Nitrogen oxides (NOx) consist of a variety of reactive gases composed of different levels of nitrogen and oxygen and are most typically formed from the combustion of certain fuels (oil, coal, gasoline, diesel) at high temperatures. Industrial processes, power plants and mobile sources (including passenger cars, trucks, buses, ships, aircraft, locomotives and construction and farm equipment) are among the most significant sources of NOx in the U.S. As a contributor to ozone (smog), particle pollution, haze, toxic air pollution, global warming, acid rain and the eutrophication of water bodies, NOx poses considerable threats to public health and the environment and is linked with a large number of adverse impacts on the respiratory system, as well as with the many ill effects associated with all of the pollution problems to which it contributes.
NO2 is used as the indicator for the larger group of NOx. EPA established primary (to protect public health) and secondary (to protect public welfare) National Ambient Air Quality Standards for NO2 for the first time in 1971, both at 53 parts per billion (ppb) averaged annually. The agency has reviewed the NOx standards twice since then but has chosen not to revise this annual standard. In January 2010, EPA set an additional primary standard of 100 ppb averaged over 1 hour to ensure the protection of public health including for sensitive populations such as those with asthma, children and older adults. The agency is currently engaged in the review of both the primary and secondary NOx standards and expects to issue decisions on whether or not to revise them in 2016 and 2018, respectively.
NOx emissions are controlled through a number of federal, state and local regulatory programs directed at the various sources that emit NOx.