H2S is naturally formed when bacteria break down organic material in the absence of oxygen. This formation can occur deep within the earth, such as in volcanoes and in crude petroleum, or relatively close to the surface in swamps, sewers, wells, and hot springs. Known by other names such as Hydrogen Sulphide, Sulfane, Sulphur Hydride, Dihydrogen Monosulfide, Sulfurated Hydrogen, Sewer Gas and Stink Damp; it is a highly toxic and colorless flammable gas that has a smell similar to rotten eggs. It should be noted, however, that using smell as a way of determining the presence of H2S is not reliable, as the sense of smell can be quickly eliminated in the presence of the gas.

H2S can seriously injure or kill exposed individuals. The gas is heavier than air, so it tends to drift to the bottom of poorly ventilated spaces such as in well holes and underground tanks. Inhalation of the escaping gas from an open orifice in the ground usually, but not always, results in death. At lower levels of concentration, a burning sensation in the eyes, nose, throat and lungs can be felt, where at higher levels of concentration a single breath can be fatal. Second man fatalities can occur when trying to rescue a fallen victim as by the time it is known that H2S is the cause it is too late for the rescuer. Training programs and proper equipment must be provided for all workers who deal with the presence of H2S gas in their environment even if the gas is remotely expected to be encountered.