Hydrogen sulfide (H2S) is a colorless, flammable gas with a distinctive “rotten egg” smell. This poisonous, corrosive substance is produced naturally in the absence of oxygen, in swamps and other stagnant waters, by hot springs and volcanoes, and during the microbial breakdown of organic materials such as animal waste or human sewage. Hydrogen sulfide is also released into the environment during oil and natural gas drilling and production, in landfills, septic systems and wastewater treatment plants. It is also a byproduct of many industrial processes, including paper and rayon textile production, coke ovens and tanneries.
Most H2S exposures are due to industrial accidents. However, since January 2008, hydrogen sulfide has been implicated in suicide attempts in Japan, where website instructions for making the gas using household chemicals have led to several deaths. The method involves mixing toilet cleaner with insecticide or bath salts.
People can usually smell H2S at concentrations up to 0.3 parts per million (ppm), though levels this low may not cause symptoms or adverse health effects. At higher concentrations, the odor becomes less noticeable as the sense of smell is killed (olfactory desensitization).
At toxic levels, inhalation can lead to a variety of symptoms including headaches, fatigue and loss of coordination. Exposure to hydrogen sulfide can also exacerbate asthma symptoms. Most of the effects of hydrogen sulfide disappear once exposed individuals are in fresh air or the source is eliminated. If symptoms persist, medical attention is required.