In this study, the effects of aerobic exercise on the upper airways and their defense mechanisms were investigated in athletes. The athletes ran in two different environments: the downtown streets of the city of São Paulo (Street), more polluted, and an urban forest (Forest), less polluted. Thirty-eight young healthy athletes ran for 45 min d−1 randomly during five consecutive days, with an interval of 48 h before changing environment. Clinical parameters and respiratory tract defense markers were evaluated before and after the first run on Mondays (1 d) and on Fridays (5 d). Street presented higher mean PM2.5 concentrations (65.1  ±  39.1 µg m−3p  <  0.001) and lower temperature (22.0 °C, p  =  0.010) than Forest (22.6  ±  15.3 µg m−3 and 22.8 °C). After 1 d Street running, subjects showed an increment in heart rate (p  <  0.001). At day 5, there was twice the number of athletes with impaired nasal mucociliary clearance (MCC) in the Street runners group when compared to the Forest runners group. Exhaled breath condensate pH values increased in the Forest group, with significant differences between groups in day 1 (p  =  0.006) and day 5 (p  <  0.001), despite the fact that both groups showed values within the normal range. After exposure to both environments, the number of cells in the nasal lavage fluid was reduced after exercise (p  =  0.014), without alterations in cell type and IL-8 and IL-10 concentrations. Aerobic exercise can either maintain or acutely enhance MCC and it may help to regulate inflammatory responses in the airways. Here we show that exercise practice in polluted outdoor environment, over a 5 d period, impairs MCC. In contrast, athletes running in the less polluted environment (Forest) show higher exhaled breath condensate pH values when compared to those who exercised in a more polluted environment (Street).