Association of Exposure to Biomass Fuels with Occurrence of Chronic Obstructive Pulmonary Disease in Rural Western China: A Real-World Nested Case-Control Study

Figures & data

Figure 1 The flow chart for sampling strategy. In stage one, we stratified administrative divisions in Mianyang into two levels based on gross domestic product (GDP) data from the Mianyang Bureau of Statistics. In stage two, we used GDP data to further stratify towns or subdistricts located in Fucheng District and Jiangyou City into two sublevels. In stage three, we selected all villages or residential communities. In stage four, proportional random sampling was used to select 15% of permanent residents at least 40 years old in each village or residential community for enrollment in the study.

Table 1 Demographic Characteristics and Lung Function of the People Over 40 Years Old in Rural Western China

				Total		No COPD		COPD		P
				n	%	n	%	n	%
Participants				11,398	100.00	10,898	100.00	500	100.00	--
Age (years)		40~49		2661	23.35	2631	24.14	30	6.00	<0.001^a
		50~59		4067	35.68	3974	36.47	93	18.60
		60~69		3261	28.61	3064	28.12	197	39.40
		≥70		1409	12.36	1229	11.28	180	36.00
		Mean		57.67±9.58		57.31±9.42		65.70±9.61		--
Sex		Male		4848	42.53	4506	41.35	342	68.40	<0.001
		Female		6550	57.47	6392	58.65	158	31.60
Ethnic groups		Han		11,263	98.82	10,767	98.80	496	99.20	0.684^b
		Ethnic minorities		38	0.33	36	0.33	2	0.40
Educational level		Primary school or less		7185	63.04	6810	62.49	375	75.00	<0.001^a
		Middle and high school		4072	35.73	3956	36.30	116	23.20
		College and higher		72	0.63	70	0.64	2	0.40
Urbanisation		Urban residents		1912	16.77	1843	16.91	68	13.60	0.060
		Rural residents		9487	83.23	9055	83.09	432	86.40
BMI (kg/m^2)		<18.5		255	2.24	223	2.05	32	6.40	<0.001^a
		18.5~23.9		5277	46.30	5007	45.94	270	54.00
		23.9~27.9		4420	38.78	4270	39.18	150	30.00
		≥28		1445	12.68	1398	12.83	47	9.40
Smoking history		Never-smoker		9110	79.93	8743	80.23	367	73.40	<0.001
		Former-smoker		239	2.10	222	2.04	17	3.40
		Current-smoker		2049	17.98	1933	17.74	116	23.20
Smoking exposure (pack-years)		0		9110	79.93	8743	80.23	367	73.40	<0.001^a
		1–9		32	0.28	32	0.29	0	0.00
		10–19		65	0.57	63	0.58	2	0.40
		≥20		2053	18.01	1929	17.70	124	24.80
History of tuberculosis				93	0.82	81	0.74	12	2.40	0.001
History of chronic bronchitis				363	3.18	270	2.48	93	18.60	<0.001
History of asthma				135	1.18	102	0.94	33	6.60	<0.001
History of emphysema				89	0.78	61	0.56	28	5.60	<0.001
Chronic cough during childhood				83	0.73	74	0.68	9	1.80	0.010
Cough		Frequent		112	0.98	80	0.73	32	6.40	<0.001^a
		Sometimes		6025	52.86	5782	53.06	243	48.60
		Rare		5261	46.16	5036	46.21	225	45.00
Sputum		Frequent		70	0.61	59	0.54	11	2.20	<0.001^a
		Sometimes		3082	27.04	2930	26.89	152	30.40
		Rare		8246	72.35	7909	72.57	337	67.40
Wheezing		Frequent		50	0.44	31	0.28	19	3.80	<0.001^a
		Sometimes		1254	11.00	1166	10.70	88	17.60
		Rare		10,094	88.56	9701	89.02	393	78.60
Chest tightness		Frequent		47	0.41	30	0.28	17	3.40	<0.001^a
		Sometimes		1216	10.67	1139	10.45	77	15.40
		Rare		10,135	88.92	9729	89.27	406	81.20
Family history of chronic bronchitis/COPD/emphysema				125	1.10	112	1.03	125	25.00	0.004
FEV1(L)				96.32±25.88		97.88±24.88		62.37±23.91		<0.001^c
FVC(L)				98.00±20.90		98.70±20.26		82.88±27.83		<0.001^c
Post-bronchodilator FEV:FVC ratio				81.12±11.51		82.13±10.59		60.57±8.07		<0.001^c
House ventilation		Poor		19	0.17	15	0.14	4	0.80	0.720^a
		Normal		3197	28.05	3064	28.12	133	26.60
		Good		8182	71.78	7819	71.75	363	72.60
Dust		Much		218	1.91	195	1.79	23	4.60	0.964^a
		Normal		5751	50.46	5513	50.59	238	47.60
		Little		5429	47.63	5190	47.62	239	47.80
Exposure to coal		No		5968	52.36	5707	52.37	261	52.20	0.964
		Yes		5430	47.64	5191	47.63	239	47.80
Exposure to biomass fuels		No		1327	11.64	1295	11.88	32	6.40	<0.001
		Yes		10,071	88.36	9603	88.12	468	93.60
Exposure time of biomass fuels		No		1327	11.64	1295	11.88	32	6.40	0.02^a
		<6 months		164	1.44	159	1.46	5	1.00
		1~2 years		218	1.92	213	1.96	5	1.00
		3~5 years		829	7.27	779	7.15	50	10.00
		5~10 years		740	6.49	690	6.33	50	10.00
		More than 10 years		8120	71.24	7762	71.22	358	71.60
Kitchen ventilation		Poor		106	0.93	94	0.86	12	2.40	0.583
		Normal		5181	45.46	4955	45.47	262	52.40
		Good		6111	53.61	5849	53.67	226	45.20
Occupational exposure		No		164	1.44	10,745	98.60	489	97.80	0.173
		Yes		11,234	98.56	153	1.40	11	2.20
Exposure to secondhand smoke		No		8078	70.87	7716	70.80	138	27.60	0.480
		Yes		3320	29.13	3182	29.20	362	72.40

