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
Shale gas in the United States thickness of the Barnett varies from 100 to , but most economic wells are located where the shale is between 300 and thick. The success of the Barnett has spurred exploration of other deep shales. In 2007, the Barnett shale (Newark East) gas field produced of gas, making it the second-largest source of natural gas in the United States. The Barnett shale currently produces more than 6% of US natural gas production. In April 2015, Baker Hughes reports that there are 0 rig currently in activity at the Barnett gas field. (That doesn't mean 0 production as there are still