Table 1.

General information of total study population (n = 19,064).

Characteristic	Mean ± SD or n (%)
Age (years, mean ± SD)	18.3 ± 0.7
Sex (n, %)
Male	9813 (51.5%)
Female	9251 (48.5%)
Second-hand smoking (n, %)
Never	15,100 (79.2%)
<1 day/week	2597 (13.6%)
≥1 day/week	1367 (7.2%)
Family annual income (CNY, n, %)
<10,000	2046 (10.7%)
10,000–29,999	4151 (21.8%)
30,000–49,999	3316 (17.4%)
50,000–99,999	4211 (22.1%)
100,000–199,999	3825 (20.1%)
≥200,000	1515 (7.9%)
BMI (n, %)
<19.00	3781 (19.8%)
19.01–24.00	11,800 (61.9%)
24.01–28.00	2514 (13.2%)
>28.01	969 (5.1%)
Bath frequency (n, %)
≤1 time/week	2525 (13.2%)
2–4 times/week	6900 (36.2%)
5–7 times/week	8670 (45.5%)
≥8 times/week	969 (5.1%)
Average humidity over past 6 years (%)	59.2 ± 13.2
Average temperature over past 6 years (°C)	28.7 ± 0.5