[Are nitrogen oxides major atmospheric pollutants?]

Nitrogen oxides (N2O, NO and NO2) are at different levels important molecules in biological, ecological and public health terms. They are implicated in several fundamental planetary equilibria: climate and greenhouse effect, stratospheric and tropospheric ozone. NO is a very important cellular bio-mediator considered as weakly toxic for the pulmonary function, as N2O. The main toxicological questions are coming from NO2 whose toxicity has been studied with experimental (cellular, animal and human models) and epidemiological approaches. Its pulmonary toxicity appears relatively weak in animal and healthy human for concentrations in the order of 0.5-11 ppm or more. But NO2 nocivity must be considered for several reasons: first the inter-individual variability of the human response, second the increased sensitivity of asthmatics, chronic obstructive pulmonary population but also children and women, third the increased toxicity of NO2 associated with other (allergenic or not) aerocontaminants. These results lead to apply to NO2 a caution principle in indoor or outdoor air pollution; in the second case, WHO and European Union defined references values.