Table 2.

Subgroup analysis in the random-effects model.

Variable	Studies	Estimated Effect Size (HR or OR)	95% CI	p-value	I2 Value, %
Sex
Female only	29 studies [15,16,17,20,27,28,30,31,32,33,34,35,36,37,40,41,42,43,45,47,49,50,52,53,54,55,57,58,59]	1.25	1.14–1.37	<0.001	56.14
Male only	4 studies [17,44,54,57]	1.59	0.91–2.79	0.100	44.27
Cancer type
Breast cancer	15 studies [16,27,28,30,35,36,37,40,41,42,45,46,50,52,53]	1.24	1.10–1.39	<0.001	73.90
Lung cancer	12 studies [15,29,39,43,47,49,51,55,56,58,59,60]	1.25	1.03–1.51	0.026	48.20
Bladder cancer	2 studies [18,57]	0.89	0.63–1.25	0.492	0.00
Pancreatic cancer	2 studies [19,34]	0.83	0.59–1.17	0.291	0.00
Cervical cancer	1 study [20]	2.18	1.15–4.16	0.018	70.54
Endometrial cancer	1 study [33]	0.83	0.62–1.11	0.205	0.00
Gastric cancer	1 study [44]	0.93	0.58–1.49	0.756	0.00
Hepatic cancer	1 study [38]	0.49	0.31–0.79	0.003	0.00
Rectal cancer	1 study [17]	1.01	0.70–1.46	0.949	0.00
Renal cancer	1 study [54]	0.90	0.60–1.34	0.593	0.00
Head and neck cancer	1 study [48]	2.17	0.56–8.37	0.262	0.00
Lymphoma	1 study [31]	1.34	1.06–1.71	0.017	0.00
All cancer	1 study [32]	0.90	0.86–0.94	<0.001	0.00
Methodological quality
High quality	26 studies [15,16,27,29,30,31,32,33,34,35,36,38,40,41,43,44,45,46,49,52,53,54,56,57,58,59]	1.15	1.02–1.30	0.023	84.00
Low quality	14 studies [17,18,19,20,28,37,39,42,47,48,50,51,55,60]	1.19	1.02–1.39	0.031	59.81
Country
Western	32 studies [17,18,19,20,27,29,30,31,32,33,34,35,36,37,38,39,40,42,43,44,45,46,47,48,50,52,53,54,55,56,57,60]	1.13	1.01–1.25	0.026	79.20
Asian	8 studies [15,16,28,41,49,51,58,59]	1.33	1.10–1.61	0.003	57.24
Period of exposure to secondhand smoking
Childhood	15 studies [28,30,31,32,36,38,43,45,46,47,51,53,55,56,59]	1.00	0.81–1.23	0.998	83.93
Adulthood	28 studies [15,17,18,19,20,28,29,30,31,34,36,37,38,41,45,46,47,50,51,52,53,54,55,56,57,58,59,60]	1.03	0.77–1.39	0.840	95.90
Lifetime	22 studies [16,27,29,30,31,33,35,36,37,38,39,40,42,44,45,46,47,48,49,53,56,59]	1.14	1.02–1.27	0.023	70.89