OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.

September 28, 1995

Mr. Robert J. Smith
Vice President, Operations
Courtaulds Fibers, Inc.
Post Office Box 141
Axis, Alabama 36505

Dear Mr. Smith:

This is in response to your letter of August 1, to Lana Graves, Area Director of the Occupational Safety and Health Administration's (OSHA) Mobile Area Office. That letter was forwarded to our office for response. In that letter, you requested the appropriate method for assessing hydrogen sulfide peak exposure levels.

In your letter, you mentioned that your were using both the instantaneous and 10 minute time period sampling techniques. You stated that you felt the appropriate method for measuring hydrogen sulfide was the 10 minute sampling period method alone.

In answering your question, please refer to 29 CFR 1910.1000(b)(2) which requires that an employee's exposure to any substance listed in Table Z-2 shall not exceed at any time the acceptable ceiling concentration limit, except for a time period and up to a concentration not exceeding the maximum duration and concentration allowed in the acceptable maximum peak column. The example given in 1910.1000(b)(3) states that "...an employee may be exposed to a concentration of Substance A (with a 10 ppm TWA, 25 ppm ceiling and 50 ppm peak) above 25 ppm (but never above 50 ppm) only for a maximum period of 10 minutes."

Hydrogen sulfide can be a severe acute hazard, and in reviewing the USA Standard Z 37.2-1966 it was noted that hydrogen sulfide is an extremely toxic and irritating gas and a significant property of the gas is it's temporary paralytic effect on the olfactory nerve. High concentrations can result in severe consequences before the odor is detected. Sampling methods are currently available and should be used for measuring both the 10 minute and instantaneous levels of hydrogen sulfide in the workplace. For these reasons, it is felt that your current policy of measuring both the 10 minute and instantaneous levels of this gas is appropriate.

Thank you for your interest in occupational safety and health. If you have any further questions, please contact OSHA's Office of Health Compliance at [(202) 693-2190].


Ruth McCully, Director
Office of Health Compliance Assistance




August 23, 1995





FROM:              Lana S. Graves
                   AREA DIRECTOR

SUBJECT:           Courtaulds Fibers, Inc. Request for a Definition of:
                   Hydrogen Sulfide Peak Exposures and Appropriate
                   Sampling Strategy

Please find attached a letter from Courtaulds Fibers requesting a definition for peak exposure and the appropriate sampling method to be used to assess hydrogen sulfide peak exposure levels. My industrial hygiene staff has spoken with Keith Motley of the Salt Lake Technical Center Health Response Team and with Richard Fairfax of your staff concerning the Agency's definition of PEAK exposure levels and the appropriate sampling method for assessing these levels. We have verbally communicated the Agency's position regarding these issues to Courtaulds Fibers on several occasions, however, the company would like the Agency's position in writing.

If you need any additional information, please contact John Deifer, Leigh Jackson, or Gail Davis of my staff. Thank you for your assistance regarding this matter.




August 1, 1995




Ms. Lana S. Graves
Area Director
Occupational Safety and Health Administration
3737 Government Blvd, Suite 100
Mobile, Alabama 36693

Dear Ms. Graves:

Could you please provide us with OSHA's definition of the appropriate method of assessing hydrogen sulphide (H(2)S) peak exposure levels. At our recent meeting, you suggested that the OSHA National Director of Compliance programs would be able to do this.

We believe that part 1910.1000(b)(2) and Table Z-2 in particular indicate that 10 minute sampling, utilizing an approved methodology outlined in the OSHA CD-ROM Chemical Sampling Information guidesheet, is the appropriate method for gathering peak exposure data.

While we see some value in using a meter to determine instantaneous H(2)S levels for reference only, we do not believe that instantaneous meter readings serve as peak samples.

We hope to get an interpretation very soon so that we can meet the deadlines set out in our Informal Settlement Agreement. In the meantime, we will gather exposure data using both the instantaneous and the 10 minute average techniques.

