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
Gas venting or controlled combustion of gases from coal mining and of so-called biogases. In practice, it is impossible or impractical to capture and filter the air from coal mining (esp open-cast) and to completely degas hydrocarbon product and/or water produced from the ground which in some cases may be disposed at surface for and used after treatment. Melting of methane hydrates in the world's oceans or sea ice would also constitute uncontrolled gas leakage to atmosphere. Gas venting Gas venting is the intentional safe release, without combustion, into the earth's atmosphere of gases that cannot be readily recycled into a production