followed by respiratory
    inactivity (apnoea). At higher concentrations, hydrogen sulfide exerts
    an immediate paralysing effect on the respiratory centres. Death due
    to asphyxia is the certain outcome, unless spontaneous respiration is
    re-established or artificial respiration is promptly provided. This
    sequence of events represents the most important toxic effect of
    hydrogen sulfide.

        Acute hydrogen sulfide intoxication can be defined as the effects
    from a single exposure to massive concentrations of hydrogen sulfide
    that rapidly produce signs of respiratory distress. Concentrations
    exceeding about 1500 mg/m3 (1000 ppm) produce such acute effects.
    Subacute hydrogen sulfide intoxication is the term applied to the
    effects of continuous exposure for up to several hours to
    concentrations ranging from 150 to 1500 mg/m3 (100-1000 ppm). In
    this range of exposure, eye irritation is the most commonly observed
    effect. However, some reports have indicated that the threshold for
    eye irritation occurs after several hours of exposure to hydrogen
    sulfide at levels of 16-32 mg/m3 (10.5-21.0 ppm). Pulmonary oedema
    may be a more important and potentially fatal complication of subacute
    hydrogen sulfide intoxication. Chronic intoxication is a largely
    subjective state characterized by fatigue and believed by some to be a
    consequence of intermittent exposure to hydrogen sulfide
    concentrations of 75-150 mg/m3 (50-100 ppm). Not all research
    workers accept the existence of such a condition.

        The characteristic "rotten egg" odour of hydrogen sulfide is well
    known. The threshold of perception of this odour varies considerably
    depending on individual sensitivity, but, under laboratory conditions,
    it ranges from 0.0008 to 0.20 mg/m3 (0.0005-0.13 ppm). Above about
    225 mg/m3 (150 ppm), the gas exerts a paralysing effect on the

    olfactory apparatus, thus neutralizing the value of its odour as a
    warning signal. At these concentrations, the odour of the gas has been
    reported to be sickeningly sweet.  Occupational exposure

        Exposure to hydrogen sulfide in high concentrations occurs in
    numerous occupations. Workers in the oil, gas, and petrochemical
    industries are occasionally exposed to hydrogen sulfide in
    concentrations sufficient to cause acute intoxication. In one survey
    of the petrochemical industry, among 221 cases of hydrogen sulfide
    poisoning, the overall mortality was 6% and a high proportion of
    victims exhibited neurological signs and symptoms. Forty percent of
    all cases required some form of respiratory assistance; 15% developed
    pulmonary oedema.

        Persistent sequelae following acute intoxication have been
    reported among workers in a number of occupations including sewer
    workers, chemical plant employees, farmers, shale-oil workers, and
    laboratory attendants. Most victims who develop sequelae experience a
    state of unconsciousness during the acute phase of their illness.
    However, sequelae following acute intoxication without unconsciousness
    have also been reported.  Exposure of the general population

        Several episodes of general population exposure to hydrogen
    sulfide have been reported. The effects of such exposure have ranged
    from minor nuisance to serious illness and death. In a small community
    adjacent to an oilfield installation, large quantities of hydrogen
    sulfide were released into the air when an oilfield flare
    malfunctioned. Three hundred and twenty persons were hospitalized, 22
    of whom died. Nine exhibited manifestations of pulmonary oedema. Four
    victims developed neurological sequelae. The air levels of hydrogen
    sulfide were not measured.

        A community of 40 000 people, located in the vicinity of a large
    geothermal field, was exposed in some areas to hydrogen sulfide levels
    in air (measured on a continuous basis over a 5-month period)
    exceeding 0.08 mg/m3 (0.05 ppm), for, on average, 35% of the time.
    Although fatal cases of hydrogen sulfide intoxication associated with
    improper ventilation in geothermal steam-heated dwellings in this area
    were occasionally reported until 1962, the major problem has been the
    nuisance caused by the odour of the gas. In a moderately sized
    community, hydrogen sulfide was released from a small industrial waste
    lagoon resulting in a 1-h average concentration of hydrogen sulfide in
    air of 0.45 mg/m3 (0.3 ppm). Complaints were mostly related to the
    odour of hydrogen sulfide gas. However, the severity of complaints of
    nausea, vomiting, headache, loss of appetite, and disturbed sleep
    exceeded the mere nuisance level.

        No community studies of the long-term, low-level effects of
    hydrogen sulfide exposure have been reported.

    1.1.6  Evaluation of health risks

        Hydrogen sulfide in ambient air in concentrations of the order of
    the odour threshold has not been shown to have any significant
    biological activity in man or animals. In controlled laboratory
    studies, the odour threshold for hydrogen sulfide has been reported to
    range from 0.0008 to 0.20 mg/m3 (0.0005-0.13 ppm). Little
    information is available on the odour detection limits for hydrogen
    sulfide either under experimental field conditions or in the ambient
    air. However, the Task Group considered that a level of 0.008 mg/m3
    (0.005 ppm) averaged over 30 min should not produce odour nuisance in
    most situations. In the occupational setting, the earliest toxic
    response appears to be eye irritation, which has been reported to
    occur at 16-32 mg/m3 (10.5-21.0 ppm) after several hours' exposure.
    The occupational exposure guidelines for hydrogen sulfide recommended
    by the Task Group included the adoption of a level of 10 mg/m3
    (7 ppm) as a workshift time-weighted average value together with a
    short-term exposure limit of 15 mg/m3 (10 ppm), the latter to be
    determined as a 10-min or less, average value.

    1.2  Recommendations for Further Studies

        Measurements of hydrogen sulfide concentrations in the ambient air
    should be included in studies of the levels in air of other common
    gaseous contaminants, such as the oxides of sulfur and nitrogen.
    Studies in areas remote from man-made emission sources would provide
    background data for the development of models for long-distance
    transport and diffusion, for the evaluation of biological decay
    processes from natural sources, and for developing a clearer
    understanding of global sulfur cycles. More studies are required to
    elucidate processes involving chemical and photochemical oxidation
    reactions of hydrogen sulfide. Studies are also necessary to develop
    methods for the personal dosimetry measurement of hydrogen sulfide
    that do not require wet chemical techniques.

        Studies should be conducted in experimental animals on the
    cumulative neural effects of repeated and/or continuous long-term
    hydrogen sulfide exposure at concentrations that induce subacute or
    chronic intoxication. The cardiac sequelae after acute intoxication
    should be investigated in intact animals and in those with pre-induced
    cardiac damage. Studies of the toxicokinetics of absorbed hydrogen
    sulfide are needed and other studies should be initiated to test for
    the metabolic generation of hydrogen sulfide from sulfur-containing
    organic compounds.

        Case studies should be made of patients who have suffered acute
    hydrogen sulfide intoxication to examine the long-term effects on the
    myocardium. Efforts should be made to estimate the dose of hydrogen
    sulfide associated with acute poisoning. Prospective studies of new
    workers to investigate the effects of long-term exposure to
    concentrations of hydrogen sulfide likely to be encountered in the
    work place would be valuable. These studies should include
    considerations of morbidity and mortality, the incidence of cancer and
    teratogenic effects, and studies of changes in pulmonary function with
    time. Continuing environmental studies should play a major part in
    these prospective studies, in order to provide dose-response data,
    where possible. Similar studies should be initiated among the general
    population in a geothermal area, taking advantage of the natural
    conditions provided, for example, by the situation in Rotorua, New


    2.1  Chemical and Physical Properties

        Hydrogen sulfide is a flammable colourless gas with the
    characteristic odour of rotten eggs. It burns in air with a pale blue
    flame and, when mixed with air, its explosive limits are 4.3% to 46%
    by volume. Its autoignition temperature is 260°C. The relative
    molecular mass of hydrogen sulfide is 34.08. Its density is 
    1.5392 g/litre at 0°C and 760 min. The ratio density of hydrogen
    sulfide compared with air is 1.19. One gram of hydrogen sulfide
    dissolves in 187 ml of water at 10°C, in 242 ml of water at 20°C, in
    314 ml of water at 30°C, and in 405 ml of water at 40°C (calculated
    from Weast, 1977-78). It is also soluble in alcohol, ether, glycerol,
    and in solutions of amines, alkali carbonates, bicarbonates and
    hydrosulfides. The vapour pressure of hydrogen sulfide is 
    18.75 × 105 Pa at 20°C and 23.9 × 105 Pa at 30°C. Its melting
    point is -85.5°C and its boiling point is -60.3°C (Macaluso, 1969;
    Windholz, 1976).

        Hydrogen sulfide can undergo a large number of oxidation
    reactions, the type and rate of the reaction and the oxidation
    products depending on the nature and concentration of the oxidizing
    agent. The principal products of such reactions are sulfur dioxide,
    sulfuric acid, or elemental sulfur. Aqueous solutions of chlorine,
    bromine, and iodine may react with hydrogen sulfide to form elemental
    sulfur. In the presence of oxides of nitrogen, the oxidation of
    hydrogen sulfide in the gas phase may result in the formation of
    sulfur dioxide or sulfuric acid but, in aqueous solution (pH 5-9), the
    primary product is elemental sulfur (Macaluso, 1969).

        Hydrogen sulfide dissociates in aqueous solution to form 2
    dissociation states involving the hydrosulfide anion (HS-) and the
    sulfide anion (S=). The pKa in 0.01-0.1 mol/litre solutions at 18°C
    is 7.04 for HS- and 11.96 for S=. At the physiological pH of 7.4,
    about one-third of the total sulfide remains as the undissociated acid
    and about two-thirds as the HS- ion. The undissociated hydrogen
    sulfide in solution is in dynamic equilibrium at the air-water
    interface with gaseous hydrogen sulfide (National Research Council,
    USA, 1977).

