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
Atmosphere of Jupiter comprehensively understood of those of all the gas giants because it was observed directly by the "Galileo" atmospheric probe when it entered the Jovian atmosphere on December 7, 1995. Other sources of information about Jupiter's atmospheric composition include the Infrared Space Observatory (ISO), the "Galileo" and "Cassini" orbiters, and Earth-based observations. The two main constituents of the Jovian atmosphere are molecular hydrogen () and helium. The helium abundance is relative to molecular hydrogen by number of molecules, and its mass fraction is , which is slightly lower than the Solar System's primordial value. The reason for this low abundance is