November 1993

(Hydrogen Sulfide, Carbon Dioxide, Methane, Ammonia, Biological Agents)
By: Gary M. Hutter, Ph.D, P.E. CSP

Potential Exposure Sources and Conditions:
  • Sewer & Sewer Access Points
  • Wastewater Treatment Facilities
  • Underground Vaults
  • Swamps/ Wetlands
  • Activated Sludge Reactors
  • Sewage Digesters
  • Manure Pits
  • Leather Tanning
  • Trenches/ Excavations Near Sewer Lines
  • Landfills
  • Wet & Dry Wells
  • Septic Tanks & Systems
  • Anaerobic Conditions
  • Methane Fermentation Reaction
  • Lift Stations
  • Certain Soils
  • Petroleum & Natural Gas

Sewer gas is a generic name for the collection of gases and airborne agents that often accompany sewage and the natural processes and reactions associated with sewage processing and the decomposition of organic materials. The major components of sewer gas can include: nitrogen (N2), hydrogen sulfide (H2S), carbon dioxide (CO2), methane (CH4), ammonia (NH3), biological organisms, water vapor, and other chemicals discharged to the effluent stream.1,2 The presence and concentration of any of these components can vary with time, composition of the sewage, temperature, and pH.

All of the above gases are colorless at the concentrations commonly encountered in sewage systems.

  • Natural biological organisms and pathogens from sewage can become airborne, primarily through agitation, or other physical actions on the accompanying liquid, but generally these microbes are short-lived when suspended in air.

The materials which may occupy these locations may be variable mixtures of liquids, gases and mists. There may be little perceptible indication of decreased oxygen levels; there may be no warning odors for the most toxic gas; and there may be a possibility for a fire or explosion hazard. Exposure to airborne biological contaminants often requires the generation of mists containing these materials.

The major adverse health effects and hazards from exposure to sewer gases are:

  1. Poisoning from H2S, Asphyxiation from displaced or consumed oxygen,
  2. Decreased vigilance or fatigue due to reduced oxygen levels (from CO2 and CH4),
  3. Biological contamination, and
  4. Fires and explosions from methane gas, H2S or other flammable gases.

Hydrogen sulfide has been reported as the leading cause of sudden death in the work place.4 At concentrations in air of approximately 300 ppm, H2S can be immediately deadly. It is absorbed primarily through the lungs, but can also be adsorbed to a limited extent through the skin and mucous membranes. Chronic health effects caused by repeated exposures have not been established. Common symptoms to non-acute exposure levels include eye irritation, fatigue, headache, and dizziness.

Carbon dioxide is a simple asphyxiant (displaces oxygen) and a stimulant for the respiratory system. A concentration of 5% may produce headaches and shortness of breath. Background concentrations of carbon dioxide in air range from 300 to 400 ppm.

Methane is a simple asphyxiant (displaces oxygen), but does not itself cause significant physiological responses.

Table 1 contains more information on the exposure criteria for the gaseous materials.

Common biological agents found in sewer systems may be bacteria, viruses, or parasites. Table 2 lists a few of the diseases and viruses that are associated with inhalation exposure from waste water operations.


Compound Vapor Density
(air = 1.0)
ppm Odor
PEL ppm 5 STEL ppm 6
Hydrogen Sulfide
Carbon Dioxide
20 (ceiling)
50 (10 mins.only)


  • Tuberculosis
  • Histoplasmosis
  • Coxsackie A & B
  • Poliomyelitis
  • Adenovirus
  • Bacillary dysentery
  • Common cold
  • Echovirus
  • Rotavirus

  • Implications:
    The presence of sewer gas at significant concentration levels may result in dangerous conditions due to its toxic nature, its suppression of life supporting oxygen levels, and its potential explosive nature. Some components of sewer gas may result in detectable odors, but the odor warning characteristics alone may be insufficient to cause exposed individuals to leave the area. Biological contaminants can become airborne and accompany mists released from sewage-related operations.

