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
Natural gas in the United States was still increasing. The most commonly quoted producer price for natural gas is the Louisiana-based Henry Hub price, which is futures-traded on NYMEX. A barrel of oil releases about 5.8 million BTU when burned, so that 5.8 MCF of gas (at the standard one thousand BTU per cubic foot) releases about the same energy as a barrel of oil. Sometimes gas containing 5.8 million BTU is defined as a "barrel of oil equivalent for energy calculation purposes. When describing reserves or production, however, the oil and gas industry more commonly uses the rounded number of 6 MCF of gas (or