With the epidemiological studies suggesting increased mortality associated with particles (see Chapter 2), there has been increasing interest in indoor particle concentrations compared to outdoor levels. A number of studies have examined this over the years and are summarized in a review by Wallace (1996).

In general, if there are no indoor sources of particles, the levels indoors tend to reflect those outdoors. For example, application of a mass balance model to measurements of indoor and outdoor particle concentrations in Riverside, California, indicated that 75% of PM2 5 and 65% of PMI() in a typical home were from outdoors (Wallace, 1996). Similar conclusions were reached by Koutrakis et al. (1991, 1992) for homes in two counties in New York. For example, they report that 60% of the mass of particles in homes is due to outdoor sources. However, the contribution to various individual elements in the particles varies from 22% for copper to f00% for cadmium.

There are some differences in indoor levels of particulate matter in areas with low outdoor compared to high outdoor levels. In the case of high outdoor levels, the indoor concentrations tend to be somewhat lower than those outdoors; for example, Colome et al. (f992)

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