Considerable work has been done to elucidate the effects of polluted air, most of which has studied acute effects of particles. Studies suggest that the effects of longer term exposures are more than just the daily sum of the acute effects. Because most of the studies of acute effects have examined changes in health status occurring within days of the exposure, this article takes a broad definition of long-term exposure to include averaging times of months to years. It concludes that health effects increase as length of exposure increases, but much of that increase occurs within the first year.