Notes: ^aCochran–Armitage trend test; ^bFisher’s exact test. ^cStudent’s t test.

Abbreviations: COPD, chronic obstructive pulmonary disease; BMI, body mass index; FEV1, forced Expiratory Volume in the first second; FVC, Forced Vital Capacity.

Table 2 Demographics and Risk Factors for Chronic Obstructive Pulmonary Disease Aged 40 years Older, Stratified by Exposure History of Biomass Fuels

Participants		Entire Population			Exposure to Biomass Fuels			Non-Exposure to Biomass Fuels
		N	COPD	Incidence (%)	N	COPD	Incidence (%)	N	COPD	Incidence (%)
Participants		11,398	500	4.39	10,071	468	4.65	1327	32	2.41
Age (years)	40~49	2661	30	1.13	2322	29	1.25	339	1	0.29
	50~59	4067	93	2.29	3526	87	2.47	541	6	1.11
	60~69	3261	197	6.04	2905	182	6.27	356	15	4.21
	≥70	1409	180	12.78	1318	170	12.90	91	10	10.99
	P for trend^a			<0.001			<0.001			<0.001
Sex	Male	4848	342	7.05	4353	321	7.37	495	21	4.24
	Female	6550	158	2.41	5718	147	2.57	832	11	1.32
	P for difference			<0.001			<0.001			0.001
Ethnic groups	Han	11,263	496	4.4	9947	464	4.66	1316	32	2.43
	Ethnic minorities	38	2	5.26	35	2	5.71	3	0	0.00
	P for difference^b			0.684			0.678			1.000
Educational level	Primary school or less	7185	375	5.22	6500	350	5.38	685	25	3.65
	Middle school and above	4144	118	2.85	3506	111	3.17	638	7	1.10
	P for difference			<0.001			<0.001			0.007
Urbanisation	Urban residents	1912	68	3.56	1637	64	3.91	274	4	1.46
	Rural residents	9487	432	4.55	8434	404	4.79	1053	28	2.66
	P for difference			0.060			0.126			0.375
BMI (kg/m^2)	<18.5	255	32	12.55	232	32	13.79	23	0	0.00
	18.5~23.9	5277	270	5.12	4671	254	5.44	606	16	2.64
	23.9~27.9	4420	150	3.39	3886	137	3.53	534	13	2.43
	≥28	1445	47	3.25	1282	45	3.51	163	2	1.23
	P for trend^a			<0.001			<0.001			0.434
Smoking history	Never-smoker	9110	367	4.03	8014	338	4.22	1096	29	2.65
	Former-smoker	239	17	7.11	218	17	7.80	21	0	0.00
	Current-smoker	2049	116	5.66	1839	113	6.14	210	3	1.43
	P for trend^a			<0.001			<0.001			0.201
Smoking exposure (pack-years)	0	9110	367	4.03	8014	338	4.22	1096	29	2.65
	1~9	32	0	0.00	28	0	0.00	4	0	0.00
	10~19	65	2	3.08	57	2	3.51	8	0	0.00
	≥20	2053	124	6.04	1837	121	6.59	216	3	1.39
