Attributing health effects to individual particulate matter constituents

There is a general consensus in the scientific community that fine particulate matter (PM2.5) composition plays a significant role in the health effects attributed to PM; indeed, components may be more important than PM concentration alone in explaining health responses. As evidence linking composition to health impacts continues to accumulate in the epidemiological, toxicological, and controlled human exposure arenas, it is becoming more urgent from a policy perspective to determine which components or combination of components are most harmful to human health. While several reviews on this topic have focused on specific types of studies, such as source apportionment studies or those using concentrated ambient particles (CAPs), no review has been published that holistically examines the evidence from all component-based studies. In particular, while source apportionment is a useful tool for developing an understanding of potential contributing sources to PM2.5 in certain areas, it is also subject to significant limitations, and therefore results from those studies need to be interpreted with care. We reviewed published studies that (1) included at least two PM components; (2) did not only group them statistically into factors; and (3) quantitatively determined the relationship between the components and health effects. The majority of the studies examined yielded significant findings for specific components of PM, but not for PM concentration, demonstrating that PM alone does not drive health responses. Overall, the epidemiological studies did not fully exonerate any major component class of PM2.5 mass, but did demonstrate that more scrutiny needs to be given to carbon-containing PM components (elemental and organic carbon), as growing evidence suggests these are most strongly associated with adverse health outcomes. The results of controlled human exposure studies are consistent with this premise, although only two such studies were available for consideration. Toxicological studies suggest that several elements, including aluminum, silicon, vanadium, and nickel, are most closely associated with health impacts, although many other elements, as well as carbon-containing components, have been implicated as well. There are no PM components for which there is unequivocal evidence of zero health impact. However, care must be taken in interpreting these results as it is unclear whether a component per se is responsible for health impacts or is an index or surr