    2.2  Atmospheric Chemistry

        The atmospheric chemistry of hydrogen sulfide and other sulfur
    compounds involves chemical and photochemical oxidation reactions of
    emissions from both natural and man-made sources. The eventual
    oxidation products are sulfuric acid (H2SO4) and/or sulfate ion

        There have been relatively few studies of the persistence and
    conversion of hydrogen sulfide under atmospheric conditions.
    Krasovitskaja and her co-workers (Krasovitskaja et al., 1965) studied
    the relationship between concentrations of hydrogen sulfide, sulfur
    dioxide, carbon monoxide, and hydrocarbons, and the distance from
    their industrial sources. Hydrogen sulfide concentrations dropped by a
    factor of 2 between the immediate neighbourhood of the source and a
    2.5 km radius. A further decrease in concentration ranging from 30% up
    to a factor of 8 occurred between the 2.5 km and 20 km radii. These
    decreases were, in general, greater than those observed for any of the
    other pollutants measured. Andersson et al. (1974) reported studies
    concerning the photolysis of hydrogen sulfide and its reaction with
    sulfur dioxide, as well as its reactions with atomic and molecular
    oxygen and with ozone.

        Junge (1963) calculated that the residence time of hydrogen
    sulfide was approximately 1.7 days in the presence of an ozone level
    of 0.05 mg/m3. A similar residence time was estimated by Katz (1977)
    using data from the global budget of the sulfur cycle presented by
    Kellogg et al. (1972). Robinson & Robbins (1970) found a residence
    time in relatively clean air of about 2 days, compared with only about
    2 h in a polluted urban atmosphere.

        Considerably lower values than those of the previously mentioned
    investigators, based on the global sulfur budget, have been presented
    by Granat et al. (1976), on the basis of a very much lower release of
    sulfur compounds from the biological decay of organic matter from land
    and sea. Clearly, this represents a subject that requires further
    studies involving the measurement of atmospheric concentrations in
    relatively clean areas on land and elsewhere.

    2.3  Sampling and Analytical Methods

        Because levels of hydrogen sulfide in air, which are of interest
    as far as human health in concerned, range from highly concentrated
    industrial gas streams to ambient air pollution levels, numerous
    analytical methods have been applied. A recent review of current
    methods can be found in Becker (1979). A further complication in
    determining air levels of hydrogen sulfide is that according to the
    air monitoring application, sampling may be on either an intermittent
    or a continuous basis. Intermittent samples have been taken in plastic
    bags, evacuated bottles, Tutweiler burets (Shaw, 1940), and detector
    tubes (West, 1970). Continuous samples have been taken by exposing
    chemically treated paper tapes (Sanderson et al., 1966; Peregud et
    al., 1971) or ceramic tiles (Gilardi & Manganelli, 1963) to air, by
    pumping air through a lead acetate solution, by bubbling air through
    impingers containing absorbing or colorimetric solutions (Goldman et
    al., 1940), and by using long-duration detector tubes or electronic
    detectors (West, 1970; ACGIH, 1972; Thompkins & Becker, 1976).

    Qualitative hydrogen sulfide detection has been based on the
    blackening of coins, keys, lead-based paint, and lead-acetate treated
    papers. More recently, direct reading instruments have been developed
    that make real-time monitoring possible. In some of these instruments,
    a 2-step absorption-reaction procedure is involved whereas in others,
    the gas reacts directly with, for example, metal-oxide-coated chips,
    the electrical properties of which change in response to various gas
    concentrations. Gas chromatographic methods of analysis have been
    developed and are particularly used by oil and gas production
    companies (Stevens et al., 1971). A recent report of the US National
    Institute for Occupational Safety and Health (NIOSH, 1977) summarizes
    the current situation with special reference to automatic and/or
    portable samplers. The report concludes that wet chemical methods are
    attractive because of their specificity and precision, but that they
    are less desirable on the basis of the portability and maintenance
    characteristics of the equipment. Direct reading solid state devices,
    on the other hand, are portable and relatively rugged but are often
    nonspecific and susceptible to cross-sensitivities.

        However, the practical importance of such cross-sensitivities
    depends a great deal on the type of study. In ambient air pollution
    studies in which hydrogen sulfide can be excepted to be in the 
    0.0015-0.075 mg/m3 (0.001-0.050 ppm) concentration range,
    interference may be of much greater practical concern than in
    industrial settings in which concentrations may reach from 30 to 
    75 mg/m3 (20 to 50 ppm) or more, on occasion, and in which the
    presence and identity of interfering compounds are often known.

        Because of the diversity of circumstances under which hydrogen
    sulfide has to be determined, only the two principal methods of
    analysis for hydrogen sulfide are described in detail in the following
    section. Questions of sampling and analysis suitable for several
    specific practical applications are also discussed.

    2.3.1  The methylene blue method

        The methylene blue colorimetric method has been evaluated and
    recommended by various research workers (Jacobs, 1965; Bamesberger &
    Adams, 1969) and by some institutions such as the US National
    Institute for Occupational Safety and Health (NIOSH, 1977). This
    method has also been proposed by the International Organization for
    Standardization (ISO, 1978). The Intersociety Committee of the
    American Public Health Association has published detailed procedures
    of this method for assessing hydrogen sulfide both in the ambient air
    and in workplace air (Intersociety Committee, 1977a).

        Although light, mercaptans, sulfides, nitrogen dioxide, and sulfur
    dioxide can cause interference, and instruments incorporating both the
    absorption and reaction functions are not portable, the methylene blue
    method appears to combine adequate specificity with good accuracy and
    precision and extreme sensitivity. It can be used with either manual

    or automatic sample collectors and, in the latter case, with
    continuous sampling, levels of hydrogen sulfide as low as 0.003 g/m3
    (0.002 ppm) can be detected (Levaggi et al., 1972).

        In the Intersociety Committee method, hydrogen sulfide is
    collected by aspirating a measured volume of air through an alkaline
    suspension of cadmium hydroxide. The sulfide is precipitated as
    cadmium sulfide to prevent air oxidation of the sulfide, which occurs
    rapidly in an aqueous alkaline solution. Arabinogalactan is added to
    the cadmium hydroxide slurry to minimize the photodecomposition of the
    precipitated cadmium sulfide. The collected sulfide is subsequently
    determined by spectrophotometric measurement of the methylene blue
    produced by the reaction of the sulfide with a strongly acid solution
    of  N, N-dimethyl- p-phenylenediamine and ferric chloride. The
    analysis should be completed within 24-26 h of collection of the

        This method is intended for the determination of hydrogen sulfide
    concentrations in the range of 0.0012-0.1 mg/m3 (0.0008-0.07 ppm).
    For concentrations above 0.08 mg/m3 (0.05 ppm), the sampling period
    can be reduced or the volume of liquid increased either before or
    after aspirating. Excellent results have been obtained using this
    method for air samples containing hydrogen sulfide concentrations in
    the range of 7.5-75 mg/m3 (5-50 ppm). This method is also useful for
    the measurement of source emissions. For example, 100 ml cadmium
    sulfide-arabinogalactan medium in Greenberg-Smith impingers and 5-min
    sampling periods have been used successfully.

        The methylene blue reaction is highly specific for sulfide at the
    low concentrations usually encountered in ambient air. Strong reducing
    agents (e.g., sulfur dioxide) inhibit colour development. Even
    solutions containing several micrograms of sulfide per millilitre show
    this effect and must be diluted to eliminate colour inhibition. If
    sulfur dioxide is absorbed to give a sulfite concentration in excess
    of 10 µg/ml, colour formation is retarded. Up to 40 µg/ml of this
    interference, however, can be overcome by adding 2-6 drops 
    (0.5 ml/drop) of ferric chloride instead of a single drop for colour
    development, and extending the reaction time to 50 min. On the other
    hand, nitrogen dioxide gives a pale yellow colour with the sulfide
    reagents at concentrations of 0.5 µg/ml or more. No interference is
    encountered when 0.57 mg/m3 (0.3 ppm) of nitrogen dioxide is
    aspirated through a midget impinger containing a slurry of cadmium
    hydroxide-cadmium sulfide-arabinogalactan. If hydrogen sulfide and
    nitrogen dioxide are simultaneously aspirated through cadmium
    hydroxide-arabinogalactan, slurry, lower results are obtained,
    probably because of the gas-phase oxidation of the hydrogen sulfide
    prior to precipitation as cadmium sulfide.

        Using permeation tubes as a source of hydrogen sulfide, a relative
    standard deviation of 3.5% and a recovery of 80% have been
    established. The overall sampling and analytical precision is 12.1%
    relative standard deviation.

        Hydrogen sulfide is readily volatilized from an aqueous solution,
    when the pH is below 7.0. Alkaline aqueous sulfide solutions are very
    unstable because the sulfide ion is rapidly oxidized by exposure to
    the air.

        Cadmium sulfide is not appreciably oxidized, even when aspirated
    with pure oxygen in the dark. However, exposure of an impinger
    containing cadmium sulfide to laboratory or more intense light sources
    produces immediate and variable photodecomposition. Losses of 50%-90%
    of sulfide have been routinely reported by a number of laboratories.
    Even though the addition of arabinogalactan to the absorbing solution
    controls the photodecomposition, it is necessary to protect the
    impinger from light at all times. This is achieved by the use of low
    actinic glass impingers, paint on the exterior of the impingers, or
    aluminium foil wrapping.

        The Intersociety Committee (1977a) has described the apparatus,
    reagents, and calibration methods suitable for use with midget
    impinger samplers and appropriate for determining air concentrations
    of 0.0012-0.1 mg/m3 (0.0008-0.07 ppm). In this range, calibration is
    recommended using PTFE permeation tubes. In the higher concentration
    range relevant to workroom air, the National Institute for
    Occupational Safely and Health recommends that calibration be carried
    out using commercially available cylinders of hydrogen sulfide in dry
    nitrogen (NIOSH, 1977).