	P for trend^a			<0.001			<0.001			0.251
History of tuberculosis	Yes	93	12	12.90	84	12	14.29	9	0	0.00
	No	11,193	483	4.32	9888	451	4.56	1305	32	2.45
	P for difference			0.001			<0.001			1.000
History of chronic bronchitis	Yes	363	93	25.62	327	87	26.61	36	6	16.67
	No	10,877	399	3.67	9615	373	3.88	1262	26	2.06
	P for difference			<0.001			<0.001			<0.001
History of asthma	Yes	135	33	24.44	126	31	24.60	9	2	22.22
	No	10,600	435	4.10	9397	406	4.32	1203	29	2.41
	P for difference			<0.001			<0.001			0.020
History of emphysema	Yes	89	28	31.46	80	27	33.75	9	1	11.11
	No	10,654	442	4.15	9448	412	4.36	1206	30	2.49
	P for difference			<0.001			<0.001			0.208
Chronic cough during childhood	Yes	83	9	10.84	71	9	12.68	12	0	0.00
	No	11,315	491	4.34	10,000	459	4.59	1315	32	2.43
	P for difference			0.010			0.005			1.000
Family history of chronic bronchitis/COPD/emphysema	Yes	125	13	10.40	115	12	10.43	10	1	10.00
	No	7573	330	4.36	6533	302	4.62	1040	28	2.69
	P for difference			0.004			0.011	1040	28	0.245
House ventilation	Poor	19	4	21.05	16	4	25.00	3	0	0.00
	Normal	3197	133	4.16	2856	127	4.45	341	6	1.76
	Good	8182	363	4.44	7199	337	4.68	983	26	2.64
	P for trend^a			0.927			0.934			0.341
Dust	Much	218	23	10.55	177	16	9.04	41	7	17.07
	Normal	5751	238	4.14	5282	230	4.35	469	8	1.71
	Little	5429	239	4.40	4612	222	4.81	817	17	2.08
	P for trend^a			0.282			0.993			0.005
Exposure to coal	No	5968	261	4.37	5344	229	4.29	624	32	5.13
	Yes	5430	239	4.40	4727	239	5.06	703	0	0.00
	P for difference			0.964			0.071			<0.001
Kitchen ventilation	Poor	106	12	11.32	89	8	8.99	17	4	23.53
	Normal	5181	262	5.06	4741	220	4.64	440	6	1.36
	Good	6111	226	3.70	5241	240	4.58	870	22	2.53
	P for trend^a			0.236			0.497			0.363
Occupational exposure	No	11,234	489	4.35	9928	458	4.61	1306	31	2.37
	Yes	164	11	6.71	143	10	6.99	21	1	4.76
	P for difference			0.173			0.163			0.403
Exposure to secondhand smoke	No	8078	362	4.48	7135	332	4.65	943	30	3.18
	Yes	3320	138	4.16	2936	136	4.63	384	2	0.52
	P for difference			0.480			0.997			0.003

Notes: ^aCochran–Armitage trend test; ^bFisher’s exact test.

Abbreviations: COPD, chronic obstructive pulmonary disease; BMI, body mass index.

