Analysis of the associations of indoor air pollution and tobacco use with morbidity of lung cancer in Xuanwei, China

Indoor air pollution emitted by smoky coal combustion in unventilated fire pits used to cause high lung cancer mortality in Xuanwei. Stove improvements were implemented from the 1970s to the 1980s. However, the present lung cancer mortality rate in Xuanwei shows almost no significant declining trend. Tobacco use is another established risk factor for lung cancer. Smoking prevalence and secondhand smoke (SHS) exposure rate are both high in Xuanwei. Therefore, in this study we evaluated the relationship among indoor air pollution over 30 years ago, tobacco use, and lung cancer risk, to further explore the competitive effects of these two risk factors. A case-control study design was used. We constructed an unconditional logistic regression model to evaluate the relationship among indoor air pollution, tobacco use, and lung cancer risk, adjusting the covariates and with an interactive term between the two key variables. We further quantitatively assessed the maximum decrease in the indoor air pollution effect when facing competition from tobacco use via a sensitivity analysis. First, the effect of indoor air pollution on lung cancer without considering tobacco use was estimated. Then, we calculated a “bias factor” and divided the effect estimation by this factor. We found a strong delayed effect of indoor air pollution over 30 years ago in each subgroup with different tobacco use history. The effects of tobacco use were relatively small, but in areas without smoky coal combustion and indoor air pollution over 30 years prior, the lung cancer risk caused by tobacco use became much stronger and statistically significant. Moreover, we assessed that the effects of tobacco use on lung cancer could reduce a maximum of 18%-30% of the effects of indoor air pollution, but did not influence their statistical significance; the competitive effect from ever smoking was stronger than that from SHS exposure. At present in Xuanwei, delayed effect of the indoor air pollution over 30 years ago remains the major risk factor for lung cancer. Concomitantly, the adverse effect of tobacco use on lung cancer is becoming more apparent; local governments should start considering public health activities for smoking cessation promotion and SHS exposure prevention.