Table 1. Selected characteristics of the subjects by stove use history in Xuanwei, 1976–1996: n (%) or means (95% CI).

	Change to stove with chimney
	Yes (n=12 529)	No (n=9127)	Total (n=21 701)
Person-years	242 154	159 831	401 985
Age on 1 January, 1976, years	38.8 (38.6–39.0)	40.4 (40.2–40.6)	39.5 (39.3–39.6)
Age at exit from follow-up, years	58.1 (57.9–58.3)	57.8 (57.6–58.0)	58.0 (57.9–58.1)
Lung cancer deaths, 1976–1996	706 (5.6)	527 (5.7)	1233 (5.7)
Age at lung cancer death	56.7 (56.1–57.3)	53.9 (53.2–54.7)	55.5 (55.0–56.0)
People who ever used smoky coal	11 319 (90.3)	3795 (41.4)	15 114 (69.6)
Tons per year	3.44 (3.42–3.46)	2.88 (2.84–2.91)	3.30 (3.28–3.31)
Age at stove improvement	41.8 (41.6–42.0)
Years from stove improvement to exit from follow-up	16.3 (16.2–16.4)
Ever smoked	11 658 (93.0)	8352 (91.1)	20 010 (92.2)
Age started smoking	21.5 (21.4–21.5)	20.7 (20.6–20.8)	21.1 (21.1–21.2)
Years of smoking among smokers	33.3 (33.2–33.5)	34.3 (34.1–34.5)	33.8 (33.6–33.9)
Ever quit smoking	173 (1.4)	89 (1.0)	262 (1.2)

Abbreviation: CI=confidence interval.