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
Trace gas rate of removal. The Junge (empirical) relationship describes the relationship between concentration fluctuations and residence time of a gas in the atmosphere. It can expressed as fc = "b"/τ, where fc is the coefficient of variation, τ is the residence time in years, and "b" is an empirical constant, which Junge originally gave as 0.14 years. As residence time increases, the concentration variability decreases. This implies that the most reactive gases have the most concentration variability because of their shorter lifetimes. In contrast, more inert gases are non-variable and have longer lifetimes. When measured far from their sources and sinks,