Table 3.

Average intensity of coal use during follow-up and risk of ischaemic heart disease and stroke mortality among non-smoking women in the Xuanwei cohort, 1976–2011

	Ischaemic heart disease cases	HR (95% CI)a	P-value	Stroke cases	HR (95% CI)a	P-value
Lifetime smoky or smokeless coal users
<2.5 tons/year	28	1.0 (ref)		132	1.0 (ref)
2.5–<4 tons/year	47	1.7 (1.1–2.8)	0.03	156	1.1 (0.9–1.5)	0.29
≥4 tons/year	30	3.7 (2.1–6.5)	<0.0001	75	1.2 (0.9–1.7)	0.27
P-trend			<0.0001			0.22
P-heterogeneity (coal type)b			0.005			0.84
Lifetime and exclusive smoky coal users
<2.5 tons/year	14	1.0 (ref)		72	1.0 (ref)
2.5–<4 tons/year	35	1.5 (0.8–2.9)	0.23	109	0.9 (0.7–1.2)	0.44
≥4 tons/year	10	1.5 (0.6–3.5)	0.40	62	1.0 (0.7–1.4)	0.92
P-trend			0.29			0.85
Lifetime and exclusive smokeless coal users
<2.5 tons/year	8	1.0 (ref)		41	1.0 (ref)
2.5–<4 tons/year	9	2.1 (0.8–5.6)	0.12	35	1.6 (1.0–2.6)	0.04
≥4 tons/year	17	7.9 (3.5–17.8)	<0.0001	13	1.1 (0.6–2.2)	0.78
P-trend			<0.0001			0.33

^aAdjusted for education (categorical variable no formal school vs any formal school), birth cohort [categorical six-level variable: 1917–21 (ref.), 1922–26, 1927–31, 1932–36, 1937–41, 1942–46, 1947–51], use of a portable stove or stove with chimney (time-dependent variable categorized as ‘1’ for ages when either stove type was used), cumulative years of cooking (<20, 20–40, >40 years), average number of hours spent indoors doing activities other than sleeping (continuous time-dependent variable), average number of rooms in home during lifetime (continuous time-dependent), average number of people in home during lifetime (continuous time-dependent), follow-up period (retrospective or prospective follow-up) and age.

^b P-heterogeneity calculated including cross-product terms (average intensity of coal use x coal type) in models that only included lifetime and exclusive users of smoky and smokeless coal users.