Table 3 The Occurrence of COPD in People Over 40 Years Old in Rural Western China, by GOLD Stage

				GOLD Stage I		GOLD Stage II		GOLD Stage III-IV		P
				n	%	n	%	n	%
Participants				126	25.20	204	40.80	170	34.00	--
Age (years)		40~49		8	6.35	15	7.35	7	4.12	<0.001^a
		50~59		31	24.60	44	21.57	18	10.59
		60~69		50	39.68	81	39.71	66	38.82
		≥70		37	29.37	64	31.37	79	46.47
		Mean		64.33±9.44		65.53±9.97		68.11±8.87		--
Sex		Male		87	69.05	136	66.67	119	70.00	<0.001
		Female		39	30.95	68	33.33	51	30.00
Ethnic groups		Han		125	99.21	203	99.51	168	98.82	0.826
		Ethnic minorities		0	0.00	1	0.49	1	0.59
Educational level		Primary school or less		89	70.63	152	74.51	134	78.82	<0.001^a
		Middle and high school		35	27.78	49	24.02	32	18.82
		College and higher		0	0.00	0	0.00	2	1.18
Urbanisation		Urban residents		14	11.11	28	13.73	26	15.29	0.198
		Rural residents		112	88.89	176	86.27	144	84.71
BMI (kg/m^2)		<18.5		2	1.59	8	3.92	22	12.94	<0.001^a
		18.5~23.9		72	57.14	103	50.49	95	55.88
		23.9~27.9		41	32.54	73	35.78	36	21.18
		≥28		10	7.94	20	9.80	17	10.00
Smoking history		Never-smoker		75	59.52	159	77.94	133	78.24	<0.001^a
		Former-smoker		4	3.17	4	1.96	9	5.29
		Current-smoker		47	37.30	41	20.10	28	16.47
Smoking exposure (pack-years)		0		75	59.52	159	77.94	133	78.24	0.030^a
		1–9		0	0.00	0	0.00	0	0.00
		10–19		0	0.00	1	0.49	1	0.59
		≥20		50	39.68	43	21.08	31	18.24
History of tuberculosis				1	0.79	5	2.45	6	3.53	<0.001^b
History of chronic bronchitis				14	11.11	25	12.25	54	31.76	<0.001
History of asthma				5	3.97	11	5.39	17	10.00	<0.001
History of emphysema				2	1.59	6	2.94	20	11.76	<0.001^b
Chronic cough during childhood				1	0.79	4	1.96	4	2.35	0.012^b
Cough		Frequent		5	3.97	12	5.88	15	8.82	<0.001^a
		Sometimes		57	45.24	97	47.55	89	52.35
		Rare		64	50.79	95	46.57	66	38.82
Sputum		Frequent		0	0.00	3	1.47	8	4.71	<0.001^a
		Sometimes		44	34.92	64	31.37	44	25.88
		Rare		82	65.08	137	67.16	118	69.41
Wheezing		Frequent		3	2.38	4	1.96	12	7.06	<0.001^a
		Sometimes		18	14.29	42	20.59	28	16.47
		Rare		105	83.33	158	77.45	130	76.47
Chest tightness		Frequent		0	0.00	6	2.94	11	6.47	<0.001^a
		Sometimes		17	13.49	37	18.14	23	13.53
		Rare		109	86.51	161	78.92	136	80.00
Family history of chronic bronchitis/COPD/emphysema				2	1.59	10	4.90	1	0.59	<0.001^b
FEV1(L)				93.31±8.93		65.65±8.00		35.52±9.37		<0.001^c
FVC(L)				115.01±15.78		87.22±12.79		53.84±16.27		<0.001^c
Post-bronchodilator FEV:FVC ratio				64.63±3.44		60.50±5.98		53.09±8.56		<0.001^c
House ventilation		Poor		2	1.59	2	0.98	0	0.00	0.003^a
		Normal		37	29.37	67	32.84	29	17.06
		Good		87	69.05	135	66.18	141	82.94
Dust		Much		5	3.97	12	5.88	6	3.53	0.807^a
		Normal		56	44.44	97	47.55	85	50.00
		Little		65	51.59	95	46.57	79	46.47
Exposure to coal		No		58	46.03	109	53.43	94	55.29	0.440
		Yes		68	53.97	95	46.57	76	44.71
Exposure to biomass fuels		No		6	4.76	16	7.84	10	5.88	0.002
		Yes		120	95.24	188	92.16	160	94.12
Kitchen ventilation		Poor		3	2.38	107	52.45	5	2.94	0.845^a
		Normal		55	43.65	92	45.10	79	46.47
		Good		69	54.76	5	2.45	86	50.59
Occupational exposure		No		124	98.41	201	98.53	164	96.47	0.148
		Yes		2	1.59	3	1.47	6	3.53
Exposure to secondhand smoke		No		73	57.94	154	75.49	135	79.41	<0.001
		Yes		53	42.06	50	24.51	35	20.59
mMRC		0		100	79.37	157	76.96	81	47.65	<0.001
		1		4	3.17	8	3.92	0	0.00
		2		13	10.32	14	6.86	0	0.00
		3		9	7.14	24	11.76	87	51.18
		4		0	0.00	1	0.49	2	1.18