I would be happy to meet with OSHA at any time to discuss the appropriate methods for peak exposure measurement.

Thank you for your assistance in this interpretation.



Robert J. Smith
Vice President, Operations





USA Standard Acceptable Concentrations of Hydrogen







1. Scope and Purpose


1.1 This standard applies to occupational exposures to hydrogen sulfide in all places of employment.

1.2 In the application of this standard, it should always be borne in mind that high concentrations of hydrogen sulfide or prolonged exposure to moderate concentrations (20-30 parts per million) may dull the sense of smell so that absence of odor should not be considered as indicating a nonhazardous concentration.


2. Properties of Hydrogen Sulfide


2.1 General Properties. Hydrogen sulfide is a colorless, transparent gas with a characteristic rotten-egg odor at low concentrations. At higher concentrations it has a sweetish odor and at still higher concentrations, an odor may not be detected.

2.2 Physical and Chemical Properties [1](1) Chemical formula: H(2)S Molecular weight: 34.08 Specific gravity: 1.192 (dry air=1) Vapor pressure: 19.6 atmospheres at 25 degrees C Explosive limits: lower 4 percent volume in air upper 44 percent volume in air Ignition temperature: 500 degrees F (burns with a pale flame) Solubility: Water--2.9 volumes of gas per volume of water at 20 degrees C 1 milligram per liter = 717 parts per million (ppm) 1 ppm = 0.00139 milligram per liter 1 grain per 100 cubic feet = 16.51 ppm

2.3 Toxic Properties. Hydrogen sulfide is an extremely toxic and irritating gas. In sufficiently high concentrations it can cause instant death by blocking the oxidative processes of tissue cells [2.3] and by reducing the oxygen-carrying capacity of the blood [2.4]. Free hydrogen sulfide in the blood depresses the nervous system and larger amounts can paralyze the nervous system so that in acute poisoning death is due to respiratory failure and asphyxiation. Because hydrogen sulfide is oxidized quite rapidly to sulfates in the body, no after effects occur in cases of recovery from acute exposures unless oxygen deprivation of the nervous system is prolonged [4].

Hydrogen sulfide is irritating to the eyes and respiratory tract. The eye irritations--conjunctivitis, pain, lacrimation, and photophobia--may persist for several days [5]. Respiratory tract symptoms include coughing, pain in breathing, and pain in the nose and throat [6]. Repeated exposures to hydrogen sulfide can result in chronic poisoning. Eye irritation, respiratory tract irritation, slow pulse rate, lassitude, digestive disturbances, and cold sweats occur [7]. The temporary paralytic effect of hydrogen sulfide on the olfactory nerve is probably its most significant property. High concentrations can cause collapse and death before the odor is detected. Exposure to low concentrations may dull the sense of smell so that odor cannot and should not be relied on as an indication of hazard. Subjective olfactory responses to various concentrations of hydrogen sulfide have been summarized as follows [6]:

    0.02 ppm   No odor
    0.13 ppm   Minimal perceptible odor
    0.77 ppm   Faint but readily perceptible odor
    4.6  ppm   Easily detectable, moderate odor
   27.0  ppm   Strong, unpleasant odor, but not intolerable

Physiological responses to various concentrations of hydrogen sulfide have been reported as follows [6]:

      10 ppm   Beginning eye irritation

  50-100 ppm   Slight conjunctivitis and respiratory tract irritation
               after 1 hour exposure.

     100 ppm   Coughing, eye irritation, loss of sense of smell
               after 2-15 minutes.  Altered respiration, pain in
               the eyes, and drowsiness after 15-30 minutes
               followed by throat irritation after 1 hour.
               Several hours exposure results in gradual increase
               in severity of these symptoms and death may occur
               within the next 48 hours.

 200-300 ppm   Marked conjunctivitis and respiratory tract
               irritation after 1 hour of exposure.

 500-700 ppm   Loss of consciousness and possibly death in 30
               minutes to 1 hour.

