Asphyxiant gas can smell the odorants, and assigning a different smell to each gas may be impractical. Another difficulty is that most odorants (e.g., the thiols) are chemically reactive. This is not a problem with natural gas intended to be burned as fuel, which is routinely odorized, but a major use of asphyxiants such as nitrogen, helium, argon and krypton is to protect reactive materials from the atmosphere. The dangers of excess concentrations of nontoxic gases has been recognized for centuries within the mining industry. The concept of black damp (or "stythe") reflects an understanding that certain gaseous mixtures could lead to
Suicide in the United Kingdom (4 per cent). Hanging is the most common method used by women, closely followed by self poisoning. Inhalation of domestic gas was the most common method of suicide during the mid-twentieth century. It was completely eliminated by the 1990s as a result of the replacement of coal gas containing toxic carbon monoxide by the non-poisonous natural gas. Later, suicide by inhalation of carbon monoxide from car exhausts became common, but has declined since the introduction of catalytic converters. Age-standardised rates generally fell between 1981 and 2007, with rates in subsequent years increasing to reach a peak of 11.1 deaths per