transformations of particulate matter in the atmosphere produce airborne particles of different sizes and chemical composition. For example, the particles can contain heavy metals, acids, biological or biogenic material, or other organic and inorganic compounds.

Although particulate matter is regulated as a single pollutant by EPA, it consists of a mixture of materials that are far more complex than regulated gaseous pollutants, such as ozone or carbon monoxide. Particles that can be inhaled into the respiratory tract span a range of aerodynamic diameters from molecular clusters as small as 0.001 µm up to larger particles of 10 µm or more in diameter. The numbers of particles and their chemical composition can vary within specific particle size fractions from location to location and over time, depending on the types of source emissions and atmospheric conditions.

Concern about airborne particulate matter in recent years has been driven largely by epidemiological studies that have reported relatively consistent associations between outdoor particulate-matter levels and adverse health effects. However, assessing the specific health risks resulting from exposures to airborne particulate matter, and distinguishing these effects from those produced by gaseous copollutants, involves substantial scientific uncertainty about the influence of copollutants and weather, about whether some particulate-matter fractions (size or chemical) might be more highly associated with health risks, and about the nature of dose-response relationships between particulate matter and health. Many previous analyses have not considered the simultaneous presence of all of the gaseous criteria air pollutants (sulfur dioxide, nitrogen dioxide, carbon monoxide, and ozone) and potentially important weather factors in estimating the association between particulate matter and health, and the treatment of such factors has not been uniform across previous studies. It will be important to understand how these factors influence estimates of particulate-matter risks to health and to learn whether the relationships are consistent across study areas. There is limited information about the physical, chemical, or biological properties of particles that might cause the observed adverse health effects, and information is also limited on the mechanisms of toxicity and the locations, activities, and intensity of actual human exposures to such particles. To date, researchers have