Evidence from animal toxicological studies indicates that long-term exposure to NO2 at concentrations above current ambient concentrations has adverse effects. In population studies NO2 has been associated with adverse health effects even when the annual average NO2 concentration complied with the WHO-2000 annual guideline value of 40 µg/m3. Also some indoor studies suggest effects on respiratory symptoms among infants at concentrations below 40 µg/m3. Together these results support a lowering of the annual NO2 guideline value. However, NO2 is an important constituent of combustion-generated air pollution and is highly correlated with other primary and secondary combustion products, it is unclear to what extent the health effects observed in epidemiological studies are attributable to NO2 itself or to other correlated pollutants. The current scientific literature, therefore, has not accumulated sufficient evidence to change the WHO 2000 guideline value of 40 µg/m3 for annual NO2 concentration.

Many short term experimental human toxicology studies show acute health effects at levels higher than 500 µg/m3, and one meta-analysis has indicated effects at levels exceeding 200 µg/m3. The current scientific literature has not accumulated evidence to change from the WHO 2000 guideline value of 200 µg/m3 for 1-hour NO2 concentration.

There is still no robust basis for setting an annual average guideline value for NO2 through any direct toxic effect. Epidemiological evidence has emerged, however, that increases the concern over health effects associated with outdoor air pollution mixtures that include NO2. These studies have shown for example, that bronchitic symptoms of asthmatic children increase in association with annual NO2 concentration, and that reduced lung function growth in children is linked with increased NO2 concentrations within communities already at current North American and European urban ambient air levels. Recently published studies document that NO2, as a marker of a complex mixtures of traffic-related combustion pollution, can have higher spatial variation than particle mass. In addition, these studies reported adverse effects on the health of children living in the areas characterized by higher levels of NO2 even when the overall level was low. Furthermore, recent studies on indoor NO2 concentrations have added evidence on adverse effects of NO2 on respiratory symptoms in children. The WHO AQG 2000 annual average NO2 guideline value of 40 µg/m3 is within the exposure ranges reported in these investigations. They also show that these associations cannot be completely explained by co-exposure to PM, but that other components in the mixture (such as organic carbon and nitrous acid vapor) might explain part of the association. Since such components are not routinely measured, and NO2 concentrations in ambient air are readily available, it seems reasonable to retain a prudent annual average limit value for NO2. Such a limit takes into account that there may be direct toxic effects of chronic NO2 exposure at low levels. In addition, the annual guideline value may help to control complex mixtures of combustion-related pollution (mainly from road traffic).

In experimental studies the lowest level of nitrogen dioxide exposure reported in more than one laboratory shows a direct effect on pulmonary function in asthmatics at 560 µg/m3. Studies of bronchial responsiveness among asthmatics indicate an increase in responsiveness at levels upwards from 200 µg/m3. The WHO AQG 2000 short term NO2 guideline of 200 µg/m3 is not challenged by more recent studies, and should therefore remain.