ble subpopulations. Equipment and protocols for this purpose were available before the committee’s first report, and the new funding made available for this topic led to a clear advance in the available evidence. Although monitoring methods are being developed to assess exposures of susceptible subpopulations to hazardous PM components (topic 2), more substantial advances are needed in assessing the components themselves (topic 5) before fully implementing topic 2.

Topic 6, dosimetry of particles, is of narrow scope, and an understanding of particle dosimetry in the lung had already been well-established. Dosimetry models have been enhanced in the past few years, although not yet sufficiently developed for those with chronic heart and lung disease.

Research methods have been further elaborated, and insights have been gained into the statistical modeling of data on air pollution and health (topic 10). Substantial methodological research has yielded new analytical strategies and an enhanced understanding of several issues, including measurement error and possibly mortality displacement. Methods have been described for combining large amounts of data to detect the effects of air pollution with greater sensitivity. In addition, new methodological issues in time-series analyses have been identified and solutions proposed.

Regarding the combined effects of PM and gaseous copollutants (topic 7), epidemiological and toxicological research has provided little indication that PM effects vary with levels of other major pollutants in ambient air; however, much research on topic 7 is needed. New knowledge about PM health effects in susceptible subpopulations (topic 8) has been developed in the past 5 years. Despite such advances in knowledge, substantial uncertainties still need to be addressed concerning those subpopulations.

Finally, a critical information gap, which is related to the characteristics of particles determining risks to health and the sources of more hazardous particles, remains largely unaddressed. An understanding of health risks in relation to particle characteristics lies largely in the domains of topics 5 and 9, and information on their sources and concentrations is the focus of topics 3 and 4. Progress on topic 5 has been slow, despite its central place in moving forward on the committee’s agenda. In the final chapters of this report, we offer recommendations on how to move forward more quickly on this topic.