    2.3.2  Gas chromatography with flame photometric detection

        An Intersociety Committee method also exists for the determination
    of hydrogen sulfide using gas chromatography (Intersociety Committee,
    1977b). This method requires the use of a gas chromatograph equipped
    with a flame photometric detector. A narrow-band optical filter
    selects the 394 ± 5 mm sulfur line. Gas chromatography separates
    sulfur compounds of low relative molecular mass before detection, and
    thereby allows individual quantitative measurement of sulfur-
    containing gases such as hydrogen sulfide, sulfur dioxide, methyl
    mercaptan, and dimethyl sulfide.

        In situations where minimal quantities of reduced sulfur compounds
    other than hydrogen sulfide are present, flame photometry can be used
    directly, in which case the hydrogen sulfide concentration is
    approximately the same as the total sulfur compounds measured. An
    absorbant is usually required to selectively remove sulfur dioxide,
    when flame photometry is used without separation of individual
    compounds by gas chromatography before detection.

        The limits of detection of this method, at twice the noise level
    for hydrogen sulfide, sulfur dioxide, methyl mercaptan, and dimethyl
    sulfide, range from 0.005 to 0.013 mg/m3 (0.0035-0.009 ppm).
    Sensitivity can be increased by the use of sample concentration
    techniques such as a freeze-out loop in the gas chromatograph sampling
    line. The upper limit of detection is 0.5 mg/m3 (0.35 ppm) but it
    can be extended by sample dilution to higher ranges, if necessary.
    Although no data are available on the precision and accuracy of the
    method for atmospheric samples, repetitive sampling of standard
    reference gases containing a hydrogen sulfide concentration of 
    0.08 mg/m3 (0.055 ppm) and a sulfur dioxide concentration of 
    0.104 mg/m3 (0.036 ppm) gave a relative standard deviation of less
    than 3% of the amount present (Intersociety Committee, 1977b).

        An advantage of flame photometric detection is that chemical
    solutions are not necessary, and the only required reagent is hydrogen
    for the flame. However, the need for a compressed hydrogen supply may
    be a disadvantage in certain situations. The analyser is calibrated
    using hydrogen sulfide, sulfur dioxide, methyl mercaptan, and dimethyl
    sulfide permeation tubes, and a dual-flow gas dilution device capable
    of producing reference standard atmospheres as low as the limits of
    detection of the method. Because the photomultiplier tube output is
    logarithmically proportional to the sulfur concentration, conversion
    can be done by either plotting the response against concentration on a
    logarithmic scale or by using a logarithmic-linear amplifier. Using
    either of these techniques, the range has been established at
    approximately 0.13-0.5 mg/m3 (0.09-0.35 ppm) with a 1% noise level
    (Intersociety Committee, 1977b).

        Several commercial flame photometric detection analysers are now
    available (with and without separation of the sulfur compounds by gas
    chromatography before detection). This method of analysis for hydrogen
    sulfide is suitable for use as a laboratory method for calibration
    purposes or for continuous monitoring in stationary field settings.

    2.3.3  Automatic monitors in stationary field settings

        Paper tapes impregnated with lead acetate have been widely used
    for making measurements in the field (Denmead, 1962; Thom & Douglas,
    1976; Institute of Hygiene and Epidemiology, 1978). A measured volume
    of air is filtered through the tape and the optical density of the
    discoloured area is compared with an unexposed area of the same tape.
    Numerous criticisms of these procedures have been reported and it is
    clear that the presence of any substance capable of oxidizing the lead
    sulfide can lead to substantial errors (Sanderson et al., 1966).
    Various modifications of the basic method have been suggested to
    minimize errors and increase sensitivity by Siu et al. (1971),
    including the substitution of mercuric chloride for lead acetate as
    proposed by Paré (1966). Natusch et al. (1974) evaluated various paper

    tape methods and concluded that tapes impregnated with silver nitrate
    are highly suitable for the determination of hydrogen sulfide
    concentrations in the range of 0.0015-75 mg/m3 (0.001-50 ppm).
    Moreover, they state that monitors using silver nitrate tape are
    simple, specific, portable, capable of unattended operation and
    inexpensive. However, silver nitrate tape systems remain subject to
    photodecomposition, an important deficiency for many field uses.

        Continuous monitors based on various wet chemical procedures have
    been developed. For example, some sulfur dioxide monitors based on
    amperometric principles can be used to monitor hydrogen sulfide by
    replacing a silver screen (which normally filters out H2S) with a
    barium acetate scrubber that removes any sulfur dioxide in the
    influent airstream. These devices are reported to have good
    maintenance characteristics and are suitable for use in remote areas.
    However, like most continuous instruments based on wet chemistry, they
    are relatively expensive (Lawrence Berkeley Laboratory, 1976).

        Continuous monitors using the methylene blue method have also been
    developed (Levaggi et al., 1972). These units are attractive in that
    they possess the inherent sensitivity of the methylene blue method,
    but they require sophisticated support facilities and highly trained
    personnel for reliable operation. In this respect, the newer metal
    oxide-coated-chip semiconductor devices appear promising for field
    use. However, there are few published reports of field experience to
    date (Thompkins & Becker, 1976).

    2.3.4  Direct-reading portable detection systems

        Semiquantitative methods of hydrogen sulfide detection based on
    lead acetate-treated papers on tiles have been reported and are said
    to be sensitive to levels of about 1 mg/m3 (0.7 ppm) (Gilardi &
    Manganelli, 1963). However, long-duration detector tubes for hydrogen
    sulfide, suitable for use in the occupational environment, are now
    available, which are inexpensive and responsive over a wide range of
    concentrations (1-84 mg/m3; 0.7-56 ppm) and overcome some of the
    deficiencies of the lead acetate detectors (Leichnitz, 1977). The US
    National Institute for Occupational Safety and Health has investigated
    the quality and reproducibility of detector tubes available from
    various US manufacturers and reports that it is possible to obtain
    tubes that meet the quality specifications of the Institute (Johnson,
    1972). Detector tubes for hydrogen sulfide are susceptible to
    interference from other sulfides, sulfur dioxide, and nitrogen
    dioxide, but, generally such interference would result in false
    positive readings.

        Various portable direct-reading hydrogen sulfide meters available
    on the US market have been evaluated by the National Institute for
    Occupational Safety and Health (Thompkins & Becker, 1976). These
    instruments are intended principally for industrial hygiene surveys

    and, in particular, for ascertaining the degree of general compliance
    with occupational health standards for hydrogen sulfide. The
    instruments surveyed operated on solid-state electrochemical
    principles, wet alectrochemical principles, and in one case, on a
    photoionization principle. In terms of response time, calibration
    stability, and reliability, the photoionization instrument was
    regarded as superior, but it was the least specific of the instruments
    evaluated. The solid-state instruments tended to have slow response
    times and accuracy deficiencies but were very reliable and rugged. The
    wet alectrochemical meters ranked highly in terms of accuracy,
    response time, and calibration stability but were somewhat less

    2.3.5  Manual collection and analysis of air samples in occupational

        In 1943, the American Public Health Association Sub-Committee on
    Chemical Methods in Air Analysis recommended collecting hydrogen
    sulfide with cadmium chloride in 2 simple petticoat bubblers in series
    followed by titration with iodine, using starch as an indicator or
    using an excess of iodine and back-titrating with sodium thiosulfate
    solution (Goldman et al., 1943). In 1965, the AIHA Analytical Guide
    (AIHA, 1965) listed 3 methods for determining hydrogen sulfide in air:
    (a) iodine oxidation using a Tutweiler buret; (b) cadmium sulfate and
    iodine in a midget impinger; and (c) formation of cadmium sulfide
    colloid using 2 midget impingers in series followed by conversion to
    methylene blue. The iodine methods are susceptible to interference at
    hydrogen sulfide levels expected in occupational settings. More
    recently, to ascertain employee exposure to hydrogen sulfide, the
    National Institute for Occupational Safety and Health recommended the
    collection of breathing-zone samples with a midget impinger and
    analysis by the methylene blue method (NIOSH, 1977).

        In recent years, there have been developments in the use of solid
    adsorbents for the collection of sulfur gases. This technique for
    sample collection could be used in association with the gas
    chromatography using flame photometric detection for the measurement
    of hydrogen sulfide and other low relative molecular mass or self-
    containing gases (Black et al., 1978). This could ultimately lead to
    the development of solid-state personal dosimeters for hydrogen
    sulfide as an alternative to those that involve wet chemistry.


    3.1  Natural Sources

        Hydrogen sulfide is one of the principal compounds involved in the
    natural sulfur cycle in the environment (National Research Council,
    USA, 1979). As indicated in Fig. 1, it occurs in volcanic gases and is
    produced by bacterial action during the decay of both plant and animal
    protein (Cooper et al., 1976). Many bacteria, fungi, and actinomycetes
    release hydrogen sulfide into the environment during the decay of
    compounds containing sulfur-bearing amino acids and in the direct
    reduction of sulfate. The heterotroph  Proteus vulgaris is an example
    of a common bacterium that produces hydrogen sulfide, when grown in
    the presence of protein (National Research Council, USA, 1979).

        The reduction of sulfate to hydrogen sulfide can be accomplished
    by members of 2 genera of anaerobic bacteria,  Desulfovibrio and
     Desulfotomaculum. The organic substrates for these organisms are
    usually short chain organic acids that are provided by the
    fermentative activities of other anaerobic bacteria or more complex
    organic material. Hence, hydrogen sulfide production can be expected
    in conditions where oxygen is depleted, organic material is present,
    and sulfate is available (National Research Council, USA, 1979).

