Airborne particulates come mostly from the exhaust of cars, trucks, and power plants.
So-called secondary particles are formed from pollutant gases, including sulfur dioxide and nitrogen oxides. They cause respiratory and cardiovascular diseases and have been linked to increased rates of mortality. Such a model would make it possible to prioritize abatement measures for different environmental problems, and to identify those measures that contribute to the solution of several problems. At many stages in the chain, the scientific picture is unclear. The range of uncertainties must be defined.