Introduction: Hydrogen sulfide (H2S) is a colorless gas. At low concentrations it has an obnoxious odor similar to rotten eggs. It is soluble in water. It is produced in nature primarily through the decomposition of organic matter by bacteria. It is a constituent of natural gas, petroleum, sulfur deposits, volcanic gases and sulfur springs. Hydrogen sulfide is oxidized by photo chemically-generated free radicals, especially hydroxyl radicals. It has a half-life in air ranging from 12 to 37 hours, but varies depending upon photoactive pollutants and temperature. The half-life in air during very cold and dry winter conditions can exceed 37 hours. In the United States about 125,000 employees in 73 industries are potentially exposed to Hydrogen sulfide. However, domestic exposure can occur from various sources as follows: Ambient air near petroleum refineries, and sewage treatment plants; sewers (sewer gas); hot water tanks; and septic tanks. Sewer gas refers to the odor associated with sewers, waste treatment plants and septic tanks.
Sewer gas contains hydrogen sulfide and reduced sulfur compounds, such as methyl and dimethyl sulfide, ethyl and diethyl sulfide. These organo-sulfur compounds add to the toxicity of the hydrogen sulfide in the sewer gas. The most dangerous aspect of hydrogen sulfide results from olfactory accommodation and/or olfactory paralysis. This means that the individual can accommodate to the odor and is not able to detect the presence of the chemical after a short period of time. Olfactory paralysis occurs in workers who are exposed to 150 ppm or greater. This occurs rapidly, leaving the worker defenseless. Unconsciousness and death have been recorded following prolonged exposure at 50
ppm.
Hydrogen sulfide is a mitochondrial poison. Its action on mitochondria is similar to that of cyanide through inhibition of cytochrome oxidase (iron containing protein). This prevents the utilization of oxygen with an uncoupling of oxidative phosphorylation,reducing the ability of mitochondria to produces the energy molecule, Adenosine Triphosphate (ATP). In addition, hydrogen sulfide binds to hemoglobin in red blood cells interfering with oxygen transport. Exposure to hydrogen sulfide occurs primarily by inhalation but can also occur by ingestion (contaminated food) and skin (water and air). Once taken into the body, it is rapidly distributed to various organs, including the central nervous system, lungs, liver, muscle, etc.
Recently, Chinese Wall Board has been implicated as releasing hydrogen sulfide and causing corrosion of copper pipes. Research has shown that iron reducing bacteria are present in the dry wall. These bacteria are capable of using sulfur in the gypsum as an energy source, producing hydrogen sulfide and other reduces sulfur compounds (See Hooper et al, 2010).
Odor and Concentrations: Subjective olfactory responses to various concentrations of hydrogen sulfide are summarized as follows: (see Note on page 5 regarding odor threshold)
0.0.005-0.01 pp, - ATSDR, 1999
0.13 ppm Minimal perceptible odor
0.77 ppm Faint, but readily detectable odor
4.6 ppm Easily detectable odor, moderate odor
27.0 ppm Strong, unpleasant odor, but not intolerable.
Physiological Responses to Acute Exposures: Physiological responses to acute exposure to hydrogen sulfide have been reported as follows: