Solar UVB radiation, total ozone and the aerosol optical thickness (AOT) of the atmosphere were measured at various sites in Brazil during season of 1995. Smoke from biomass burning caused very significant AOT and up to an 81% reduction in UVB at Cuiabá, hundreds of kilometers from the most widespread burning. Little or no smoke was apparent upwind of the major burning regions at and near Manaus, and UVB was close to the expected values. There is an increased incidence of respiratory, cardiopulmonary and other diseases associated with severe air pollution, but the responsible biological mechanisms are unknown. The bactericidal effects of solar UVB are well known, and significantly reduced UVB resulting from severe air pollution in regions where UVB levels are ordinarily high might enhance the survivability of pathogenic organisms in air and water and on surfaces exposed to sunlight.