Xuan Wei County, China has an unusually high lung cancer mortality rate that cannot be attributed to tobacco use or occupational exposure. Most Xuan Wei residents, especially women, are exposed domestically to smoke from unvented, open pit coal/wood fires used for cooking and heating. Samples collected during combustion of smoky coal, smokeless coal, and wood were examined for physical and chemical characteristics and for mutagenicity in the Ames Salmonella mutagenesis assay. Samples collected in two communes in which lung cancer rates are high and smoky coal is the predominant fuel contained high concentrations of submicron particles of high organic content. Indoor levels of polycyclic aromatic hydrocarbons in these communes were comparable to occupational exposure levels. Samples from homes in a wood-burning commune that has a low lung cancer rate, on the other hand, mostly consisted of larger particles of lower organic content. Extracts of these samples, containing mostly polar compounds displayed little mutagenicity. (Copyright (c) 1987 by the American Association for the Advancement of Science.)