        From a microbiological point of view, the production of hydrogen
    sulfide is balanced by processes involving a variety of bacteria,
    found in soil and water, that can oxidize hydrogen sulfide to
    elemental sulfur. Among these are the filamentous bacteria,  Beggiatoa
    and  Thiothrix. Photosynthetic bacteria belonging to the families
    Chromatiaceae and Chlorobiaceae oxidize hydrogen sulfide to elemental
    sulfur and sulfate in the presence of light and the absence of oxygen.
    Reduced sulfur compounds are also oxidized in nature by members of the
    genus  Thiobacillus. The end result of this oxidative activity is the
    production of sulfate which, once formed, is extremely stable to
    further chemical activity in nature (National Research Council, USA,

        As a result of these various biogeochemical processes, hydrogen
    sulfide occurs in and around sulfur springs and lakes and is almost
    continuously present as an air contaminant in some geothermally active

    3.2  Sources Associated with Human Activity

        There are various circumstances under which naturally occurring
    hydrogen sulfide is released by human activity. For example, hydrogen
    sulfide occurring in association with natural gas and/or crude oil
    deposits in some areas may be released during extraction and drilling
    operations. The sulfur content of crude oils ranges from 0 to 5% and
    some natural gas deposits have been reported to comprise up to 42%

    FIGURE 1

    hydrogen sulfide (Espach, 1950). Coals can contain sulfur levels of up
    to 80 g/kg and, occasionally, conditions arise in which hydrogen
    sulfide is formed within such deposits. Thus, special precautions must
    be taken in some mining operations as well as in the drilling and
    extraction of natural gas and crude oils with significant sulfur

        Hydrogen sulfide can also be released by activities surrounding
    the development and use of geothermal resources. At the Cerro Prieto
    geothermal power generating plant in Baja California, Mexico, for
    example, hydrogen sulfide levels are sufficiently high to necessitate
    special ventilation to protect electrical systems, and alarms for the
    protection of personnel (Mercado, 1975).

        During industrial operations, hydrogen sulfide can be formed
    whenever elemental sulfur or certain sulfur-containing compounds come
    into contact with organic materials at high temperatures. It is
    usually produced as an undesirable by-product, though it is also used
    as an important reagent or desirable intermediate compound in some
    industrial processes such as the manufacture of sulfides, sodium
    hydrosulfide, and various organic sulfur compounds. Examples of
    processes in which hydrogen sulfide occurs as a by-product include the
    production of coke from sulfur-containing coal, the production of
    carbon disulfide, the manufacture of viscose rayon in the Kraft
    process for producing wood pulp (Macaluso, 1969) and sulfur extraction
    by the Frasch process.

        In refining sulfur-containing crude oils, about 80%-90% of the
    divalent sulfur compounds of hydrogen and carbon are converted to
    hydrogen sulfide. Both the hydrogen sulfide produced and that
    occurring in other industrial, geothermal, or natural gas streams can
    be recovered by one of a number of processes that can be classified as
    either absorption-desorption processes or processes involving
    oxidation to oxides or to elemental sulfur. The bulk of hydrogen
    sulfide recovered in industrial processes is used to produce elemental
    sulfur or sulfuric acid (Macaluso, 1969).

        Large quantities of hydrogen sulfide are used in the production of
    heavy water, which is employed as a moderator in some nuclear power
    reactors. The process is based on enrichment of the deuterium content
    of water by hydrogen sulfide in a gas/liquid ion exchange system,
    followed by separation of heavy water and water by fractional
    distillation (McGraw-Hill Encyclopedia of Science and Technology,

        In the tanning industry, hydrogen sulfide is produced in the
    process by which hair or wool is removed from the hides. This
    typically involves deliming by adding ammonium chloride or ammonium
    sulfate followed by pickling with sulfuric acid, and takes place in
    large rotating drums. The gases evolved, including hydrogen sulfide,

    are released from the drums on opening the hatches either to add
    chemicals or to unload the treated hides, and also from the waste
    waters (ILO, 1971).

        As in the natural environment, hydrogen sulfide can be generated
    by bacterial action in industrial or community settings in malodorous
    and sometimes dangerous amounts.

        In some countries, such as India and Sri Lanka, hydrogen sulfide
    is produced in the process by which coconut fibres are separated from
    the husk. This procedure involves the decomposition of the husks in
    shallow ponds. The hydrogen sulfide is produced as a result of
    microbiological decay processes.


    4.1  Concentrations in Outdoor Air

        There are few published data on either natural background or urban
    air levels of hydrogen sulfide. Robinson & Robbins (1970) estimated
    the average ambient air level of hydrogen sulfide to be 0.0003 mg/m3
    (0.0002 ppm). This estimate supports the data of Minster (1963) who,
    when sampling over a 2“-year period in northwest London, reported that
    air levels of hydrogen sulfide were generally below 0.00015 mg/m3
    (0.0001 ppm), under clear, fresh conditions. Minster (1963) also
    reported that average summer levels ranged from 0.00015 to 0.0007
    mg/m3 (0.0001-0.0005 ppm) and average winter levels from 0.0007 to
    0.0015 mg/m3 (0.0005-0.001 ppm). Data collected from this same
    station during the London fog of December 1962 indicated a hydrogen
    sulfide concentration of up to 0.046 mg/m3 (0.033 ppm) on 6 December
    during heavy fog (each sampling time was 32 min).

        Measurements summarized by the US National Air Pollution Control
    Administration show concentrations ranging from below 0.001 mg/m3 to
    0.006 mg/m3 (0.0007 to 0.0042 ppm) at various urban locations in the
    USA in the period 1951-64 (Miner, 1969). However, as sensitive and
    standardized methods of sampling and analysis for hydrogen sulfide
    were lacking during this period, there is some doubt about the
    reliability of these data. Furthermore, the averaging times for the
    data are not available.

        Much higher concentrations of hydrogen sulfide have been measured
    near point sources. In California, peak concentrations as high as 
    0.20 mg/m3 (0.13 ppm) were measured near a pulp and papermill at the
    time of its commissioning (California Air Resources Board, 1970).
    After operating for several months, levels fell to 0.015 mg/m3
    (0.010 ppm) or less. The averaging time was not reported. Near a brick
    works in Boom, Belgium, air levels of hydrogen sulfide were monitored
    over a 6-month period. During this time, the average 24-h
    concentration of hydrogen sulfide was 0.005 mg/m3 (0.003 ppm) with
    occasional daily averages in excess of 0.017 mg/m3 (0.011 ppm)
    (Institute of Hygiene & Epidemiology, 1978).

        A major accidental release of hydrogen sulfide occurred at Poza
    Rica, Mexico, in 1950 (McCabe & Clayton, 1952). Although no data could
    be collected on environmental levels during this episode, numerous
    fatalities occurred indicating that exposure levels were most likely
    in excess of 1500-3000 mg/m3 (1000-2000 ppm). Further details of
    this episode are given in section 6.3.

        In the geothermally active areas in and around the city of
    Rotorua, New Zealand, airborne concentrations of hydrogen sulfide are
    usually sufficient to cause noticeable odours (Thom & Douglas, 1976).
    At one site, for one day, a 1-h mean concentration of up to

    2.0 mg/m3 (1.4 ppm) was reported (Thom & Douglas, 1976). Continuous
    measurements taken at another site over a period of 5 months showed
    that a concentration of 0.08 mg/m3 (0.05 ppm) was exceeded, on
    average, 35% of the time. It was also found that there were
    considerable seasonal variations in the hydrogen sulfide levels,
    reflecting the fluctuating steam-use patterns and also changes in the
    dispersive nature of the atmosphere. During the mid-winter months of
    the 1978 monitoring period, a concentration of hydrogen sulfide in air
    of 0.08 mg/m3 (0.05 ppm) was exceeded more than 55% of the time,
    whereas, during warmer months, this concentration was exceeded less
    than 20% of the time (Rolfe, 1980).

        Also in New Zealand, fine discharge of industrial and domestic
    liquid wastes into an inlet near Auckland created conditions in which
    hydrogen sulfide levels were sufficient to cause paint blackening and
    complaints of offensive odours. Continuous air monitoring was
    conducted for 21 months. These data indicated that 40-min average
    hydrogen sulfide concentrations in air of from 0.8 to 1.4 mg/m3
    (0.5-0.96 ppm) occurred at some time during the worst months of the
    year at all the sites monitored (Denmead, 1962).

    4.2  Concentrations in Work Places

        Under normal operating conditions, concentrations of hydrogen
    sulfide in the air in work places are believed to be less than 
    10-15 mg/m3 (7-10 ppm), the 8-h time weighted average that most
    national authorities have set as their occupational exposure standard

        It is well known, however, that hazardous exposures to hydrogen
    sulfide can occur under accidental circumstances in industries in
    which gas streams with a high hydrogen sulfide content exist.
    Furthermore, as hydrogen sulfide is slightly heavier than air, it can
    accumulate in toxic concentrations in low-lying areas, even when
    generated or leaking at very low rates. However, in such cases, the
    environmental levels of hydrogen sulfide have usually only been
    measured after the accidents in question, or have been determined by
    simulation or reenactment. Concentrations that have been reported
    range from 150 mg/m3 (100 ppm), in which a worker lost consciousness
    while sawing ebonite boards (Brown, 1969), to 18 000 mg/m3 
    (12 000 ppm) in a case in which a truck driver died while cleaning the
    tank of a vehicle used to transport industrial waste (Simson &
    Simpson, 1971). In an outdoor setting, 4 workmen lost consciousness
    while digging a pit in marshy land in which hydrogen sulfide
    concentrations in air of 442-810 mg/m3 (295-540 ppm) were measured 5
    days later (Anonymous, 1952). Alexander (1974) reported hydrogen
    sulfide concentrations as high as 0.037 mg/litre (24.8 ppm) in a
    sewage stabilization pond and 10.0-13.2 mg/m3 (6.7-8.8 ppm) in the
    air 15 m from this pond. In a report by Ahlborg (1951) on hydrogen

    sulfide poisoning in the Swedish shale oil industry, concentrations of
    hydrogen sulfide measured at various locations in the plant ranged
    from 30 to 900 mg/m3 (20-600 ppm), though men seldom worked where
    the high levels occurred.

