The rural Xuan Wei County, Yunnan Province, China, has an unusually high lung cancer mortality rate that cannot be attributed to tobacco smoke or occupational exposure. The lung cancer rate is associated with 'smoky' coal, in contrast to wood or 'smokeless' coal burned in unventilated homes. The study was conducted to characterize and compare mouse skin tumorigenicity of the coal and the wood combustion emissions and to link the resulting animal data to human lung cancer. Indoor air particles were collected from a central commune where the lung cancer mortality rate is high and smoky coal is the major fuel used, and also from a south western commune where lung cancer mortality rate is low and wood and smokeless coal are the major fuels used. The organic extracts of these indoor air particles were analyzed for polycyclic aromatic hydrocarbons (PAHs) and assayed for skin tumor initiation activity and complete carcinogenicity in SENCAR mice. Mouse skin was initiated with 1,2,5,10, and 20 mg of organic extracts of the emission particles during the first week, and one week after initiation the mice were promoted with 12-0-tetradecanoylphorbol-13-acetate (TPA, 2 microgram/mouse) applied topically twice a week for 26 weeks. The results showed that the smoky coal sample is the most active among the three combustion emission samples.