Notes: ^aCochran–Armitage trend test; ^bFisher’s exact test.^c ANOVA.

Abbreviations: GOLD, Global Initiative for Chronic Obstructive Lung Disease guidelines; COPD, chronic obstructive pulmonary disease; BMI, body mass index; FEV1, forced Expiratory Volume in the first second; FVC, Forced Vital Capacity.

Figure 2 The incidence of COPD at different biomass fuel exposure times. Bars represent the incidence of COPD, and error bars 95% CI. The horizontal axis represents the exposure time of biomass fuels.

Table 4 Multivariate Analysis for COPD Associated with Biomass Fuels Occurrence in Rural Western Chinese Population Over 40 Years Old

		Age and Sex Adjusted			Multivariable Adjusted
		OR	95% CI	P	OR	95% CI	P
Age	40~59	1.00	--	--	1.00	--	--
	≥60	4.27	3.47~5.26	<0.001	2.79	2.12~3.67	<0.001
Sex	Female	1.00	--	--	1.00	--	--
	Male	2.67	2.19~3.24	<0.001	2.75	2.07~3.64	<0.001
Ethnic groups	Han	1.00	--	--	1.00	--	--
	Ethnic minorities	2.03	0.47~8.70	0.343	4.99	1.09~22.75	0.038
Educational level	Primary school or less	1.00	--	--	1.00	--	--
	Middle school and above	0.68	0.54~0.85	0.001	0.82	0.71~0.94	0.006
Urbanisation	Urban residents	1.00	--	--	1.00	--	--
	Rural residents	1.23	0.95~1.61	0.121	1.21	0.84~1.75	0.298
BMI (kg/m^2)	<18.5 (underweight)	3.39	2.25~5.12	<0.001	2.74	1.54~4.87	0.001
	18.5~23.9 (normal weight)	1.00	--	--	1.00	--	--
	≥23.9 (overweight and obese)	1.45	1.20~1.76	<0.001	1.41	1.10~1.79	0.006
Smoking history	No	1.00	--	--	1.00	--	--
	Yes	1.03	0.61~1.76	0.909	1.41	0.67~2.97	0.368
History of tuberculosis	No	1.00	--	--	1.00	--	--
	Yes	3.50	1.84~6.65	<0.001	1.70	0.59~5.08	0.343
History of asthma	No	1.00	--	--	1.00	--	--
	Yes	5.36	3.51~8.20	<0.001	1.63	0.81~3.29	0.171
Chronic cough during childhood	No	1.00	--	--	1.00	--	--
	Yes	2.40	1.16~4.93	0.018	0.66	0.23~1.88	0.437
Family history of chronic bronchitis/COPD/emphysema	No	1.00	--	--	1.00	--	--
	Yes	3.14	1.72~5.76	<0.001	3.14	1.54~6.41	0.002
House ventilation	Poor	1.00	--	--	1.00	--	--
	Good	0.13	0.04~0.42	0.001	0.60	0.41~0.88	0.009
Dust	Much	1.00	--	--	1.00	--	--
	Little	0.89	0.72~1.07	0.196	0.92	0.77~1.10	0.363
Exposure to coal	No	1.00	--	--	1.00	--	--
	Yes	1.11	0.93~1.34	0.253	1.08	0.85~1.37	0.532
Exposure to biomass fuels	No	1.00	--	--	1.00	--	--
	Yes	2.96	2.45~3.67	<0.001	2.58	2.23~3.05	<0.001
Kitchen ventilation	Poor	1.00	--	--	1.00	--	--
	Good	0.35	0.19~0.69	0.001	0.19	0.08~0.42	<0.001
Occupational exposure	No	1.00	--	--	1.00	--	--
	Yes	1.46	0.77~2.75	0.246	1.05	0.44~2.51	0.908
Exposure to secondhand smoke	No	1.00	--	--	1.00	--	--
	Yes	1.01	0.83~1.24	0.897	0.81	0.61~1.06	0.128

Abbreviations: OR, odds ratio; 95% CI, 95% confidence interval; BMI, body mass index; COPD, chronic obstructive pulmonary disease.

