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The purpose of this study is to examine the association between hourly air pollution on hourly physical activity (PA) and sedentary behavior (SB) among college students in Beijing, China. The secondary aim was to examine such associations varied at specific time. A total of 340 participants were recruited from the Tsinghua University, in Beijing, China. Accelerometers provided PA measures, including moderate-to-vigorous physical activity (MVPA), walking steps, energy expenditure and sedentary time for 7 consecutive days. Corresponding air pollution data by the Beijing Municipal Ecological Environment Bureau in the closed site (Wan Liu site) in Tsinghua University were collected including average hourly air quality index (AQI) and PM 2.5 (μg/m³). Associations were estimated using linear individual fixed-effect regressions. We also conducted an air pollution risk perception survey among 2307 freshmen (76.6% males) who were enrolled in Tsinghua in 2016, and the survey was done in May 22–26, 2017. A one level increase in hourly air quality index (AQI) was associated with a reduction in 1-h PA by 0.083 (95% confidence interval [CI] = -0.137, −0.029) minutes of MVPA, 8.8 (95% CI = −15.0, −2.6) walking steps, 0.65 (95% CI = -1.03, −0.27) kcals of energy expenditure. A 10 μg/m³ increase in air pollution concentration in hourly PM 2.5 was associated with a reduction in 1-h PA by 0.021 (95% confidence interval [CI] = -0.033, −0.010) minutes of MVPA, 2.2 (95% CI = −3.5, −0.9) walking steps, 0.170 (95% CI = -0.250, −0.089) kcals of energy expenditure an increase in 1-h sedentary behavior 0.045 (0.005, 0.0845). At specific time, stronger negative associations of AQI and PM 2.5 air pollution with PA at 8 a.m., 4 p.m., 5 p.m. and 7 p.m. Similarly, stronger positive associations of 1 h AQI and PM 2.5 air pollution with SB at 8 a.m., 9 a.m., 11 a.m., and 7 p.m. A total of 94.9% participants (n = 2235) responded "yes" to air pollution change activities in the survey, which may partially explain PA change. Air pollution may discourage physical activity and increases sedentary behavior among freshman students living in Beijing, China. This is preliminary study. The impact of air pollution on physical activity and sedentary behavior at a specific time may be different. There are large variations in walking steps in specific time. Trend for walking steps in specific time in one day among the freshman. Image 1 • Air pollution may discourage physical activity behavior. • Air pollution may increase sedentary behavior. • The impact of air pollution on health behavior may be different at a specific time. [ABSTRACT FROM AUTHOR]