Wiki and OSHA Fact Sheet for Hydrogen Sulfide Gas

Wikipedia has an entry on 

Sewer Gas

"Sewer gas is a complex mixture of toxic and non-toxic gases produced and collected in sewage systems by the decomposition of organic household or industrial wastes, typical components of Sewage

Sewer gases may include hydrogen sulfide, ammonia, methane, carbon dioxide, sulfur dioxide and nitrogen oxides. Improper disposal of petroleum products such as gasoline and mineral spirits contribute to sewer gas hazards. Sewer gases are of concern due to their odor, health effects, and potential for creating fire or explosions.

. . .

Health effects

In most homes, sewer gas may have a slightly unpleasant odor, but does not often pose a significant health hazard. Residential sewer pipes primarily contain the gases found in air (nitrogen, oxygen, carbon dioxide, etc). Often, methane is the gas of next highest concentration, but typically remains at non-toxic levels, especially in properly vented systems. However, if sewer gas has a distinct “rotten egg” smell, especially in sewage mains, septic tanks, or other sewage treatment facilities, it may be due to hydrogen sulfide content, which can be detected by human olfactory senses in concentrations as low as parts per billion. Exposure to low levels of this chemical can irritate the eyes, cause a cough or sore throat, shortness of breath, and fluid accumulation in the lungs. Prolonged low-level exposure may cause fatigue, pneumonia, loss of appetite, headaches, irritability, poor memory, and dizziness. High concentrations of hydrogen sulfide (>150 ppm) can produce olfactory fatigue, whereby the scent becomes undetectable. At very high concentrations (>300 ppm), hydrogen sulfide can cause loss of consciousness and death." 

[I have removed the superscripted hypertext links to the references to prevent this blog from exploding with html - author]

OSHA's Fact Sheet on Hydrogen Sulfide

Hydrogen sulfide is a colorless, flammable, extremely hazardous gas with a “rotten egg” smell. Some common names for the gas include sewer gas, stink damp, swamp gas and manure gas. It occurs naturally in crude petroleum, natural gas, and hot springs. In addition, hydrogen sulfide is produced by bacterial breakdown of organic materials and human and animal wastes (e.g., sewage). Industrial activities that can produce the gas include petroleum or [my edit ]natural gas drilling and refining, wastewater treatment, coke ovens, tanneries, and kraft paper mills. Hydrogen sul-fide can also exist as a liquid compressed gas.

Hazardous properties of H2S gas Hydrogen sulfide is heavier than air and may travel along the ground. It collects in low-lying and enclosed, poorly-ventilated areas such as basements, manholes, sewer lines, underground telephone vaults and manure pits. For work within confined spaces, use appropriate pro-cedures for identifying hazards, monitoring and enter-ing confined spaces.

The primary route of exposure is inhalation and the gas is rapidly absorbed by the lungs. Absorption through the skin is minimal. People can smell the “rotten egg” odor of hydrogen sulfide at low concentrations in air. However, with continuous low-level exposure, or at high concentrations, a person loses his/her ability to smell the gas even though it is still present (olfactory fatigue). This can happen very rapidly and at high concentrations, the ability to smell the gas can be lost instantaneously. Therefore, DO NOT rely on your sense of smell to indicate the continuing presence of hydrogen sulfide or to warn of hazardous  concentra-tions. In addition, hydrogen sulfide is a highly flammable gas and gas/air mixtures can be explosive. It may travel to sources of ignition and flash back. If ignited, the gas burns to produce toxic vapors and gases, such as sulfur dioxide.

Contact with liquid hydrogen sulfide causes frostbite. If clothing becomes wet with the liquid, avoid ignition sources, remove the clothing and isolate it in a safe area to allow the liquid to evaporate. 

Health effects of H2S exposure Hydrogen sulfide is both an irritant and a chemical asphyxiant with effects on both oxygen utilization and the central nervous system. Its health effects can vary depending on the level and duration of exposure. Repeated exposure can result in health effects occurring at levels that were previously tolerated without any effect.

Low concentrations irritate the eyes, nose, throat and respiratory system (e.g., burning/tearing of eyes, cough, shortness of breath). Asthmatics may ex-perience breathing difficulties. The effects can be de-layed for several hours, or sometimes several days, when working in low-level concentrations. Repeated or prolonged exposures may cause eye inflammation, headache, fatigue, irritability, insomnia, digestive disturbances and weight loss.

Moderate concentrations can cause more severe eye and respiratory irritation (including coughing, diffi-culty breathing, accumulation of fluid in the lungs), headache, dizziness, nausea, vomiting, staggering and excitability.

High concentrations can cause shock, convulsions, inability to breathe, extremely rapid unconsciousness, coma and death. Effects can occur within a few breaths, and possibly a single breath. 

Protection against H2S exposure

Before entering areas where hydrogen sulfide may be present: 
1. Air must be tested for the presence and concen-tration of hydrogen sulfide by a qualified person using air monitoring equipment, such as hydrogen sulfide detector tubes or a multi-gas meter that detects the gas. Testing should also determine if fire/explosion precautions are necessary.

2. If the gas is present, the space/area must be vent-ilated continually to remove the gas.

3. If the gas cannot be removed, the person entering the space/area must use appropriate respiratory protection and any other necessary personal protective
equipment, rescue and communication equipment. OSHA’s Confined Spaces standard contains specific requirements for identifying, monitoring and entering confined spaces. 

Entering dangerous H2S atmospheres A level of H2S gas at or above 100 ppm is Immediately Dangerous to Life and Health (IDLH). Entry into IDLH atmospheres can only be made using: 
1) a full facepiece pressure demand self-contained breathing apparatus (SCBA) with a minimum service life of thirty minutes,
2) a combination full facepiece pressure demand supplied-air respirator with an auxiliary self-contained air supply.

If H2S levels are below 100 ppm, an air-purifying respirator may be used, assuming the filter cartridge or  [my edit] canister is appropriate for hydrogen sulfide. A full facepiece respirator will prevent eye irritation. If air concentrations are elevated, eye irritation may become a serious issue. If a halfmask respirator is used, tight fitting goggles must also be used. Workers in areas containing hydrogen sulfide must be mon-itored for signs of overexposure. NEVER attempt a rescue in an area that may contain hydrogen sulfide without using appropriate respiratory protection and without being trained to perform such a rescue. 

This is one in a series of informational fact sheets highlighting OSHA programs, policies or standards. It does not impose any new compliance requirements. For a comprehensive list of compliance requirements of OSHA standards or regulations, refer to Title 29 of the Code of Federal Regulations. This information will be made available to sensory impaired individuals upon request. The voice phone is (202) 693-1999; teletypewriter (TTY) number: (877) 889-5627.
U.S. Department of Labor
(800) 321-OSHA
For more complete information:
DSG 10/2005

Lighting farts, that's one thing, this is entirely another. In some places in the texts quoted above, I have added the word OR in place of the / - e.g., petroleum/gas is made petroleum or gas - and I have noted it in the text in context.