air concentrations of PM and gases to actual human exposures that include indoor environments. Most people spend the majority of their time indoors exposed to a mixture of particles that penetrate from outdoors or are generated indoors. Personal exposures to certain air pollutants have consistently been found to differ from estimates based on corresponding outdoor concentrations. The differences are largely due to the variable contributions of outdoor air to indoor environments, the indoor fate of outdoor contaminants, and the substantial contribution of indoor sources and sinks to total personal PM exposures (Lioy et al. 1990). Studies have found that a significant fraction (50–90%) of small indoor particles have outdoor origins (Koutrakis et al. 1992; Clayton et al. 1993; Thomas et al. 1993). Once indoors, particles may deposit on surfaces, or they can be altered through volatilization, as with ammonium nitrate, or through reactions with other pollutants present indoors, as with neutralization of sulfuric acid by ammonia. Particles are further generated by myriad indoor particle sources, including cooking, resuspension, cleaning, tobacco smoking, pets, insects, and molds. The emission rates of most indoor-particle sources, however, have not been adequately quantified. Furthermore, factors that affect the contribution of outdoor particles to indoor concentrations have not been well characterized.
In its first report, the committee concluded that information is needed on relationships between particulate matter in outdoor air and personal exposure to particulate matter, especially for subpopulations that may be susceptible to the effects of PM exposures, such as the elderly, individuals with respiratory or cardiovascular disease, and children. Hypothesis-driven exposure studies must be designed to provide fundamental information on actual human breathing-zone exposures to PM and gaseous pollutants (NRC 1991). The recommended studies should be used to determine the exposure metrics that are most suitable for establishing exposure-response relationships. To attain these goals, the following specific research activities were recommended in the committee's first report:
Field studies that quantify the contributions to personal exposures to PM and gaseous pollutants attributed to outdoor ambient air and to the penetration of ambient air indoors.