Tar level of cigarettes smoked and risk of smoking-related diseases

Abstract

Background: Opinions differ on the relationship between tar level and risk of smoking-related disease. However, except for lung cancer, few reviews have evaluated the epidemiological evidence. Here the relationship of tar level to risk of the four main smoking-related diseases is considered. Methods: Papers comparing risk of lung cancer, COPD, heart disease or stroke in smokers of lower and higher tar yield cigarettes were identified from reviews and searches, relative risk estimates being extracted comparing the lowest and highest tar groups. Meta-analyses investigated heterogeneity by various study characteristics. Results: Twenty-six studies were identified, nine of prospective design and 17 case–control. Two studies grouped cigarettes by nicotine rather than tar. Seventeen studies gave results for lung cancer, 16 for heart disease, five for stroke and four for COPD. Preferring relative risks adjusted for daily amount smoked, where adjusted and unadjusted estimates were available, combined estimates for lowest versus highest tar (or nicotine) groups were 0.78 (95% confidence interval 0.70–0.88) for lung cancer, 0.86 (0.81–0.91) for heart disease, 0.77 (0.62–0.95) for stroke and 0.81 (0.65–1.02) for COPD. Lower risks were generally evident in subgroups by publication period, gender, study design, location and extent of confounder adjustment. Estimates were similar preferring data unadjusted for amount smoked or excluding nicotine-based estimates. Conclusions: Despite evidence that smokers substantially compensate for reduced cigarette yields, the results clearly show lower risks in lower tar smokers. Limitations of the evidence are discussed, but seem unlikely to affect this conclusion.