Chronic Effects of High Level Hydrogen Sulfide Exposure

Hydrogen sulfide (H2S) occurs naturally in crude petroleum, volcanic gases, natural gas and hot springs. It also comes from the breakdown of human and animal wastes, as well as organic matter. Petroleum refineries, paper mills and tanneries may produce hydrogen sulfide. According to the Occupational Safety and Health Administration (OSHA), 50 parts-per-million (ppm) for 10 minutes is the maximum allowed exposure. While there is ample evidence of the acute effects of being exposed to that concentration of this gas, there is no conclusive evidence that there are chronic effects from these exposures.

Loss of Smell

• Short-term hydrogen sulfide exposure leads to loss of smell.

  In the study, "Long-Term Effects on the Olfactory System of Exposure to Hydrogen Sulfide," authors Alan R. Hirsch and Gilberto Zavala report that a high percentage of patients had reduced or limited ability to smell odors as much as three years after exposure to hydrogen sulfide. The study did not rule out the possibility there were other factors that could have contributed to the loss of smell.

Neurological

• Hydrogen sulfide is a central nervous system toxin. It is absorbed quickly into the blood through the lungs, and is then transported to the brain. It inhibits an enzyme necessary for cellular respiration, thereby depriving the brain of the oxygen needed for normal function. According to the government of Alberta, Canada, long-term nervous system effects include fatigue, anxiety, irritability, and impaired learning and memory. However, in a study of the available Canadian data, Dr. Sheldon Roth and Verona Goodwin found that, "few of the human studies obtained good exposure data that could be clearly associated with the reported effects."

Other Effects

• There is lack of conclusive evidence that the symptoms and conditions associated with hydrogen sulfide exposure up to the OSHA maximum levels, such as eye sensitivity to light, cough, nose and throat pain, headache, dizziness, nausea and vomiting, are long-lasting once the exposure ends.

  Some studies suggest that there is an increased risk for cardiovascular disease, but according to the "Health Effects of Hydrogen Sulfide: Knowledge Gaps" study, these risk conclusions were severely limited because the studies did not "control for co-exposures or exposure measures." The same report questioned the dissimilarity between the reported reproductive effects of hydrogen sulfide in animals and humans, and suggested that further research is needed for clarification.

  Depending on the individual, exposure to more than 100 ppm can lead to death, with the death threshold commonly reported to be between 500 ppm and 800 ppm.