The Characteristics of Survey Participants.
Table 1 presents the characteristics of the participants. Participants were 340 freshmen. Among the 340 participants, accounting for more than two third (70.58%) were male. Participants were compliant wearing GT3X accelerometer 42291 hours over 7 continuous days. The mean age of the participants was 18.33 (SD = 1.02). The mean participant’s BMI was 21.61 kg/m2 (SD = 3.07), male were significantly higher than female (P < 0.001). Only 0.88% of participants were reported smoker and 2.93% was reported drinkers. The mean self-rated physiological health scores were 5.54 (SD = 1.78) and the mean self-rated mental health scores were 6.51 (SD = 1.88).
The Air Pollution Variations.
Table 2 shows the variations of air pollution measures during the study period. The mean AQI value at one-hour was 106.08 (SD = 88.66). There were 10,977 (26.63%), 14,980 (36.35%), 6,476 (15.71%), 2,329 (5.65%), 4,868 (11.81%) and 1,583 (3.84%) one hour AQI at “good” AQI (0-50) , “moderate” AQI (51-100), “Unhealthy for sensitive groups” AQI (101-150), “unhealth” AQI (151-200), “very unhealthy” AQI (201-300) and “Harzardous” AQI (>300), respectively. The mean concentration of PM2.5 value at one-hour was 68.02 μg/m3 (SD = 66.77). A majority proportion (59.11%) of average PM2.5 were in concentrations exceeding the U.S. Environmental Protection Agency (EPA) PM2.5 standard of 35µg/m3.
The PA and SB Variations.
Table 3 presents the mean variations of participants’ PA and SB in the study. As illustrated, there are large variations in PA and SB. For example, the mean minutes of the participant’s one-hour MVPA was 3.53 (SD = 4.50). One-hour mean minutes of MVPA ranged largely from 1.64 (SD = 2.69) at 11 pm to 6.11 (SD = 8.32) at 5 pm in the participants (P < .001). The mean steps of the participant’s one-hour walking was 394.46 (SD = 627.24). One-hour mean steps of walking ranged largely from 147.03 (SD = 302.91) at 11 pm to 709.78 (SD = 947.25) at 5 pm in the participants (P < .001). Similarly, the mean kcals participant’s one-hour energy expenditure was 394.46 (SD = 627.24). One-hour mean steps of walking ranged largely from 9.20 (SD = 19.53) at 11 pm to 38.04 (SD = 66.19) at 5 pm in the participants (P < .001). Whereas the mean minutes participant’s one-hour SB was 31.48 (SD = 19.69). One-hour mean minutes of SB ranged largely from 38.56 (SD = 18.16) at 11 pm to 25.44 (SD = 17.07) at 12 am in the participants (P < .001).
Impact of AQI on PA and SB.
Table 4 shows the estimated effects of Air quality index (AQI) on individual-level outcomes hourly PA and SB using linear individual fixed-effect regressions. AQI was found to be significantly negatively associated with one-hour PA among participants. A one level increase in AQI was linked with a significant reduction in minutes of one-hour MVPA, in steps of one-hour walking, in kcals of one-hour energy expenditure by 0.083 (95% confidence interval [CI] = -0.137, -0.029), 8.822 (95% confidence interval [CI] = -15.028, -2.617), 0.653 (95% confidence interval [CI] = -1.033, -0.273), respectively (P < .001).
The impact of AQI on individual-level one-hour PA at a specific time was different. AQI was found to be higher negatively associated with participants’ one-hour PA at 8 am, 4 pm, 5 pm and 7 pm. In specific, a one level increase in AQI was linked with a significantly reduction in steps of one-hour walking at 8 am, 4 pm and 7 pm by 35.345 (95% confidence interval [CI] = -59.468, -11.220), 74.738 (95% confidence interval [CI] = -108.273, -41.204), 23.516 (95% confidence interval [CI] = -45.675, -1.357), respectively. However, AQI was found to be positively associated with participants’ one-hour PA at 10 am and 3 pm. There was no significant relationship between AQI and one-hour SB among participants.
Impact of PM2.5 on PA and SB.
Table 5 indicates the estimated effects of air pollution concentration in PM2.5 on individual-level outcomes hourly PA and SB using linear individual fixed-effect regressions. PM2.5 was found to be significant negatively associated with one-hour PA among participants. A 10µg/m³ increase in PM2.5 was associated with a significant reduction in minutes of one-hour MVPA, in steps of one-hour walking, in kcals of one-hour energy expenditure by 0.021 (95% confidence interval [CI] = -0.033, -0.010), 2.232 (95% confidence interval [CI] = -3.548, -0.916) and 0.170 (95% confidence interval [CI] = -0.250, -0.089), respectively (P < .001).
The impact of PM2.5 on individual-level one-hour PA at specific time was also different. PM2.5 was found to be higher negatively associated with participants’ one-hour PA at 8 am, 4 pm, 5 pm and 7 pm. In specific, 10µg/m³ increase in PM2.5 was linked with a significantly reduction in steps of one-hour walking at 8 am, 4 pm, 5pm and 7 pm by 6.134 (95% confidence interval [CI] = -11.274, -0.995), 10.871 (95% confidence interval [CI] = -17.444, -4.298) and 9.654 (95% confidence interval [CI] = -17.632, -1.676) and 4.973 (95% confidence interval [CI] = -9.588, -0.002), respectively. However, PM2.5 was found to be positively associated with participants’ one-hour PA at 10 am, 2 pm and 3 pm. Air pollution concentration in PM2.5 was found to be significant positively associated with SB among participants. A 10µg/m³ increase in PM2.5 was associated with a significant increase in minutes of one-hour SB by 0.045 (95% confidence interval [CI] = -0.005, -0.0845).
The impact of PM2.5 on individual-level one-hour SB at specific time was also different. PM2.5 was found to be higher positively associated with participants’ one-hour SB at 9 am, 11 pm, 5 pm and 7 pm. In specific, a 10µg/m³ increase in PM2.5 was linked with a significantly increase in minutes of one-hour SB at 9 am, 11 pm, 5pm and 7 pm by 0.247 (95% confidence interval [CI] = 0.067, 0.427), 0.342 (95% confidence interval [CI] = 0.139, 0.544), 0.192 (95% confidence interval [CI] = 0.030, 0.355) and 0.201 (95% confidence interval [CI] = 0.048, 0.355), respectively. However, PM2.5 was found to be negatively associated with participants’ one-hour PA at 10 am and 3 pm.