        More recently, the US National Institute for Occupational Safety
    and Health reported that, in viscose rayon churn rooms, spinning
    tanks, and drying and storage cells, workers were mainly exposed
    during the working day to hydrogen sulfide concentrations of 
    23 mg/m3 (15 ppm) or less with occasional peaks of 150 mg/m3 
    (100 ppm) (NIOSH, 1977).

        In the USA, it has been estimated that there are 125 000 employees
    potentially exposed to hydrogen sulfide (NIOSH, 1977), Table 1 is a
    list of occupations in which such exposure can occur ranging according
    to occupation from rare exposure to low concentrations, to frequent
    exposure to concentrations very near those associated with adverse
    health effects.

    Table 1.  Examples of occupations with potential exposure to
              hydrogen sulfidea

    Animal fat and oil processors          Lithographers
    Animal manure removers                 Lithopone makers
    Artificial-flavour makers              Livestock farmers
    Asphalt storage workers                Manhole and trench workers
    Barium carbonate makers                Metallurgists
    Barium salt makers                     Miners
    Blast furnace workers                  Natural gas production and
    Brewery workers                          processing workers
    Bromide-brine workers                  Painters using polysulflde
    Cable splicers                           caulking compounds
    Caisson workers                        Papermakers
    Carbon disulfide makers                Petroleum production and
    Cellophane makers                        refinery workers
    Chemical laboratory workers,           Phosphate purifiers
      teachers, students                   Photo-engravers
    Cistern cleaners                       Pipeline maintenance workers
    Citrus root fumigators                 Pyrite burners
    Coal gasification workers              Rayon makers
    Coke oven workers                      Refrigerant makers
    Copper-ore sulfidizers                 Rubber and plastics processors
    Depilatory makers                      Septic tank cleaners
    Dyemakers                              Sewage treatment plant workers
    Excavators                             Sewer workers
    Felt makers                            Sheepdippers
    Fermentation process workers           Silk makers
    Fertilizer makers                      Slaughterhouse workers
    Fishing and fish-processing workers    Smelting workers
    Fur dressers                           Soapmakers
    Geothermal-power drilling and          Sugar beet and cane processors
      production workers                   Sulfur spa workers
    Gluemakers                             Sulfur products processors
    Gold-ore workers                       Synthetic-fibre makers
    Heavy-metal precipitators              Tank gaugers
    Heavy-water manufacturers              Tannery workers
    Hydrochloric acid purifiers            Textile printers
    Hydrogen sulfide production            Thiophene makers
      and sales workers                    Tunnel workers
    Landfill workers                       Well diggers and cleaners
    Lead ore sulfidizers                   Wool pullers
    Lead removers

    a  From: NIOSH (1977).


        Very little information is available on the effects of low level
    concentrations of hydrogen sulfide gas on experimental animals; most
    published data have emphasized the effects of exposure to lethal or
    near-lethal concentrations of the gas. According to Evans (1967) and
    Smith & Gosselin (1979), the effects of high doses of hydrogen sulfide
    and high doses of cyanide are very similar. Both inhibit the enzyme
    cytochrome  c oxidase [EC]. This was demonstrated in studies
    using purified preparations of the enzyme (Smith & Gosselin, 1979).

        When sodium sulfide at 0.1, 0.25, and 0.32 mmol/kg body weight was
    administered intraperitoneally to mice, sulfide was not exhaled
    (Susman et al., 1978) suggesting that it was inactivated primarily by
    metabolism. In studies on the inhalation of hydrogen sulfide in rats,
    cats, rabbits, and dogs, the nervous centres were first excited arid
    then paralysed; pupils first contracted and then dilated; blood
    pressure was first raised, then lowered; and respiration first
    increased and then halted (Evans, 1967; Haggard, 1925). The findings
    of Lehmann (1892) in the cat, dog, and rabbit, of Haggard (1925) in
    the dog, and of Sayers et al. (1925) in the canary, rat, guineapig,
    dog, and goat, are quite consistent: at 150-225 mg/m3 (100-150 ppm),
    signs of local irritation of eyes and throat after many hours of
    exposure; at 300-450 mg/m3 (200-300 ppm), eye and mucous membrane
    irritation after inhalation for 1 h and slight general effects with
    prolonged inhalation; at 750-1050 mg/m3 (500-700 ppm), local
    irritation and slight systemic symptoms in less than 1 h and possible
    death after several hours' exposure; at 1350 mg/m3 (900 ppm), grave
    systemic effects within 30 min and death in less than 1 h; at 
    2250 mg/m3 (1500 ppm), collapse and death within 15-30 min; and at
    2700 mg/m3 (1800 ppm), immediate collapse, respiratory paralysis,
    and death.

        When mice were exposed repeatedly (4 times for 2 h, at 4-day
    intervals) to a hydrogen sulfide concentration in air of 150 mg/m3
    (100 ppm), the critical inhibition of terminal cytochrome  c oxidase
    appeared to be cumulative. This effect was accompanied by a cumulative
    decrease in cerebral RNA synthesis (Savolainen et al., 1980).
    Experimental studies on rabbits indicated that either a single or
    repeated exposure for 1.5 h per day (5 consecutive days) to a hydrogen
    sulfide concentration in air of 105 mg/m3 (70 ppm) caused
    electrocardiographic (ECG) changes (Kósmider et al., 1967).

        Though some small differences in susceptibility to hydrogen
    sulfide gas were exhibited among the species studied by Sayers et al.,
    (1925), canaries being the most sensitive and goats the most
    resistant, the interspecies differences were slight. It is agreed
    among investigators that the effects of hydrogen sulfide gas on the
    nervous system represent the most important aspect of its toxicity
    (Haggard, 1925; Evans, 1967). Beck et al. (1979) showed that ethanol

    in doses of 0.33-0.66 g/kg significantly shortened the time to loss of
    consciousness in rats exposed to a hydrogen sulfide concentration in
    air of 1200 mg/m3 (800 ppm) for 30 min. The induction of
    methaemoglobinaemia by the injection of sodium nitrite had both
    protective and antidotal effects against hydrogen sulfide poisoning in
    mice, armadillos, rabbits, and dogs (Smith & Gosselin, 1979).

        Water containing a hydrogen sulfide concentration as low as 
    0.86 mg/litre was toxic to trout after exposure for 24 h (McKee &
    Wolf, 1971).


        Adequate systematic studies of the relationship between hydrogen
    sulfide exposure and health status in the general population have not
    been carried out. Controlled exposure of human subjects to
    concentrations of hydrogen sulfide gas exceeding about 75 mg/m3 
    (50 ppm) has been deemed to involve excessive risk because of the
    possibility of injury to the lungs (Sayers et al., 1925; National
    Research Council, USA, 1979). Furthermore, except for studies related
    to odour threshold, controlled exposures of human subjects to very low
    concentrations of the gas, for example, below 1.5 mg/m3 (1.0 ppm)
    have not been reported. Thus, the information presented in this
    section has mainly been derived from reports of accidental and
    industrial exposures to hydrogen sulfide. A general discussion of the
    toxicology of hydrogen sulfide has been included, because a basic
    understanding of the subject is necessary for a discussion of the role
    of the gas as an industrial and community hazard.

    6.1  General Toxicological Considerations

        The following observations have been derived from reports of
    studies involving man. However, for clarification, some studies on
    experimental animals have also been included. In general, both animals
    and man respond in a very similar fashion to toxic concentrations of
    hydrogen sulfide. It is both an irritant and an asphyxiant gas 
    (Table 2) that induces local inflammation of the membranes of the
    human eye and respiratory tract (Yant, 1930). It has been shown that
    eye irritation, the most commonly reported effect of hydrogen sulfide
    exposure, can occur after several hours' exposure to concentrations of
    16-32 mg/m3 (10.5-21.0 ppm) (Elkins, 1939; Nesswetha, 1969).
    However, pulmonary tract irritation is, potentially, a more serious
    reaction. When inhaled by dogs, hydrogen sulfide exerted an irritant
    action through the entire respiratory tract, though the deeper
    structures suffered the greatest damage (Haggard, 1925). Inflammation
    of these deeper structures may result in pulmonary oedema.

        Exposure to hydrogen sulfide gas did not induce important effects
    on the human skin nor was any appreciable absorption through intact
    skin observed (Yant, 1930). However, Petrun (1966) reported that when
    the skin of rabbits was exposed to hydrogen sulfide at concentrations
    of 1050 and 2100 mg/m3 (700 and 1400 ppm), trace amounts of hydrogen
    sulfide were found in the exhaled air of the rabbits. No quantitative
    information was given.

        Hydrogen sulfide gas is rapidly absorbed through the lung. Like
    hydrogen cyanide, it is a potent inhibitor of cytochrome  c oxidase
    that interferes with tissue use (Smith & Gosselin, 1979). As a result,
    the oxidative metabolism may slow to the point where tissue metabolic
    demands cannot be met. In the central nervous system, the result may
    be paralysis of the respiratory centres. Respiratory arrest and death
    from asphyxia would be the natural outcome.

