What Is a NOx Number?

If your car has failed an emissions test, there's a chance a high NOx number was to blame. Nitrogen oxides, represented by the chemical symbol NOx, area a class of compounds regulated by the Environmental Protection Agency.

What are Nitrogen Oxides?

• Nitrogen oxides are chemical compounds containing nitrogen, which has the chemical symbol "N," and oxygen, which has the chemical symbol "O." The notation NOx refers to the entire family of compounds. For individual compounds, the subscript "x" is replaced with a specific number indicating the number of oxygen atoms per nitrogen atom. Nitrogen dioxide, for example, appears as NO2.

Why Does the EPA Regulate NOx?

• Nitrogen oxides contribute to air pollution in the form of ozone and smog. They also can cause lung irritation, particularly in those with asthma. The EPA first set national standards for NOx levels in 1971, and those same standards are still used today. Nitrogen dioxide is measured as an indicator for the entire group of compounds. The national air quality standard is 0.053 parts per million, or 53 ppb, averaged annually.

Where do NOx Emissions Come From?

• In 2005, on-road vehicles accounted for about 36 percent of all NOx emissions in the United States. Non-road equipment produced 23 percent and electricity generation 21 percent to make the top three sources nationwide, according to the EPA. The primary sources of NOx emissions vary from state to state, however. In Alaska, for example, burning fossil fuels accounts for 44 percent of such emissions; in California, on-road vehicles account for 51 percent.

NOx and Car Emissions

• Because vehicles account for such a large percentage of NOx emissions, some states require that individual vehicles have their emissions levels tested. California is an example of a state with required "smog checks." These tests measure the levels of various regulated substances in tailpipe exhaust. The amount of NOx that is considered acceptable in emissions varies from state to state, and based on the type of vehicle and its age. As of 2000, California's highest acceptable rate for passenger cars manufactured after 1993 was 78 parts per million. The state hopes continuing smog tests will remove 100 tons of pollutants, including NOx, from the air every day, according to a publication from California's Bureau of Automotive Repair.