Fig. 1

Carboxyhemoglobin values in workers and controls, stratified by smoking status and home heating system. Group 1: Smokers who use natural gas for heating (study; n:35, control; n:36, min-max value worker and control group; 0–18 and 0–4, 95% CI of study group 5.68% to 7.55%). Group 2: Smokers who use charcoal for heating (study; n:19, control; n:19) (study; n:19, control; n:19, min-max value worker and control group; 1–15 and 0–4, 95% CI of study group 5.84% to 8.38%). Group 3: Non-smokers who use natural gas for heating (study; n:32, control; n:32, min-max worker value and control group; 0–13 and 0–4, 95% CI of study group 4.76% to 6.72%). Group 4: Non-smokers who use charcoal for heating (study; n:13, control; n:13, min-max value worker and control group; 0–14 and 0–4, 95% CI of study group 4.34% to 7.42%)