Table 1. Baseline characteristics of the study population.

	Overall subjects, n = 21305 Number (%)	Subjects with questionnaire survey, n = 5394 Number (%)
Women	7879 (37.0)	1735 (32.2)
Age (year)
18–34	1585 (7.4)	462 (8.6)
35–44	7526 (35.3)	1751 (32.5)
45–54	8268 (38.8)	2025 (37.5)
55–64	2849 (13.4)	733 (13.6)
≥65	1077 (5.1)	423 (7.8)
Region
South China	5547 (26.0)	1231 (22.8)
North China	15758 (74.0)	4163 (77.2)
Chronic symptoms
Gastrointestinal	–-	1274 (23.6)
Rhinitis	–-	636 (11.8)
Migraine	–-	511 (9.5)
Arthritis	–-	392 (7.3)
Eczema	–-	310 (5.7)
Urticaria	–-	259 (4.8)
Asthma	–-	153 (2.8)
Other	–-	721 (13.4)