H2S

Hydrogen sulfide, with the chemical formula H2S, is a colorless, flammable gas known for its characteristic rotten-egg odor at low concentrations. It is highly toxic, can cause rapid loss of consciousness, and is denser than air, meaning it can collect in low-lying areas. H2S is sparingly soluble in water and forms hydrosulfide and sulfide species in solution; it is also a weak acid with dissociation in water.

Natural and industrial sources contribute to atmospheric H2S. It occurs in volcanic and geothermal gases, petroleum and natural gas deposits, and in the decomposition of organic matter under anaerobic conditions. Industrially, it is produced or released during petroleum refining, natural gas processing, pulp and paper production, tanneries, and various chemical manufacturing processes. It is sometimes used as an industrial intermediate for sulfur-containing compounds and in the production of sulfuric acid.

Chemically, H2S is a simple, linear diatomic molecule. It is highly flammable and has a broad flammable range in air, typically about 4 to 44 percent by volume. In the environment, it oxidizes readily in the air to sulfur and sulfur dioxide, and it can react with metals to form metal sulfides. Because it is heavier than air, concentrations can accumulate in enclosed or poorly ventilated spaces.

Health and safety considerations are the primary concerns with H2S. It is highly toxic, even at low concentrations, and exposure can cause headaches, dizziness, respiratory distress, and loss of consciousness. The odor threshold can be temporary due to olfactory fatigue, so detection cannot be relied upon alone. Industrial handling relies on proper ventilation, gas detection systems, monitoring, and, in higher-risk settings, use of appropriate respiratory protection and procedures to control releases.

Environmental impact is significant because H2S is hazardous to aquatic life and can contribute to air pollution as it oxidizes to sulfur compounds.