Table 5 Subgroup Analysis of COPD Occurrence in Rural Western Chinese Population Over 40 Years Old by Sex

		Male			Female
		OR	95% CI	P	OR	95% CI	P
Age	40~59	1.00	--	--	1.00	--	--
	≥60	1.91	0.60~2.26	<0.001	2.09	1.67~2.61	<0.001
Ethnic groups	Han	1.00	--	--	1.00	--	--
	Ethnic minorities	0.00	0.00~	0.999	13.35	2.66~67.07	0.002
Educational level	Primary school or less	1.00	--	--	1.00	--	--
	Middle school and above	0.87	0.63~1.19	0.368	0.51	0.28~0.96	0.036
Urbanisation	Urban residents	1.00	--	--	1.00	--	--
	Rural residents	1.57	0.96~2.59	0.074	0.76	0.46~1.27	0.292
BMI (kg/m^2)	<18.5 (underweight)	3.36	1.57~7.17	0.002	2.03	0.86~10.54	0.107
	18.5~23.9 (normal weight)	1.00	--	--	1.00	--	--
	≥23.9 (overweight and obese)	1.62	1.19~2.21	0.002	1.02	0.68~1.52	0.942
Smoking history	No	1.00	--	--	1.00	--	--
	Yes	1.02	0.92~1.13	0.677	0.90	0.45~1.81	0.773
History of tuberculosis	No	1.00	--	--	1.00	--	--
	Yes	3.03	0.93~9.82	0.065	0.00	0.00~	0.998
History of asthma	No	1.00	--	--	1.00	--	--
	Yes	3.46	1.65~7.25	0.001	5.12	2.02~12.96	0.001
Chronic cough during childhood	No	1.00	--	--	1.00	--	--
	Yes	0.43	0.09~2.14	0.301	3.00	0.85~10.55	0.245
Family history of chronic bronchitis/COPD/emphysema	No	1.00	--	--	1.00	--	--
	Yes	2.10	0.74~5.91	0.161	5.58	2.19~14.22	<0.001
House ventilation	Poor	1.00	--	--	1.00	--	--
	Good	0.62	0.39~1.01	0.056	0.69	0.36~1.30	0.245
Dust	Much	1.00	--	--	1.00	--	--
	Little	0.86	0.69~1.07	0.182	1.03	0.76~1.38	0.867
Exposure to coal	No	1.00	--	--	1.00	--	--
	Yes	0.99	0.74~1.33	0.932	1.06	0.71~1.57	0.168
Exposure to biomass fuels	No	1.00	--	--	1.00	--	--
	Yes	1.71	1.09~2.68	<0.001	2.88	2.01~3.48	<0.001
Kitchen ventilation	Poor	1.00	--	--	1.00	--	--
	Good	1.12	0.68~1.82	0.665	1.41	0.76~2.65	0.280
Occupational exposure	No	1.00	--	--	1.00	--	--
	Yes	0.83	0.33~0.87	0.001	0.71	0.18~0.89	0.009
Exposure to secondhand smoke	No	1.00	--	--	1.00	--	--
	Yes	0.75	0.53~1.05	0.094	1.16	0.75~1.81	0.501

Abbreviations: OR, odds ratio; 95% CI, 95% confidence interval; BMI, body mass index; COPD, chronic obstructive pulmonary disease.

