Background: Short term exposure to traffic air pollution is associated with adverse respiratory outcomes. Exercise in polluted air increases pollutant uptake. Aims: The objective was to assess respiratory and systemic subclinical responses in a healthy population in Barcelona following short term exposures to traffic related air pollution and the role of moderate physical activity as a potential effect modifier. Methods: Well controlled crossover study design comparing lung function and -inflammation [Fraction of exhaled Nitric Oxide (FeNO)], and systemic inflammation markersin blood at baseline with measures repeated up to 6h post exposure to traffic related air pollution. Healthy non-smoking subjects were exposed for 2 hours to contrasting pollution levels while either cycling or resting (n=31). Each volunteer was to undergo all four conditions (high vs. low exposure; moderate physical activity vs. resting). On-site exposure monitoring included particulate matters of different sizes (UFP, PM10, PM 2.5), nitrogen oxides (NOx), and elemental carbon (EC).Data were analyzed using mixed effect models for repeated measures. Results: Physical Activity was associated with a significant increase in FEV1, (Coef. 0.034, p=0.000). These associations were robust and insensitive to adjustment for pollutants. Similarly consistent associations were seen with FeNO (Coef. 0.880, p=0.047), neutrophil percentage (Coef. 7.49%, p=0.000), and leukocytes counts (Coef. p=0.000). Interquartile increases in UFP (Coef.-0.004, p=0.001), EC (Coef.-0.004, p=0.003), NOx (Coef.-0.004, p=0.002), PM2.5 (Coef.-0.005, p=0.001), and PM10 (Coef. -0.003, p=0.002) were significantly associated with a decrease in the ratio of FEV1/FVC. Conclusions: This study suggests that moderate physical activity is slightly increasing lung function, but also lung inflammation and systemic inflammation. The decreasing ratio of FEV1 and FVC indicates a lung obstruction after traffic air pollution exposure. However, the implications of these immediate subclinical responses remain to be further explored.