Hydrogen sulfide H2S is a colorless chalcogen hydride gas with the characteristic foul odor. It is poisonous, corrosive, and flammable. Hydrogen sulfide is often produced from the microbial breakdown of organic matter in the absence of oxygen gas, such as in swamps and sewers; this process is commonly known as anaerobic digestion which is done by sulfate-reducing microorganisms. H2S also occurs in volcanic gases, natural gas, and in some sources of well water. The human body produces small amounts of H2S and uses it as a signaling molecule.