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
Industrial gas such as vacuum, which is often provided in hospital gas systems; purified compressed air; or refrigeration. Another unusual system is the inert gas generator. Some industrial gas companies may also supply related chemicals, particularly liquids such as bromine and ethylene oxide. Most materials that are gaseous at ambient temperature and pressure are supplied as compressed gas. A gas compressor is used to compress the gas into storage pressure vessels (such as gas canisters, gas cylinders or tube trailers) through piping systems. Gas cylinders are by far the most common gas storage and large numbers are produced at a "cylinder fill"