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 annual standards at the conclusion of each review. In January 2010, EPA established an additional primary standard at 100 ppb, averaged over one hour. Together the primary standards protect public health, including the health of sensitive populations - people with asthma, children, and the elderly. No area of the country has been found to be out of compliance with the current NO2 standards.
All areas presently meet the current (1971) NO2 NAAQS, with annual NO2 concentrations measured at area-wide monitors well below the level of the standard (53 ppb). Annual average ambient NO2 concentrations, as measured at area-wide monitors, have decreased by more than 40% since 1980. Currently, the annual average NO2 concentrations range from approximately 10-20 ppb.
EPA expects NO2 concentrations will continue to decrease in the future as a result of a number of mobile source regulations that are taking effect. Tier 2 standards for light-duty vehicles began phasing in during 2004, and new NOx standards for heavy-duty engines are phasing in between 2007 and 2010 model years. Current air quality monitoring data reflects only a few years of vehicles entering the fleet that meet these strict NOx standards.