PO Box 15004, Flagstaff, AZ 86011-5004
Phone: (928) 523-0526
Fax: (928) 523-1266 Andy.Bessler@nau.edu
NTAA Air Topics:
NAAQS - Nitrogen Oxides (NOx)
The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards (NAAQS) for wide-spread
pollutants from numerous and diverse sources considered harmful to public health and the environment. The Clean Air Act established
two types of national air quality standards. Primary standards set limits to protect public health, including the health of
"sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public
welfare, including protection against visibility impairment, damage to animals, crops, vegetation, and buildings. The Clean Air Act
requires periodic review of the science upon which the standards are based and the standards themselves.
EPA has set NAAQS for six principal pollutants, which are called "criteria" pollutants. They are listed below. For a
summary table of the current NAAQS, click on the following link: NAAQS Table.
Nitrogen dioxide (NO2) is one of a group of highly reactive gasses known as "oxides of nitrogen," or "nitrogen
oxides (NOx)." Other nitrogen oxides include nitrous acid and nitric acid. While EPA’s National Ambient Air Quality Standard
covers this entire group of NOx, NO2 is the component of greatest interest and the indicator for the larger group of
nitrogen oxides. NO2 forms quickly from emissions from cars, trucks and buses, power plants, and off-road equipment. In
addition to contributing to the formation of ground-level ozone, and fine particle pollution, NO2 is linked with a number of adverse
effects on the respiratory system.
EPA first set standards for NO2 in 1971, setting both a primary standard (to protect health) and a secondary standard (to protect
the public welfare) at 0.053 parts per million (53 ppb), averaged annually. The Agency has reviewed the standards twice since that
time, but chose not to revise the standards at the conclusion of each review. All areas in the U.S. meet the current (1971)