700-1,000 ppm   Rapid unconsciousness, cessation of respiration,
               and death.

1,000-2,000ppm  Unconsciousness at once, with early cessation of
               respiration and death in a few minutes. Death may
               occur even if individual is removed to fresh air at once.
3. Acceptable Concentrations

3.1 Acceptable Maximum for "Peaks" Above Acceptable Base Line for Continuous Exposure. Concentration peaks above the ceiling (see 3.2) should not be permitted if the time-weighted average exposure is close to this value. Where there are no other measurable exposures during the work day, a single ten-minute exposure to 50 ppm should not be hazardous.

3.2 Acceptable Ceiling Concentration. The acceptable concentration for protection of health for an eight-hour, five-day week shall be 20 ppm. Fluctuations are to occur below this concentration [8.9].

3.3 Acceptable Eight-Hour Time-Weighted Average. A time-weighted average is not applicable because hydrogen sulfide is an acute acting substance.

3.4 Acceptable Concentration to Avoid Discomfort. The acceptable level of hydrogen sulfide to avoid discomfort is 10 ppm. Eye irritation begins at concentrations just above this figure [10].


4. Sampling Procedure and Analytical Methods


4.1 Chemical Methods [10]. Hydrogen sulfide may be determined by absorption in (1) cadmium chloride solution and subsequent iodometric determination of cadmium sulfide; (2) alkaline zinc acetate solution with subsequent colormetric determination; (3) iodine-potassium iodide solution and subsequent titration with sodium thiosulfate. This last method is not applicable in the presence of reducing or oxidizing gases.

4.2 Detectors [11]. Commercially available devices can be used for quantative estimation of low concentrations of hydrogen sulfide by "spot-sampling." These instruments indicate the amount of the gas present by a color change in chemically-coated granules in a narrow glass tube. A strip filter paper sampler has been developed for the measurement of hydrogen sulfide [12]. Air is drawn through a lead acetate impregnated tape where the hydrogen sulfide reacts to form lead sulfide. Concentrations are determined by comparing the optical density of the black spot with standards.


References to the Text


[1] Physical constants are taken or calculated from standard references.

[2] Rodenacker, G. Mechanism of hydrogen sulfide on tissue respiration. American Medical Association Archives of Industrial Hygiene and Occupational Medicine, vol 98, 1923, p 70.

[3] Rodenacker, G. Mechanism of hydrogen sulfide poisoning. Zentral, Gewerbehgg. Unfallverhut, NF-4, vol 176, 1927, pp 211-405 [4] Haggard, H. W. The toxicology of hydrogen sulfide. Journal of Industrial Hygiene and Toxicology, vol 7, 1925, p 113.

[5] Grant, W. M. Toxicology of the Eye. Springfield, Illinois: Charles C. Thomas, 1962, p 271.

[6] Yant, W. P. Hydrogen sulphide in industry: occurrence, effects and treatment. American Journal of Public Health, vol 20, 1958, p 598.

[7] API Toxicological Reviews--Hydrogen Sulfide. New York: American Petroleum Institute, 1948.

[8] Threshold Limit Values for 1963. Lansing, Michigan: American Conference of Governmental Industrial Hygienists, 1963.

[9] Barthelmy, H. L. Ten years' experience with industrial hygiene in connection with the manufacture of viscose rayon. Journal of Industrial Hygiene and Toxicology, vol 21, 1939, p 141.

[10] Jacobs, M. D. The Analytical Chemistry of Industrial Hazards, Poisons, and Solvents. New York: Interscience Publishers, 1949, vol 1, second edition p 323.

[11] Hygienic Guide-Hydrogen Sulfide. Detroit, Michigan: American Industrial Hygiene Association, 1962.

[12] Sensenbaugh, J. D. and Hemeon, W. L. L. A low cost sampler for measurement of low concentrations of hydrogen sulfide. Air Repair, vol 4, 1954, p 5.