Table 6 Subgroup Analysis of COPD Occurrence in Rural Western Chinese Population Over 40 Years Old by Smoking History

		Never-Smoker			Smoker
		OR	95% CI	P	OR	95% CI	P
Age	40~59	1.00	--	--	1.00	--	--
	≥60	3.32	2.41–4.59	<0.001	2.80	1.73–4.54	<0.001
Sex	Female	1.00	--	--	1.00	--	--
	Male	3.07	1.26–3.66	<0.001	0.31	0.04–2.37	0.260
Ethnic groups	Han	1.00	--	--	1.00	--	--
	Ethnic minorities	9.17	1.842–45.642	0.007	0.00	0.0–	0.999
Educational level	Primary school or less	1.00	--	--	1.00	--	--
	Middle school and above	0.56	0.39–0.791	0.001	0.77	0.47–1.24	0.278
Urbanisation	Urban residents	1.00	--	--	1.00	--	--
	Rural residents	1.33	0.88–2.02	0.182	0.83	0.44–1.57	0.568
BMI (kg/m^2)	<18.5 (underweight)	1.37	1.04–1.81	0.027	1.50	0.95–2.39	0.085
	18.5~23.9 (normal weight)	1.00	--	--	1.00	--	--
	≥23.9 (overweight and obese)	2.60	1.33–5.09	0.005	6.03	2.25–16.11	<0.001
History of tuberculosis	No	1.00	--	--	1.00	--	--
	Yes	2.61	0.85–7.99	0.094	0.00	0.0–	0.999
History of asthma	No	1.00	--	--	1.00	--	--
	Yes	5.68	3.01–10.73	<0.001	1.44	0.27–7.54	0.669
Chronic cough during childhood	No	1.00	--	--	1.00	--	--
	Yes	1.59	0.53–4.77	0.407	0.41	0.03–5.00	0.448
Family history of chronic bronchitis/COPD/emphysema	No	1.00	--	--	1.00	--	--
	Yes	5.26	2.49–11.13	<0.001	0.725	0.09–5.85	0.760
House ventilation	Poor	1.00	--	--	1.00	--	--
	Good	0.60	0.38–0.95	0.029	0.65	0.31–1.39	0.266
Dust	Much	1.00	--	--	1.00	--	--
	Little	0.98	0.65–1.47	0.924	0.50	0.25–0.99	0.045
Exposure to coal	No	1.00	--	--	1.00	--	--
	Yes	1.10	0.68–1.22	0.531	1.32	0.82–2.12	0.254
Exposure to biomass fuels	No	1.00	--	--	1.00	--	--
	Yes	2.18	1.53–3.11	<0.001	0.56	0.30–1.03	0.06
Kitchen ventilation	Poor	1.00	--	--	1.00	--	--
	Good	0.76	0.49–1.17	0.205	0.99	0.45–2.19	0.989
Occupational exposure	No	1.00	--	--	1.00	--	--
	Yes	1.13	0.43–2.88	0.826	0.83	0.10–6.89	0.865
Exposure to secondhand smoke	No	1.00	--	--	1.00	--	--
	Yes	1.03	0.52–2.05	0.745	0.76	0.48–1.22	0.262

Abbreviations: OR, odds ratio; 95% CI, 95% confidence interval; BMI, body mass index; COPD, chronic obstructive pulmonary disease.

Table 7 Weight of Risk Factors for COPD Occurrence in Rural Western Chinese Population Over 40 Years Old

	OR	95% CI	P	Incidence Contribution Ratio	Weight of Contribution to Incidence W (%)	AR (%)
Age	2.79	2.12~3.67	<0.001	0.94	17.04	64.16
Sex	2.75	2.07~3.64	<0.001	0.93	16.80	63.64
Educational level	1.22	0.71~0.94	0.006	0.41	7.45	18.03
Urbanisation	1.21	0.84~1.75	0.298	0.41	7.39	17.36
Overweight and obese	1.41	1.10~1.79	0.006	0.48	8.61	29.08
Underweight	2.74	1.54~4.87	0.001	0.93	16.74	63.50
Smoking history	1.41	1.27~2.97	0.038	0.48	8.61	29.08
Family history of chronic bronchitis/COPD/emphysema	3.14	1.54~6.41	0.002	1.06	19.18	68.15
House ventilation	1.67	0.41~0.88	0.009	0.56	10.20	40.12
Exposure to biomass fuels	2.58	2.23~3.05	<0.001	1.00	18.08	61.24
Kitchen ventilation	5.26	0.08~0.42	<0.001	1.78	32.13	80.99
Exposure to secondhand smoke	1.81	1.61~2.06	0.028	0.61	11.06	44.75

Abbreviations: OR, odds ratio; 95% CI, 95% confidence interval; AR%, attributable risk percent; COPD, chronic obstructive pulmonary disease.