California Office of Environmental Health Hazard Assessment

Nitrogen dioxide (NO2) is the most abundant and toxic of the nitrogen oxides formed from combustion of fossil fuels, and ambient concentrations are related to traffic density as well as point sources. Indoor NO2 levels may exceed those found outdoors. When inhaled, NO2 persists to the lung periphery because of its relatively low solubility. Greater than 60 % of inhaled NO2 is deposited, predominantly in the centri-acinar region, and the fraction deposited increases with exercise. Epidemiological studies have found relationships between both outdoor and indoor NO2 levels and respiratory illness, decrements in lung function, and exacerbation of asthma, especially in children. Outdoor NO2 was associated with increased infant mortality and intrauterine mortality in Sao Paulo, Brazil. However, these studies are subject to exposure misclassification, and generally fail to consider a possible role of indoor and outdoor particle exposure as a confounding factor. NO2 may represent a marker for exposure to traffic- or combustion-related pollution in these epidemiological studies. Human clinical studies generally fail to show effects of exposure concentrations at or below the current California standard of 0.25 ppm