NOx is a generic term for the mono-nitrogen oxides nitric oxide NO (2NO= N2+O2) and nitrogen dioxide NO2 (2NO2=2NO+O2. They are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures. In areas of high motor vehicle traffic, such as in large cities, the amount of nitrogen oxides emitted into the atmosphere as air pollution can be quite significant. NOx gasses are formed everywhere where there is combustion - like in an engine.

SOURCES AND HEALTH EFFECTS.
The major source of NOx production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NOx during combustion.NOx reacts with ammonia, moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of such particles may cause or worsen respiratory diseases such as emphysema, bronchitis it may also aggravate existing heart disease.