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 gas pipelines. The North American market, consisting of Canada, Mexico, and the United States, all connected by a common pipeline network, has had much lower gas prices in recent years than some other major world gas markets, such as Europe (since 2010), Japan (since 2008), and Korea. The United States is connected by pipeline to Canada and Mexico. The US has long imported large quantities of gas from Canada, and exported smaller quantities to some parts of eastern Canada. In 2014, the US imported 2,634 BCF from Canada, and exported 769 BCF, so that net imports from Canada totaled 1,865