NO_x is a common term for the more reactive nitrogen oxides and includes nitric oxide (NO) and nitrogen dioxide (NO₂), but excludes, for example, nitrous oxide (N₂O). NO₂ is a reddish brown, highly reactive gas that is formed in the air by the oxidation of NO. Anthropogenic emissions from the high-temperature combustion of coal, oil, gas, and gasoline can oxidize atmospheric nitrogen (N₂) to yield the majority of NO found in the environment. Natural sources of NO₂ are soil microbial processes. In the soil the nitrification and denitrification processes pass through compounds that can break down and release NO and N₂O into the atmosphere. This is a natural process that is enhanced when nitrogen fertilizers are used to improve crop yields.

Short-term exposure to NO₂ at concentrations found in the United States can increase respiratory illness in children. There is evidence...