        In studies on dogs (Haggard, 1925), hydrogen sulfide at
    concentrations of 1500-3000 mg/m3 (1000-2000 ppm) initially
    stimulated excessively rapid breathing (hyperpnoea), because of a
    depletion in the carbon dioxide content of the blood (hypocapnia).
    This was followed by a period of respiratory inactivity (apnoea).
    Spontaneous respiration may be reestablished, if carbon dioxide
    depletion has not progressed beyond the point where prompt
    reaccumulation can act as a stimulus to the reestablishment of
    respiration. If spontaneous recovery does not occur and artificial
    respiration is not applied rapidly, death from asphyxia is the
    inevitable result (Haggard, 1925). At about 2250 mg/m3 (1500 ppm),
    the sequence of events in the dog was the same, except that the
    reaction was more pronounced; at 3000 mg/m3 (2000 ppm) there was
    respiratory paralysis after a breath or two and, in Haggard's words,
    "the victim falls to the ground as though struck down". When breathing
    ceases, generalized convulsions frequently begin. There appears to be
    no clear explanation of the cause of this picture of sudden collapse.
    According to Haggard (1925), this form of respiratory failure is not
    related to the carbon dioxide content of the blood but, rather, to the
    directly paralysing effect of hydrogen sulfide on the respiratory
    centre. Breathing is never reestablished spontaneously following this
    hydrogen sulfide-induced respiratory paralysis. Haggard noted,
    however, that because the heart continues to beat for several minutes
    after respiration has ceased, death from asphyxia can be prevented if
    artificial respiration is begun immediately and is continued until the
    hydrogen sulfide concentration in the blood decreases. This decrease
    is probably a consequence of metabolic processes, as shown in mice by
    Susman et al. (1978) rather than, as once believed, the result of the
    pulmonary excretion of the gas.

        Smith & Gosselin (1979) have called attention to the confusion
    that exists in the literature with regard to the effects of hydrogen
    sulfide on haemoglobin. They emphasize that many studies have proved
    that neither sulfhaemoglobin nor any other abnormal pigments are
    present in sufficient concentrations in the blood of animals or human
    subjects, fatally poisoned by hydrogen sulfide.

        The characteristic "rotten egg" odour of hydrogen sulfide is an
    important aspect of the toxicology of the gas. The threshold of
    perception (odour) varies considerably depending on individual
    sensitivity. Several authors have reported odour detection thresholds
    ranging from 0.0007 mg/m3 to 0.20 mg/m3 (0.0005-0.13 ppm)
    (Table 2). Thus, the odour of hydrogen sulfide gas can be a very
    sensitive indicator of its presence in low concentrations. However, at
    higher concentrations (> 225 mg/m3 (150 ppm)), hydrogen sulfide
    exerts a paralysing effect on the olfactory apparatus (Milby, 1962),
    thus neutralizing the value of its odour as a warning signal. Poda
    (1966) reported that among 42 workers, who were rendered unconscious
    from overexposure to hydrogen sulfide, majority did not smell the
    characteristic odour of the gas but noted a sickeningly sweet odour,
    very briefly, before losing consciousness.

        Table 2.  Effects of hydrogen sulfide exposure at various concentrations in air

    Effect                                                    Duration of
                                    mg/m3            ppm        exposure            Reference

    Approximate threshold         0.0007-0.2     0.0005-0.13    A few seconds       Yant (1930); Ryazanov
    for odour                                                   to less             (1962); Adams & Young
                                                                than 1 min          (1968); Leonardos et al.
                                                                                    (1969); Lindvall (1970);
                                                                                    Thiele (1979); Winneke
                                                                                    et al. (1979)

    Threshold of eye              16-32          10.5-21        6-7 h               Elkins (1939)
    irritation                                                                      Nesswetha (1969)

    Acute conjuctivitis           75-150         50-100         > 1 h               Yant (1930)
    (gas eye)

    Loss of sense of smell        225-300        150-200        2-15 min            Sayers et al. (1925)

    Local irritation and          750-1050       500-700        < 1 h               Haggard (1925)
    slight systemic symptoms;
    possible death
    after several hours

    Table 2.  (Con't)

    Effect                                                    Duration of
                                    mg/m3            ppm        exposure            Reference

    Systemic symptoms;            1350           900            < 30 min            Haggard (1925)
    death in less than 1 h

    Death                         2250           1500           15-30 min           Haggard (1925)

    a  These observations were made in experimental animals. However, there are no better quantitative
       data available concerning man with respect to exposure to hydrogen sulfide at high concentrations.
        Hydrogen sulfide intoxication in man has generally been
    categorized according to 3 clinical forms, acute, subacute, and
    chronic depending on the nature of the predominant clinical signs and
    symptoms (National Research Council, USA, 1979). The term "acute
    hydrogen sulfide intoxication" has been most often used to describe
    systemic poisoning characterized by rapid onset and predominance of
    signs and symptoms of nervous system involvement. The term "subacute
    intoxication" has been applied to episodes of poisoning in which signs
    and symptoms of eye and respiratory tract irritation were most
    prominent. "Chronic hydrogen sulfide intoxication" has been applied by
    some authors to describe a prolonged state of symptoms resulting from
    a single or repeated exposure to concentrations of hydrogen sulfide
    that do not produce clear-cut manifestations of either acute or
    subacute illness.

        In a document prepared by the National Research Council of the
    National Academy of Sciences of the USA (National Research Council,
    USA, 1979), the observation was made that application of the terms
    "acute", "subacute" and "chronic" to hydrogen sulfide exposure was
    both imprecise and misleading. However, rather than abandon these
    frequently used terms altogether, the authors suggested a series of
    clarifying definitions which are quoted in the following paragraphs,
    and will henceforth be used in this section.

        " Acute intoxication: Effects of a single exposure [seconds-
    minutes]a to massive concentrations of hydrogen sulfide that rapidly
    produce signs of respiratory distress. Concentrations approximating
    1400 mg/m3 (1000 ppm) are usually required to cause acute

         Sub-acute intoxication: Effects of continuous exposure [up to
    several hours]a to mid-level 140 to 1400 mg/m3 (100 to 1000 ppm)
    concentrations of hydrogen sulfide. Eye irritation (gas eye) is the
    most commonly reported effect, but pulmonary edema (in the absence of
    acute intoxication) has also been noted.

         Chronic intoxication: Effects of intermittent exposures to low
    to intermediate concentrations 70 to 140 mg/m3 (50 to 100 ppm) of
    hydrogen sulfide, characterized by "lingering", largely subjective
    manifestations of illness."

        It is important to note that these definitions do not include a
    consideration of the health consequences to man associated with
    prolonged low-level exposure to hydrogen sulfide gas, such as may be
    encountered under conditions of general urban air pollution.

        A concentration of hydrogen sulfide in drinking water as low as
    0.07 µg/litre (0.05 ppm) can affect its taste (Campbell et al., 1958).


    a  Time factors added by WHO Task Group.

    6.2  Occupational Exposure

        In certain occupations, workers are intermittently exposed to
    concentrations of hydrogen sulfide that are not only malodorous but
    can, in some situations, cause severe adverse health effects and even
    death. Usually, hydrogen sulfide is encountered in the workplace as an
    undesirable by-product of a manufacturing process, notably petroleum
    refining, viscose rayon production, sugar beet processing, and tannery
    work (Milby, 1962; NIOSH, 1977). In other occupations, for example
    cesspool cleaning and work in sewers, exposure to hydrogen sulfide may
    occur when the gas is formed as a result of the decomposition of
    sulfur-containing organic matter, in the absence of complete
    oxidation. Deaths attributed to such exposure occurred in the sewers
    of Paris in the 1780s (Mitchell & Davenport, 1924) and still occur
    under various circumstances.

        Workers in certain occupations risk exposure to naturally
    occurring hydrogen sulfide; geothermal energy workers and spa
    attendants may be included in this category.

        Acute hydrogen sulfide intoxication is a dramatic, often fatal
    event. Three men were inadvertently enveloped in a cloud of hydrogen
    sulfide gas escaping from a cylinder under high pressure; all fell, as
    if struck down, and ceased breathing. Only as a result of prompt
    resuscitation by trained onlookers did the men survive, though the two
    most seriously affected experienced violent convulsions and did not
    recover consciousness for some 30 min. None of the men suffered
    important after-effects, and none recalled having noted the
    characteristic odour of hydrogen sulfide. The hydrogen sulfide
    concentrations to which the men were exposed were estimated to be
    about 2800 mg/m3 (2000 ppm) (Milby, 1962). Twelve workmen in a plant
    that produced benzyl polysulfide were overcome by hydrogen sulfide
    gas, when a pipe used to transfer sodium sulfhydrate ruptured. The
    liquid sulfhydrate drained into a nearby sewer, where it reacted with
    acid sewage releasing hydrogen sulfide from several sewer openings in
    the immediate vicinity. Two of the 12 workmen died, probably as a
    result of respiratory arrest; 3 stopped breathing but were
    successfully resuscitated; 6 lost consciousness but recovered
    spontaneously, and 1 individual developed pulmonary oedema that
    responded to therapy (Kleinfeld et al., 1964).

        Burnett et al. (1977) reviewed 221 cases of exposure to hydrogen
    sulfide associated with the oil, gas, and petrochemical industries in
    Canada. The overall mortality was 6%; three-quarters of all victims
    experienced a period of unconsciousness and 12% were comatose. A high
    proportion of patients had other neurological signs and symptoms,
    including altered behaviour patterns, confusion, vertigo, agitation,
    or somnolence. Respiratory tract effects were second in frequency only
    to neurological manifestations. Forty percent of all cases required
    some form of respiratory assistance and 15% of all cases developed

    pulmonary oedema. Less severely affected patients complained primarily
    of headaches, sore eyes, or gastrointestinal upsets. There were no
    recognizable sequelae among the survivors. Data on environmental
    exposure levels were not reported.

