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It is estimated that about 2.4 billion people around the world, or about 40% of the world's population, depend on biomass fuels (wood, charcoal, dung, crop residue) to meet their energy needs for cooking and heating. The burden is especially high in Asia. Studies suggest that levels of pollutants including particulate matter <10 μm and polycyclic aromatic hydrocarbons indoors in homes where biomass fuels are used far exceed levels recommended as safe. Whilein vitroandin vivostudies in animal models suggest that wood smoke emission extracts are mutagenic and carcinogenic, epidemiologic studies have been inconsistent. In this review, we discuss possible carcinogenic mechanisms of action of biomass fuel emissions, summarize the biological evidence for carcinogenesis, and review the epidemiologic evidence in humans of biomass fuel emissions as a risk factor for lung cancer. Finally, we highlight some issues relevant for interpreting the epidemiologic evidence for the relationship between biomass fuel exposure and lung cancer: these include methodologic considerations and recognition of possible effect modification by genetic susceptibility, smoking status, age of exposure and histologic type.