Experimental exposures to nitrogen dioxide.

Nitrogen dioxide (N02) produced by combustion processes, is a major constituent of ambient air pollution in urban and industrial areas. It also plays a role as an indoor air pollutant and in the workplace. Indeed acute inhalation accidents in the workplace with high concentrations of N02 provided the first evidence of its adverse respiratory effects. Epidemiological studies provided evidence of an association between indoor as well as outdoor N02 exposure and the frequency or duration of respiratory illness and lung function impairment (1, 2]. However, it is well-known that interpretation of epidemiological studies in terms of causal relationships poses many problems, due to confounding variables and the complexity of air pollutant mixtures. Therefore, controlled exposure studies in human subjects are essential to the understanding of the health effects of N02.