Although hydrogen sulphide is detectable at concentrations far below those considered toxic, due to its disagreeable "rotten egg" odour, even at low concentrations it has an irritant action on the eyes and respiratory tract. Hydrogen sulphide, also known as marsh gas, and is a natural product of organic decomposition in anoxic wet muds; also of a number of industrial processes, notably petroleum refining, manufacture of pulp and paper, tanning, and sulphur dye manufacture. It is a highly toxic substance which blocks oxygen absorption by the blood. Hydrogen sulphide formed in a Intoxication may be hyperacute, acute, subacute or chronic. Inhalation of massive quantities of hydrogen sulphide will rapidly produce anoxia resulting in death by asphyxia; epileptiform convulsions may occur and the individual falls apparently unconscious, and may die without moving again. This is a syndrome characteristic of hydrogen sulphide poisoning in sewermen; however, in such cases, exposure is often due to a mixture of gases including methane, nitrogen, carbon dioxide and ammonia.