Ambient particles contain a large spectrum of individual compounds. Research findings from the Supersites Program and other atmospheric characterization studies have elegantly demonstrated the complexity of ambient particle characteristics. Research to assess hazardous PM components (topic 5) seeks to understand the comparative toxicity of particles in relation to their specific characteristics (for example, size or composition). Such information is helpful for the development of effective controls on emission sources to protect public health.

The current National Ambient Air Quality Standards (NAAQS) for PM are based on size and mass and assume that all particles have the same toxicity per unit mass irrespective of chemical composition. In the committee’s judgment, that assumption greatly oversimplifies complex biological phenomena that are influenced by PM and other pollutants. There are numerous physical and chemical characteristics of particles that are potentially relevant to their toxicity; however, to date, there is little information on the relationship between health outcomes and specific particle properties or source types.

Research to date has provided some new insights concerning particle characteristics and toxicity. For example, as discussed in Chapter 3, there are studies suggesting that health impacts of sulfate per se may not be proportional to their contribution to ambient PM mass. From the regulatory point of view, that is an important finding, because ammonium sulfate represents a significant fraction of PM, especially in the eastern United States, where it is the dominant component of secondary PM and is largely attributed to a small range of source types (for example, coal combustion). The toxicity of a range of particle components and sizes will need to be explored across the relevant health outcomes. Investigating the rest of PM components will be a challenging task, considering the complexity of PM as supported by numerous PM characterization studies (see Chapters 1 and 3); however, it is imperative that more progress be made in this area.

Without sufficiently compelling findings on the assessment of hazardous PM components, the committee’s research agenda could stall, and the possibility of standards and control strategies that go beyond the current mass-based approach would be delayed. Further research on emission characterization, development and testing of air quality models, and exposure of individuals at greatest risk needs to be linked to and redirected by advances in research on the assessment of hazardous PM components. A