        Nearly fatal cases of acute hydrogen sulfide intoxication
    associated with sequelae of varying severity have been reported. A 
    48-year-old farmer, who collapsed from hydrogen sulfide intoxication
    while shovelling manure, continued to have convulsive seizures after
    resuscitation. The ECG changes suggested myocardial infarction. The
    patient recovered but a slight, persistent dizziness remained
    (Kaipainen, 1954). A 46-year-old sewer worker, who was overcome by
    hydrogen sulfide in a manhole for 30 min, was cyanotic, suffering
    generalized spasms, and required artificial respiration. A week later,
    he could move and speak only with great effort; a month afterwards, he
    still exhibited neurological deficits. The ECG showed evidence of
    small anterolateral infarct and a right bundle branch block. Three
    months later, although ambulatory, the patient still suffered from
    anginal pain upon exertion (Hurwitz & Taylor, 1954). In a man who had
    suffered severe hydrogen sulfide intoxication with collapse and
    respiratory failure, ECG evidence of myocardial ischaemia was noted
    during the early phase of acute illness but gradually disappeared over
    a period of 15 days (Kemper, 1966). Each of these 3 events, in which
    sequelae were reported, were characterized by periods of
    unconsciousness during which hypoxia of vital issues was likely to
    have occurred and may have been the basis for the observed prolonged
    effects. Many other instances of sequelae following acute hydrogen
    sulfide intoxication have been reported. Most resemble the cases just
    mentioned, in which serious poisoning with unconsciousness preceded
    the appearance of sequelae.

        Ahlborg (1951) described 58 cases of acute hydrogen sulfide
    intoxication in Sweden's shale oil industry. There were no fatalities.
    Symptoms were generally uniform: sudden fatigue, dizziness, and
    intense anxiety followed by unconsciousness with or without
    respiratory failure. Several cases of acute poisoning were not
    associated with unconsciousness, otherwise symptoms were similar to
    those of the other cases. The diagnosis of "sequelae after acute
    hydrogen sulfide poisoning" was made in 15 cases. The majority of
    these had a history of repeated acute intoxications followed in each
    case by neurasthenic problems (fatigue, somnolence, headache, lack of
    initiative, irritability, anxiety and poor memory, and decreased
    libido), though some developed sequelae following acute intoxication
    without an intercurrent episode of unconsciousness. Many stricken
    workers developed an increase in sensitivity (aversion) to the odour
    of gas of any type, even pure gasoline vapour (Ahlborg, 1951).

        Numerous case histories of fatal hydrogen sulfide intoxication
    have been reported (Larson et al., 1964; Adelson & Sunshine, 1966;
    Simson & Simpson, 1971). Oedema of the lungs and brain are common

    post-mortem findings. The presence of detectable concentrations of
    hydrogen sulfide in the blood has been reported on several occasions
    (Larson et al., 1964; Adelson & Sunshine, 1966) and concentrations in
    the blood of fatally poisoned victims have ranged from 1.70 mg to 
    3.75 mg/litre (McAnalley et al., 1979).

        In acute hydrogen sulfide intoxication, cessation of respiration
    is an immediate threat to life. Accordingly, the provision of
    artificially assisted respiration on an emergency basis is absolutely
    critical. There is some question as to whether mouth-to-mouth
    resuscitation may create a potential health hazard to the rescuer,
    because of the presence of hydrogen sulfide in the expired air or on
    the clothing of the victim. Thus, methods of artificial respiration
    requiring less direct contact (for example, back-pressure-arm lift)
    may be prudent.

    6.3  General Population Exposure

        There are several reports of episodes of general population
    response to air contamination by hydrogen sulfide. Information derived
    from these events is consistent with observations reported among
    workers occupationally exposed to hydrogen sulfide. Table 2
    demonstrates the wide range of odour perception thresholds for
    hydrogen sulfide reported by various investigators. In view of the
    magnitude of these differences, it is not possible to state with
    certainty the concentration at which odour-related complaints can be

        A catastrophic exposure episode involving the release of large
    quantities of hydrogen sulfide into a small community was reported by
    McCabe & Clayton (1952). This occurred in 1950 in Poza Rica, Mexico, a
    city of 22 000 people located about 210 km northeast of Mexico City.
    Poza Rica was then the centre of Mexico's leading oil-producing
    district and the site of several oil field installations, including a
    sulfur-recovery plant. An early morning malfunction of the waste gas
    flare resulted in the release of large quantities of unburned hydrogen
    sulfide into the atmosphere. The unburned gas, aided by a low-level
    temperature inversion and light early morning breezes, was carried to
    a residential area adjacent to the plant area. Residents of the area
    were overcome while attempting to leave the area and while assisting
    stricken neighbours. Within 3 h, 320 persons were hospitalized and 22
    died. The most frequent symptom was loss of the sense of smell. More
    than half of the patients lost consciousness, many suffered signs and
    symptoms of respiratory tract and eye irritation and 9 developed
    pulmonary oedema. Four of the 320 victims exhibited neurological
    sequelae; 2 experienced neuritis of the acoustic nerve; 1 developed
    dysarthria; the fourth patient suffered aggravation of pre-existing
    epilepsy. The duration of these neurological sequelae was not

        There have been reports of other episodes of general atmospheric
    pollution by hydrogen sulfide evolved from both natural and industrial
    sources, but none has been as severe as the Poza Rica incident.

        In the geothermal areas of Rotorua, New Zealand, a city of 40 000
    people, steam and hot water from approximately 500 active bores are
    used to heat houses, buildings, and swimming pools, and to provide
    domestic hot water supplies and steam for cooking boxes.

        In these areas, accidental fatal cases of acute hydrogen sulfide
    intoxication associated with improper ventilation of geothermal steam-
    heated dwellings have occasionally been reported. For example, at
    least 3 deaths from acute hydrogen sulfide poisoning occurred in 1962.
    However, with the introduction of legal requirements regarding the
    safe domestic use of the geothermal resources, and possibly also as a
    result of increased public awareness of the dangers involved, no
    fatalities attributable to hydrogen sulfide have occurred in Rotorua
    since 1962 (Thom & Douglas, 1976).

        Air pollution monitoring for hydrogen sulfide has been conducted,
    during several periods over the past 15 years, at various sites in
    Rotorua. However, there has been considerable variation in the results
    obtained between the various sites, and furthermore, the
    concentrations measured showed considerable seasonal variations. Some
    of the results of these measurements are mentioned in section 4.1.

        In 1964, the Division of Air Pollution, US Public Health Service,
    reported that in the city of Terre Haute, Indiana, biodegradation of
    industrial wastes in a 14.5-ha lagoon caused the atmospheric
    concentration of hydrogen sulfide to reach a 1-h mean concentration of
    0.45 mg/m3 (0.3 ppm). As a result, 81 complaints were registered by
    the public, 41 of which were health-related. The most common
    complaints were concerned with the perception of a foul odour. The
    most common effects were nausea, interruption of sleep, burning of the
    eyes, and shortness of breath. Less common manifestations were cough,
    headache, and anorexia. Several acute asthma attacks were reported,
    but the association of these attacks with the hydrogen sulfide
    incident was not clearly established. Though no grave physical
    illnesses could be directly related to the air pollution incident, the
    investigators stressed that the odours emanating from the lagoon
    caused more than a mere nuisance (United States Public Health Service,

        The Poza Rica tragedy provides ample evidence that the accidental
    release of hydrogen sulfide into a community can be expected to cause
    systemic intoxication of varying severity. The Rotorua experience is
    notable in that it emphasizes the fact that the potential for serious,
    even fatal, hydrogen sulfide intoxication is present in active
    geothermal areas. The more common picture of general population
    exposure is exemplified by the Terre Haute incident where an industry-

    related source of low-level concentrations of hydrogen sulfide gas
    created a health problem which, although not grave, exceeded the level
    of mere nuisance.

        The potential of long-term, low-level exposure to hydrogen sulfide
    to cause pulmonary changes of the type known to be associated with
    other irritant gases such as oxides of nitrogen and sulfur has
    scarcely been studied. No epidemiological data are available, at
    present, upon which to base any sound conclusions.


        By far the most important recognized toxic effect of hydrogen
    sulfide is its ability to induce acute intoxication, characterized by
    immediate collapse, frequently accompanied by respiratory arrest and,
    without treatment, death. The scientific literature abounds with cases
    of this type, most often associated with industrial overexposure.
    However, a few cases of acute hydrogen sulfide exposure have been
    recorded in the general population as a result of the release of
    hydrogen sulfide either from an industrial process or from natural
    sources. A second form of injury associated with exposure to hydrogen
    sulfide is caused by the irritative action of the gas on the mucous
    membranes of the eyes and respiratory tract. Keratoconjunctivitis (gas
    eye) and pulmonary oedema are two most serious manifestations of this
    local irritative effect. The malodorous property of hydrogen sulfide
    gas is well recognized and this characteristic alone is believed by
    many to be capable of producing impairment of human health and well-

        Most of the information available on human health effects
    associated with exposure to various concentrations of hydrogen sulfide
    gas has come from observations on accidental and industrial exposures.
    With the exception of the Poza Rica catastrophe, information on
    general population exposures and associated health effects is sketchy
    at best. There is also little information on controlled human exposure
    to hydrogen sulfide gas, and, except for data from odour threshold
    studies, the information that is available is more than 50 years old.
    Although a small amount of information is available on the effects on
    experimental animals of high concentrations of hydrogen sulfide gas,
    there is virtually no information on the long-term, low-level effects
    of the gas on experimental animals. Furthermore, epidemiological data
    are lacking concerning the health consequences of long-term, low-level
    exposure to hydrogen sulfide, in both the general and industrial

    7.1  Exposure Levels

        General population air pollution problems associated with hydrogen
    sulfide arise mainly in connexion with malodorous conditions,
    traceable to point sources. Such sources can be industrial or, in some
    cases, polluted bodies of walter. Peak levels as high as 0.20 mg/m3
    (0.13 ppm) have been reported in the air in the neighbourhood of
    industrial sources. Hydrogen sulfide is also a common pollutant in
    geothermally active areas. At one site is a geothermal area in New
    Zealand, where continuous measurements were carried out over a 5-month
    period, a level of 0.08 mg/m3 (0.05 ppm) was exceeded, on average,
    for 35% of the time.

