Table 3– Measures of additive interaction between mean county-level residential radon concentration, cigarette smoking and other inhalable agents for chronic obstructive pulmonary disease mortality, follow-up 1982–2006, American Cancer Society Cancer Prevention Study-II^#.

	RERI	Attributable proportion	Synergy index
Cigarette smoking	0.61 (-0.38–1.73)	0.08 (-0.06–0.20)	1.11 (0.94–1.31)
Industrial exposures	-0.06 (-0.34–0.25)	-0.05 (-0.38–0.14)	0.76 (0.19–3.08)
Passive smoke	-0.08 (-0.17–0.03)	-0.06 (-0.17–0.01)	0.70 (0.39–1.26)
Ambient ozone	-0.04 (-0.18–0.11)	-0.04 (-0.22–0.07)	0.29

Data for radon concentration was obtained from the Lawrence Berkeley National Laboratory (Berkeley, CA, USA). RERI: relative excess risk due to interaction. ^#: exposures categorised as: mean county-level residential radon concentrations <148 Bq·m⁻³ and ≥148 Bq·m⁻³; cigarette smoking never or ever; industrial exposures no or yes; passive smoking in home none or any; ambient ozone concentrations <57.1 ppb and ≥57.1 ppb. Cox regression models were fitted with the baseline hazard stratified by age, race, sex and state, and adjusted for education, marital status, body mass index, body mass index squared, cigarette smoking status, cigarettes per day, cigarettes per day squared, duration of smoking, duration of smoking squared, age started smoking, passive smoking, vegetable/fruit/fibre consumption, fat consumption, industrial exposures, and occupation dirtiness index where appropriate.