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
Most common words in English about half of all written English. According to a study cited by Robert McCrum in "The Story of English," all of the first hundred of the most common words in English are of Anglo-Saxon origin, except for "people", ultimately from Latin "populus", and "because", in part from Latin "causa". Some lists of common words distinguish between word forms, while others rank all forms of a word as a single lexeme (the form of the word as it would appear in a dictionary). For example, the lexeme "be" (as in "to be") comprises all its conjugations ("is", "was", "are", "were", etc.),