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 gas extraction led to an increase in aggregate US employment of 725,000 and a 0.5 percent decrease in the unemployment rate during the Great Recession". Research shows that shale gas wells can have a significant adverse impact on some house prices, with groundwater-reliant homes declining 13% in value whereas piped-water homes will see an increase of 2–3%. The price increase of the latter is most likely due to the royalty payments that property owners get from gas extracted under their land. The issue of whether to export natural gas has split the business community. Manufacturers such as Dow Chemical are