This is the third in a series dealing with chronic diseases and early exposure to airborne mixtures from industrial releases. The purpose of this study is to increase the understanding of previously unconsidered factors in the physical environment potentially acting as risk factors for female breast cancer. Data are from the Environmental Quality Database containing lifetime residential records for about 20,000 cases, with 1 of 15 cancers and about 5000 controls. Subjects resided within 25 km of all kraft mills, sulfite mills, coke ovens, oil refineries, copper, nickel and lead/zinc smelters operating in Canada in 1967-1970, and were aged <31 years. Subjects are exposed at home to simultaneous counter-current plumes of dioxin congeners and dimethyl sulfate (DMS) during the exposure period. DMS concentration increases with time of flight from the source and [SO(2)] at 2 km. For all source types the number of cancers in an age cohort declines as the age of the cohort increases. The number of cases less than the median distance is less than the number of cases greater than the median distance. This supports the presence of a new source of risk with an origin in the plume. The crude rate of breast cancer, averaged over the 25 km of the study area for each age cohort <31 years of age, as well as source type, is least when the conditions of initial exposure are [SO(2)] > or = [DMS] and increases as [DMS] increases. The probability of an adverse effect from early, intermittent and simultaneous exposure to Dioxin and DMS, manifesting as a breast cancer after a latency period of as little as 26 years, is a function of age of first exposure, distance from the source and source type. The most susceptible age cohorts are the youngest.