BACKGROUND: The impact of nitrogen dioxide (NO2) and particulate matter with an aerodynamic diameter of less than or equal to 2.5. microns (PM2.5) exposures on lung function has been investigated mainly in children and less in adults. Furthermore, it is unclear whether short-term deviations of air pollutant concentration need to be considered in long-term exposure models. OBJECTIVES: The aims of this study were to investigate the association between short-term air pollution exposure and lung function and to assess whether short-term deviations of air pollutant concentration should be integrated into long-term exposure models. METHODS: Short-term (daily averages 0-7 d prior) and long-term (1- and 4-y means) NO2 and PM2.5 concentrations were modeled using satellite, land use, and meteorological data calibrated on ground measurements. Forced expiratory volume within the first second (FEV1) of forced exhalation and forced vital capacity (FVC) were measured during a LuftiBus assessment (2003-2012) and linked to exposure information from the Swiss National Cohort for 36,085 adults (ages 18-95 y). We used multiple linear regression to estimate adjusted associations, and additionally adjusted models of long-term exposures for short-term deviations in air pollutant concentrations. RESULTS: A 10 μg/m3 increase in NO2 and PM2.5 on the day of the pulmonary function test was associated with lower FEV1 and FVC (NO2: FEV1 - 8.0 ml [95% confidence interval: - 13.4, - 2.7], FVC - 16.7 ml [ - 23.4, - 10.0]; PM2.5: FEV1 - 15.3 ml [ - 21.9, - 8.7], FVC - 18.5 ml [ - 26.5, - 10.5]). A 10 μg/m3 increase in 1-y mean NO2 was also associated with lower FEV1 ( - 7.7 ml; - 15.9, 0.5) and FVC ( - 21.6 ml; - 31.9, - 11.4), as was a 10 μg/m3 increase in 1-y mean PM2.5 (FEV1: - 42.2 ml; - 56.9, - 27.5; FVC: - 82.0 ml; - 100.1, - 63.9). These associations were robust to adjustment for short-term deviations in the concentration of each air pollutant. CONCLUSIONS: Short- and long-term air pollution exposures were negatively associated with lung function, in particular long-term PM2.5 exposure with FVC. Our findings contribute substantially to the evidence of adverse associations between air pollution and lung function in adults. https://doi.org/10.1289/EHP7529.