Pinnacles and Pitfalls for Source Apportionment of Potential Health Effects From Airborne Particle Exposure

Abstract

Since its origins in the 1970s, source apportionment using receptor modeling has improved to a point where both the chemical mass balance and various methods of factor analysis have been applied to many urban and regional data sets to infer major sources or source classes influencing airborne particle concentrations. Recently the factors from the latter analyses have been combined with regression techniques using human health endpoints to infer source influence on health effects. This approach is attractive for air quality management when the composition of particles is known, since it provides, in principle, a means of quantifying major source influence on health consequences. The factor-based analyses have been used for both epidemiological and toxicological studies with some success. While the method is useful in many ways, it also has important limitations that include failing to identify specific sources, misidentification from comingled source factors, and inconsistency or unreasonableness of results from the same locations using different factor techniques. Examples of ambiguities evolving from these limitations are cited in this article. Ambiguity found in the literature is fostered by loosely worded terminology that does not distinguish statistically based factors from actual sources, and from health impacts inferred by single centrally located air monitors, which are assumed to represent actual exposure or dosage to airborne particles.

Notes

*Sources of Br have not been explored thoroughly. Historically, Br has been viewed as a component of vehicular emissions, since leaded gasoline included both Br and lead. However, lead has not been used in gasoline for several years at this point. Many of the studies discussed here used data from approximately 1996–1998. While Pb is a minor component of roadside dust from vehicular emissions today as a submicrometer component of brake wear (Lough et al., 2005), exploration of the literature suggests that in the United States today Br is not a component of brake, engine, or tire wear. (Br is often present in the inner tire, mainly in truck tires, but not in the part of the tire exposed to sunlight, because sunlight will degrade Br compounds.) Note that in Brown et al. (2007), the regional transport factor is much more enriched in Br than other factors, and that the source regions for this factor are border areas in California and Mexico west of Phoenix. Studies of Br sources for contemporary source apportionment studies appear to be needed.

^†The carbon component OP is determined in the thermal differentiation analysis for carbon as the charred, pyrolyzed fraction of carbon in transition between the OC components and the EC components.