Although most ultraviolet B (UVB) radiation is absorbed by stratospheric ozone, dense anthropogenic sulfate aerosols in the troposphere may further attenuate UVB in some regions. Mortality rates from colon and breast cancer tend to be much higher in areas with low levels of UVB radiation. These high rates may be due in part to inadequate cutaneous photosynthesis of vitamin D. Satellite data on atmospheric aerosols, stratospheric ozone, and cloud cover were obtained from the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA). These data were combined with age-adjusted mortality rates from 175 countries reporting to the World Health Organization. Regression was used to assess the relationship of stratospheric ozone thickness, aerosol optical depth, cloud cover, solar UVB irradiance at the top of the atmosphere, average skin exposure, and a dietary factor with colon and breast cancer mortality rates. Solar UVB irradiance at the top of the atmosphere, total cloud cover, and atmospheric aerosols had the strongest associations with mortality rates, apart from a strong influence of diet. Since 95% of circulating vitamin D is derived from current or stored products of photosynthesis, which may be nonexistent or minimal much of the year above 37°N or below 37°S, attenuation of UVB by atmospheric aerosols and clouds may have a greater than expected adverse effect on human health.