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 infrastructure emerged as environmental concerns for the rush. In Pennsylvania, controversy has surrounded the practice of releasing wastewater from "fracking" into rivers which serve as consumption reserves. Methane release contributing to global warming is a concern. The word “fracking”, slang for hydraulic fracturing, has entered the English language. The Great Shale Gas Rush refers to the growth in unconventional shale gas extraction in the early 21st-century. Pennsylvania was featured in the Academy Award-nominated, environmental documentary Gasland by Josh Fox in 2010. Most of the filming for the 2012 Gus Van Sant dramatic film, Promised Land, starring Matt Damon, took place