[Respiratory toxicity due to atmospheric pollutants. General review and a study of the relation to respiratory infections].

The main primary pollutants released into the atmosphere are sulfur dioxide (SO2), nitrogen monoxide and dioxide (NOx), particulate dust and in a less important part carbon monoxide (CO), hydrocarbons and heavy metals (Pb, Cd). Sulfur and nitrogen oxides are released from combustion of coals and fuels. Sulfates, nitrates and ozone are secondary pollutants resulting from chemical reactions within the atmosphere. While governmental directives limiting emissions have decreased SO2 and particulate matter levels, air quality in urban regions has improved in the last two decades. The role of air pollution as a risk factor for respiratory infections is difficult to address. Animal experiments demonstrate that air pollutants decrease the efficacy of lung defense mechanisms and increase the sensibility to respiratory infections. Nevertheless, because of difference in sensitivity between animal species and between exposure conditions, these effects are difficult to extrapolate to humans. Moreover, it is obvious that direct exposure studies of the sensibility of humans to respiratory infections are rare for ethical reasons. Epidemiological data addressing the role of air pollutants at usual levels can only suggest that some pollutants (SO2, suspended particulates) constitute a risk factor for respiratory infections. Since most of these studies do not include bacteriologic and virologic confirmation, it is unclear whether this respiratory morbidity is due to respiratory irritation or infection. In conclusion, we think that high concentrations of air pollutants are very likely to increase sensibility to respiratory infections in humans. There are however no sufficient data to clearly establish whether air pollution constitutes a risk factor for respiratory infections at usual ambient concentrations.