        Concentrations of hydrogen sulfide in the workplace also vary
    widely. In the shale oil industry and in viscose rayon production, for
    example, maximum levels of exposure during the work day have been

    reported to range from 23 to 30 mg/m3 (15-20 ppm). In general,
    however, massive accidental exposure to hydrogen sulfide has
    constituted the principal hazard of this gas in industrial settings.
    In many cases, such exposure has occurred because of equipment
    breakage or malfunction. However, because hydrogen sulfide is heavier
    than air, it can accumulate in lethal concentrations in low-lying or
    enclosed areas. Numerous fatalities have occurred from the slow,
    insidious accumulation of the gas in the air in both ambient and
    industrial environments.

    7.2  Experimental Animal Studies

        The toxic effects of hydrogen sulfide gas have not been studied
    extensively in experimental animals. However, in studies on a number
    of animal species including the mouse, rat, cat, dog, and goat, it has
    been shown that the primary target of hydrogen sulfide in high doses
    is the nervous system. Collapse, followed by respiratory arrest and
    asphyxia resulting from the paralysing effects of high concentrations
    of hydrogen sulfide on the respiratory centres of the central nervous
    system, is the usual sequence of events leading to death.

        Little information is available in the published literature
    concerning the effects in experimental animals of long-term, low-level
    exposure to hydrogen sulfide.

    7.3  Effects of Occupational Exposure

        Inadvertent and accidental exposure of human subjects to high
    concentrations of hydrogen sulfide has occurred among workers engaged
    in petroleum refining, viscose rayon production, sugar beet
    processing, and tannery work. Exposure levels have not been precisely
    documented in many of these situations. Reported effects range from
    the relatively less grave conditions of neurasthenic and
    otoneurological symptoms and keratoconjunctivitis to the more serious
    effects of pulmonary oedema, respiratory failure, collapse, and even
    death. From the available data, it can be estimated that exposure for
    seconds or minutes to concentrations of approximately 1400 mg/m3
    (1000 ppm) or more would cause acute intoxication, concentrations of
    140-1400 mg/m3 (100-1000 ppm) with an exposure time of up to several
    hours would produce keratoconjunctivitis and pulmonary oedema, and
    intermittent exposures to concentrations of 70-140 mg/m3 
    (50-100 ppm) could be associated with lingering, largely subjective,
    manifestations believed by some to represent chronic intoxication.
    Various studies have associated exposure to hydrogen sulfide in
    concentrations as low as 16-32 mg/m3 (10.5-21.0 ppm) for several
    hours with eye irritation in workers.

        Effects of low-level, long-term industrial exposure to hydrogen
    sulfide have not been systematically evaluated.

    7.4.  Effects of General Population Exposure

        Several episodes of exposure of the general population to hydrogen
    sulfide emanating from a specific source have been investigated and
    described. For the most part, these events involved only annoyance
    because of the odour or, at worst, minor temporary illness such as
    headache, nausea, and sleeplessness. However, as described in section
    6.3, on two occasions, general population exposure to hydrogen sulfide
    caused grave illness and even death: one at Poza Rica, Mexico, and the
    other in and around Rotorua, New Zealand.

        Unfortunately, these incidents were not studied using
    epidemiological techniques, and it is not possible to establish
    exposure-effect relationships from the data.

    7.5  Guidelines for the Protection of Public Health

        There are two aspects concerning the protection of public health
    in relation to hydrogen sulfide exposure: (a) the protection of the
    public, and occupational groups in particular, from the toxicological
    effects of such exposure; and (b) the protection of the public from
    the odour nuisance that can be associated with releases of hydrogen

        The odour threshold for hydrogen sulfide has been variously
    reported to range from 0.0007 mg to 0.20 mg/m3 (0.0005-0.13 ppm)
    (Table 2). Little information is available on the odour detection
    limits for ambient hydrogen sulfide either under experimental field
    conditions or in general population exposures. The Task Group
    considered that a level of 0.007 mg/m3 (0.005 ppm) averaged over 
    30 min should not produce odour nuisance in most situations. Some
    regulatory bodies may wish to adopt longer averaging times with
    appropriately adjusted concentration limits.

        The best estimates available suggest that eye irritation may occur
    in man after several hours' exposure to hydrogen sulfide
    concentrations of 16-32 mg/m3 (10.5-21.0 ppm). As occupational
    exposure guidelines, the Task Group recommended the adoption of
    10 mg/m3 (7 ppm) as a workshift time-weighted average value together
    with a short-term exposure limit of 15 mg/m3 (10 ppm). The short-
    term limit should be determined as a 10-min or less averaged value.
    These limits should prevent eye irritation in workers which represents
    the earliest recognized toxic response in man.

        Annex tables 1 and 2 contain various national standards or
    recommendations for ambient air quality and occupational exposure
    limits for hydrogen sulfide. As can be seen, there is considerable

    consensus regarding occupational exposure limits and the
    recommendations of the Task Group are generally consistent with the
    national values. There is less agreement regarding ambient air quality
    standards, possibly because of different values placed on the nuisance
    value of odours.


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        Annex table 1.  Ambient air quality standards for hydrogen sulfide from selected countries

                              Long-term                    Short-term
    Country                            averaging                   averaging   Reference
                      mg/m3    ppm     time(h)    mg/m3     ppm    time(min)

    Bulgaria          0.008    0.006     24       0.008     0.006      30      Newill (1977)
    China             --       --        --       0.01      0.007      20      Official
    Czechoslovakia    0.008    0.006     24       0.008     0.006      30      Newill (1977)
    German            0.008    0.006     24       0.015     0.011  10--30      Newill (1977)
    Germany,          0.005    0.004     24       0.01      0.007      30      Minister for
    Federal                                                                    Home Affairs
    Republic of                                                                (1974)
    Hungary           0.008a   0.006     24       0.008a    0.008      30      Newill (1977)
    Hungary           0.15     0.11      24       0.3       0.21       30      Newill (1977)
    Israel            0.045    0.032     24       0.15      0.11       30      Newill (1977)
    Philippines       --       --        --       0.3       0.21       30      UNEP/IRPTCe
    Poland            0.008b   0.006     24       0.0086    0.008      30      UNEP/IRPTCe
    Poland            0.02c    0.014     24       0.06c     0.04       20      UNEP/IRPTCe
    Romania           0.01     0.007     24       0.03      0.02       30      Newill (1977)
    Spain             0.004    0.003     24       0.01      0.007      30      Newill (1977)
    USSR              0.008    0.006     24       0.008     0.006      30      Newill (1977)
    Yugoslavia        0.008    0.006     24       0.008     0.006      30      Newill (1977)

    a  For highly protected and protected areas.
    b  For especially protected areas, sanatoria, health resorts, sanctuaries, and
       national parks.
    c  For protected areas, towns, and villages.
    d  Official communication from the Institute of Health, Chinese Academy of Medical
    e  Private communication.
    Note by the WHO Task Group

        Ambient Air Quality Standards have been set according to different
    criteria in different countries, and these criteria may include, but
    are not necessarily limited to the assessment of health effects.
    Moreover, the limits themselves may have different meanings such as
    the maximum acceptable level or the permissible level over a 10 to 
    30-min averaging time, etc.

        Annex table 2.  Occupational exposure standards for hydrogen sulfide
                    from selected countriesa

    Country                                  mg/m3   ppm   Standard type


    Australia                                15      10    8-h TWAb
    Belgium                                  15      10    8-h TWAb
    Bulgaria                                 10            ceiling
    China                                    10            ceiling
    Czechoslovakia -- average                10            shiftc TWAb
                      maximum                20            10-min STELd
    Finland                                  15      10    8-h TWAb
    German Democratic Republic -- average    15            8.75-h TWA
                               short-term    15            30-min STELd
    Germany, Federal Republic of             15      10    8-h TWAb
    Hungary                                  10            8-h TWAb
    Italy                                    10            shiftc TWAb
    Japan                                    15      10    shiftc TWAb
    Netherlands                              15      10    shiftc TWAb
    Poland                                   10            8-h TWAb
    Romania -- average                       10            shiftTWAb
               maximum                       15            ceiling
    Sweden                                   15      10    shifte TWAb
    Switzerland                              15      10    8 to 9-h TWAb
    USSR                                     10            less than 30-min STELd
    USA -- occupational standard                     20    ceiling except for one
                                                           10-min peak less than
                                                           50 ppm
          ACGIH                              15      10    8-h TWAb
                                             21      15    15-min STELd
    Yugoslavia                               10      7     ceiling


    a  Abstracted from: ILO (1977).
    b  TWA = time-weighted average value.
    c  = a time-weighted value averaged over the entire shift or workday.
    d  STEL = short-term exposure limit.

    See Also:
       Toxicological Abbreviations
       Hydrogen sulfide (ICSC)