Nitrogen oxides (NOX) form naturally when nitrogen and oxygen combine through bacterial action in soil, lightning, volcanic activity, and forest fires. Nitrogen oxides also result from human activities including high-temperature combustion of fossil fuels by automobiles, power plants, industry, and the use of home heaters and gas stoves. Environmental agencies particularly track the light brown gas nitrogen dioxide (NO2) because when it combines with volatile organic compounds (VOCs) in the presence of sunlight, it forms ground-level ozone.

The national average for ambient levels of nitrogen dioxide decreased by 38 percent from 1976-1997. Since 1992, all monitoring locations across the country met the national NO2 air-quality standard.