NOx is short for oxides of Nitrogen. It is a class of pollutants which includes nitrogen oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). When emitted into the atmosphere, they can cause an array of problems, including acute and chronic health problems and smog. Nitrous oxide is also a potent greenhouse gas (GHG). For these reasons, NOx emissions are regulated in many areas of the world.
Nitrogen oxides are formed during combustion as high temperatures cause nitrogen from the atmosphere to oxidize. Because only temperature and heat are needed to form NOx, even lightning strikes form the compounds. NOx emissions from industrial facilities like power plants are regulated, as are the emissions from vehicles.
The level of regulation depends on the regulatory agency and the requirements of the area; more densely populated areas tend to require higher NOx reductions than rural areas. Geographic features also contribute to atmospheric NOx buildup, which makes some areas, such as Mexico City and Los Angeles, more susceptible to smog.