Use of wood stoves and risk of cancers of the upper aero-digestive tract: a case-control study

Background Incidence rates for cancers of the upper aero-digestive tract in Southern Brazil are among the highest in the world. A case-control study was designed to identify the main risk factors for carcinomas of mouth, pharynx, and larynx in the region. We tested the hypothesis of whether use of wood stoves is associated with these cancers. Methods Information on known and potential risk factors was obtained from interviews with 784 cases and 1568 non-cancer controls. We estimated the effect of use of wood stove by conditional logistic regression, with adjustment for smoking, alcohol consumption and for other sociodemographic and dietary variables chosen as empirical confounders based on a change-in-estimate criterion. Results After extensive adjustment for all the empirical confounders the odds ratio (OR) for all upper aero-digestive tract cancers was 2.68 (95% confidence interval [CI] :2.2-3.3). Increased risks were also seen in site-specific analyses for mouth (OR = 2.73; 95% CI:1.8-4.2), pharyngeal (OR = 3.82; 95% CI:2.0-7.4), and laryngeal carcinomas (OR = 2.34; 95% CI:1.2-4.7). Significant risk elevations remained for each of the three anatomic sites and for all sites combined even after we purposefully biased the analyses towards the null hypothesis by adjusting the effect of wood stove use only for positive empirical confounders. Conclusion The association of use of wood stoves with cancers of the upper aero-digestive tract is genuine and unlikely to result from insufficient control of confounding. Due to its high prevalence, use of wood stoves may be linked to as many as 30% of all cancers occurring in the region.