The World Health Organization (WHO) estimates that about 800,000 premature deaths occur each year due to urban outdoor particle air pollution around the world. A further 1.6 million premature deaths are attributed to indoor pollution from solid fuel use for cooking and heating in developing countries. Studies in many cities have shown that more deaths occur on days of high air pollution, and the responsible pollutant is most often identified as particles. Fine particles are created by combustion processes, and can evade the body’s defenses and penetrate the lungs and bloodstream. The precise mechanisms by which particles cause morbidity and mortality are uncertain, and the critical toxic components of the particles are also unclear. Exposure studies are helping to reduce this uncertainty by determining the relationship between personal exposure and the concentrations measured at central site monitors, particularly for those people at highest risk. Exposure studies have also identified the most important sources of particles: smoking, heating, cooking, and traffic. This chapter reports on the basic studies that have provided most of our knowledge about the sources of indoor particles and the extent of human exposure. It provides the basic mathematical relationship between outdoor and indoor concentrations of particles, and presents recent data showing the range of “protectiveness” of homes with respect to outdoor pollution. “Tight” homes that reduce the penetration of outdoor particles unfortunately increase the concentrations of

indoor-generated particles, so other means of reducing exposure to particles are considered. It is shown that certain high-performance air cleaners can cut particle levels in homes by about a factor of two. This would be a useful protective step to take for persons at risk, such as those with chronic respiratory or cardiovascular problems.