The aim of this study was to assess the effect of outdoor air pollution on exhaled levels of endogenously released nitric oxide. To exclude bias from exogenous NO in the recovered exhaled air (residual NO or NO in dead volume) an experimental design was used that sampled NO of endogenous origin only. The validity of the presented experimental design was established in experiments where subjects were exposed to high levels of exogenous NO (cigarette smoke or 480 microg x m(-3) synthetic NO). Subsequent 1 min breathing and a final inhalation of NO-free air proved to be sufficient to attain pre-exposure values. Using the presented method detecting only endogenous NO in exhaled air, 18 subjects were sampled on 4 separate days with different levels of outdoor air pollution (read as an ambient NO level of 4, 30, 138 and 246 microg x m(-3)). On the 2 days with highest outdoor air pollution, exhaled NO was significantly (p<0.001) increased (67-78%) above the mean baseline value assessed on 4 days with virtually no outdoor air pollution. In conclusion, the level of endogenous nitric oxide in exhaled air is increased on days with high outdoor air pollution. The physiological implications of this findings need to be investigated further.