Background: Increasing proportions of smokers in Japan smoke less than 10 cigarettes per day (CPD). Yet, the health risks of low-intensity smoking in Asia are poorly understood.
Methods: We performed a pooled analysis of 410,294 adults from nine population-based prospective cohort studies participating in the Japan Cohort Consortium. Cigarette use data were collected at each study baseline in 1983-1994. Study specific hazard ratios (HRs) and 95% confidence intervals (CIs) for all-cause and cause-specific mortality were calculated using multivariable-adjusted Cox regression by CPD among current smokers and by age at cessation among former smokers with never smokers as the referent group. Pooled HRs and CIs were computed using a random effect model.
Results: The smoking prevalence was 54.5% in men and 7.4% in women. About 15.5% of male and 50.4% of female current smokers smoked 1-10 CPD (low-intensity). Both male and female low-intensity smokers had higher all-cause mortality risks than never smokers. Risks were further higher with increasing CPD in a dose-response manner. HRs (95% CIs) were 1.27 (0.97-1.66), 1.45 (1.33-1.59), and 1.49 (1.38-1.62) for 1-2, 3-5, and 6-10 CPD, respectively, in men; 1.28 (1.01-1.62), 1.49 (1.34-1.66), and 1.68 (1.55-1.81) for 1-2, 3-5, and 6-10 CPD, respectively, in women. Similar associations were observed for smoking-related causes of death. Among former low-intensity smokers, younger age at cessation was associated with lower mortality risk.
Conclusions: Smoking very low amounts was associated with increased mortality risks in Japan. All smokers should quit, even if they smoke very few cigarettes per day.