Back in the latter part of the 90s, the US EPA issued its first major cost/benefit analysis. While there were several flaws in this analysis, one of the major findings was that 80% of the benefits came from reductions in particulates emissions. This analysis looked at the 6 criteria pollutants…Particulates, SO2, NOx, CO, lead, and ozone. In the US, ambient air quality had improved dramatically and most of the country was in attainment relative to ambient air standards for particulates, SO2, NOx, CO, and lead. Ozone has some special issues and is influenced by automobile emissions and proximity to large bodies of water. As a result, EPA declared SO2 and NOx as precursors to particulate formation. These compounds end up as sulfate and nitrate particulates that are washed out of the atmosphere in rain (ie acid rain). As a result, emissions of all criteria pollutants are down by nearly 80% since 1970, while the economy is up by a factor of 3. Even so, the benefits are somewhat difficult to conclusively pin down. You would be hard pressed to find a death certificate that says somebody died from particulates. There have been very, very few studies that have been able to do a true, side by side comparison simply because we cannot do a controlled study on what would have happened had we not done such a good job in the US of reducing emissions. The best study that I have ever seen was done by Prof, Michael Greenstone when he was at MIT. He used infant mortality data (which is very detailed) from the period before and after the 1978 modifications to the Clean Air Act. These regulations stipulated the first attainment/non attainment areas for particulates. By selecting regions of the country with approximately the same economic conditions but were in different attainment/non attainment areas, he was able to statistically tease out the impact of particulate emissions on infant mortality, while correcting for such potential influences as smoking/non smoking, alcoholism, income, etc. The results showed a modest impact of 1000 infant deaths out of 2 million births. Since infants have the same mortality rate in the first year of life as the population as a whole, this gives some indication of the level of impact of particulate emissions. The death rate in the US is on the order of 2 million people each year, so the number of deaths attributable to air pollution in the US is somewhat exaggerated. Of course, in places like China and Southeast Asia, the ambient concentrations are much higher and will lead to higher mortality rates.