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 Spanish includes samples from all Spanish-speaking countries. The list of "1000 most frequent word forms" comes from an analysis of CREA version 3.2. Plurals, verb conjugations, and other inflections are ranked separately. Homonyms, however, are not distinguished from one another. CREA 3.2 was published in June 2008. In 2006, Mark Davies, an associate professor of linguistics at Brigham Young University, published his estimate of the 5000 most common words in Modern Spanish. To make this list, he compiled samples only from 20th-century sources—especially from the years 1970 to 2000. Most of the sources are from the 1990s. Of the 20 million