Nitrogen oxides (NOx) are for the most part emitted from combustion processes and are very commonly emitted in our everyday lives. Sources include gasoline and diesel engines (cars, trucks, generators, lawnmowers and yard equipment, snowmobiles, power boats, etc), home furnaces (gas, oil, and wood), electrical power plants, industrial processes, explosives, and fertilizers. NOx contributes to several air pollution problems.

One component of NOx that is considered a direct health risk is nitrogen dioxide (or NO2). It can cause respiratory illness and lung damage. High levels can cause edema, and prolonged high exposure can produce fatal lung damage.

NOx is a primary component to ozone (or smog) formation. Ozone forms when NOx and certain volatile organic compounds combine in the presence of strong sunlight (ultraviolet wavelengths). Reducing emissions of NOx is critical to reducing ozone.

NOx is also a major contributor to acid rain (or acid deposition). After NOx is released by a source, it travels and mixes in the atmosphere. Eventually, NOx is oxidized and acidified into nitric acid and is removed from the air through precipitation (rain, snow, etc.), settling of particles, and by condensation from the air onto surfaces, similar to the formation of dew. Acid deposition damages our forests, lakes, and wetlands by robbing them of necessary nutrients and lowering the pH levels, often to levels below what native species can tolerate. The end result of this process is not a void of wildlife, but rather a shifting of species, where a more tolerant species simply populates an area where native species once were dominant.

One other complication of NOx is the nitrification of lakes, ponds, estuaries, and other wetlands. Nitrogen acts as a fertilizer for plants, and when it is added to water, algae, and other aquatic plants may multiply excessively, displacing other inhabitants of the wetland.

Emission rates of NOx have grown slowly over the past 20 years. While emission controls have been added to power plants and automobiles (catalytic converters), the growth in the usage of these sources has essentially neutralized the progress. As a result, measured NO2 levels in New Hampshire have remained relatively stable over the last 20 years.