93.How Hydrogen Sulfide (H2S) is harmful

Hydrogen sulfide (H₂S) is a flammable, colorless gas that smells like rotten eggs. People usually can smell hydrogen sulfide at low concentrations in air, ranging from 0.0005 to 0.3 parts per million (ppm) (0.0005-0.3 parts of hydrogen sulfide in 1 million parts of air). At high concentrations, a person might lose their ability to smell it. This is important because a person might falsely think that hydrogen sulfide is no longer present; this may increase their exposure risk to air levels that may cause serious health effects.

Hydrogen sulfide health effects	The health effects of hydrogen sulfide depend on several factors such as how much hydrogen sulfide you are exposed to and the length of that exposure. Studies in workers, communities living near industrial sources of hydrogen sulfide, and volunteers suggest that the respiratory tract and nervous system are the most sensitive targets of hydrogen sulfide toxicity. No health effects have been found in humans exposed to typical environmental concentrations of hydrogen sulfide (0.00011-0.00033 parts per million [ppm]).
Respiratory effects	Exposure to low concentrations of hydrogen sulfide may cause irritation to the eyes, nose, or throat. It may also cause difficulty in breathing for some asthmatics. Respiratory distress or arrest has been found in people exposed to very high concentrations of hydrogen sulfide.
Nervous system effects	Exposure to low concentrations of hydrogen sulfide may cause headaches, poor memory, tiredness, and balance problems. Brief exposures to high concentrations of hydrogen sulfide (greater than 500 ppm) can cause a loss of consciousness. In most cases, the person appears to regain consciousness without any other effects. However, in some individuals, there may be permanent or long-term effects such as headaches, poor attention span, poor memory, and poor motor function.
Hydrogen sulfide and cancer	Hydrogen sulfide has not been shown to cause cancer in humans, and its possible ability to cause cancer in animals has not been studied thoroughly. DHHS and the International Agency for Research on Cancer (IARC) have not classified hydrogen sulfide as to its carcinogenicity. EPA has determined that data for hydrogen sulfide are inadequate for carcinogenic assessment.