    Hydrogen sulfide is formed from the bio-chemical reduction reaction of naturally occurring sulfate ions in water or from the decomposition of organic matter that contains sulfur under anaerobic conditions,8 and from reactions of metal sulfides and strong acids.9 Hydrogen sulfide generally will not form if there is an abundance of available oxygen. There is a potential for the continued biological oxidation of the hydrogen sulfide to form weak concentrations of sulfuric acid (H2SO4) or the formation of ferrous sulfide (FeS), a blackish solid residue, if iron is available. If sufficient dissolved oxygen is present, H2S will not be generated.10

    Carbon dioxide is the natural product of respiration, including that from microbial activity, and is primarily harmful due to the consumption and displacement of oxygen that accompanies its generation. This gas can also be produced under some circumstances from the reaction of some acids and the carbonates in concrete. This set of reactions is usually restricted due to such mass transfer limitations as the presence of slime and accumulation of a precipitate layer. There also is a water-carbonate system that will naturally dissolve or release CO2 from ground and surface waters.11

    Methane in sewers and similar structures generally occurs from biological activities or chemical reactions of certain organic materials. 12 Usually its concentration is below the lower explosive limit, and at that concentration range will only decrease the available oxygen concentration one percent for every five percent methane. Methane can add to the explosive vapors that may be present from other flammable and explosive chemicals that have been discharged to the system. The presence of elevated levels of nitrogen and carbon dioxide may alter the flammability limits normally published for methane in air.

    The production of these and other gases may be altered by the presence of other chemicals, changes in temperature, and pH. The rate of gas generation may significantly affect the final concentration.

    There are several chemical pathways for the formation of these gases. Their concentration may be limited by reaction kinetics, mass transfer considerations, or by dilution effects. There are several sinks and sources for these materials. These kinds of controlling factors may substantially change the potential for exposure.


    1. J.B. Barsky et al., "Simultaneous Multi-Instrumental Monitoring of Vapors in Sewer Headspaces by Several Direct-Reading Instruments," Environmental Research v. 39 #2 (April 1986): 307-320.
    2. "Characteristics of Common Gases Found in Sewers," in Operation of Wastewater Treatment Plants, Manual of Practice No. 11. Alexandria, VA, Water Pollution Control Federation, 1976, Table 27-1.
    3. R. Garrison and M. Erig, "Ventilation to Eliminate Oxygen Deficiency in Confined Space - Part III: Heavier-than-Air Characteristics," Applied Occupational and Environmental Hygiene v. 6 #2 (February 1991): 131-140.
    4. "Criteria for a Recommended Standard - Occupational Exposure to Hydrogen Sulfide," DHEW Pub. No. 77-158; NTIS PB 274-196. Cincinnati, National Institute for Occupational Safety and Health, 1977.
    5. Permissible Exposure Limit (29 CFR 1910.1000 Tables Z-1 and Z-2).
    6. Short-Term Exposure Limit (29 CFR 1910.1000 Table Z-2).
    7. Biological Hazards at Wastewater Treatment Facilities. Alexandria, VA, Water Pollution Contol Federation, 1991.
    8. J. Chwirka and T. Satchell, "A 1990 Guide for Treating HydrogenSulfide in Sewers," Water Engineering and Management v. 137 #1 (January 1990): 32-35.
    9. John Holum, Fundamentals of General, Organic and Biological Chemistry. New York, John Wiley & Sons, 1978, p. 215.
    10. J. Chwirka and T. Satchell, "1990 Guide for Treating Hydrogen Sulfide" in Sewers, Water Engineering and Management v. 137 #1 (January 1990): 32.
    11. V. Snoeyink and D. Jenkins, Water Chemistry. New York, John Wiley & Sons, 1980, p. 156.
    12. M. Zabetakis, "Biological Formation of Flammable Atmospheres," US. Bureau of Mines Report #6127, 1962.

    This is not a Material Safety Data Sheet, but rather a Reference Data Sheet that has been compiled from a number of sources, and is intended to be a concise, relatively non-technical source of information on a particular material or category of materials. It is provided in good faith and is believed to be correct as of the date compiled; however, Meridian Engineering & Technology makes no representation as to the comprehensiveness or accuracy of the information. It is expected that individuals receiving the information will exercise their independent judgment in determining its appropriateness for a particular purpose. Accordingly, Meridian Engineering & Technology will not be responsible for damages of any kind resulting from the use of or reliance upon such information.

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