Table 1. Basic demographic characteristics of study subjects.

Variable	Total (n = 3969)	Control (n = 3479)	COPD (n = 490)	χ2	P
Gender
Men	2483(62.6)	2135(61.4)	348(71.0)	17.08	<0.001
Women	1486(37.4)	1344(38.6)	142(29.0)
Resident area
Suburb	587(14.8)	486(14.0)	101(20.6)	126.12	<0.001
City	2764(69.6)	2524(72.6)	240(49.0)
Suburban junction	618(15.6)	469(13.5)	149(30.4)
Education
Illiterate	473(11.9)	345(9.9)	128(26.1)	231.77	<0.001
Primary school	351(8.8)	288(8.3)	63(12.9)
Junior high school	552(13.9)	458(13.2)	94(19.2)
Senior high school	837(21.1)	706(20.3)	131(26.73)
College or over	1756(44.2)	1682(48.4)	74(15.1)
Income per month*, RMB (USD)
<2001(309)	672(16.9)	523(15.0)	149(30.4)	86.59	<0.001
2001(309) -	289(7.3)	246(7.1)	43(8.8)
3001(464) -	298(7.5)	253(7.3)	45(9.2)
4001(618) -	457(11.5)	412(11.8)	45(9.2)
5001(772) -	2253(56.8)	2045(58.8)	208(42.5)
COPD stage
I, mild	—	—	80(16.3)
II, moderate	—		262(53.5)
III, severe	—	—	115(23.5)
IV, extremely severe	—	—	33(6.7)