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George D. Thurston,¹ Kazuhiko Ito,¹ Therese Mar,² William F. Christensen,³ Delbert J. Eatough,⁴ Ronald C. Henry,⁵ Eugene Kim,⁶ Francine Laden,⁷ Ramona Lall,¹ Timothy V. Larson,⁸ Hao Liu,⁹ Lucas Neas,¹⁰ Joseph Pinto,¹¹ Matthias Stölzel,¹² Helen Suh,⁷ and Philip K. Hopke⁶

¹Nelson Institute of Environmental Medicine, New York University School of Medicine, Tuxedo Park, New York, USA; ²Department of Environmental Health, University of Washington, Seattle, Washington, USA; ³Department of Statistics, and ⁴Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah, USA; ⁵Department of Civil and Environmental Engineering, Southern California University, Los Angeles, California, USA; ⁶Center for Air Resources Engineering and Science, Clarkson University, Potsdam, New York, USA; ⁷Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts USA; ⁸Department of Civil and Environmental Engineering, and ⁹Department of Biostatistics, University of Washington, Seattle, Washington, USA; ¹⁰National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, Chapel Hill, North Carolina, USA; ¹¹National Center for Environmental Assessment, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, USA; ¹²Institute of Epidemiology, Focus Network Aerosols and Health, National Research Center for Environment and Health (GSF), Neuherberg, Germany

Abstract
Although the association between exposure to ambient fine particulate matter with aerodynamic diameter < 2.5 µm (PM_2.5) and human mortality is well established, the most responsible particle types/sources are not yet certain. In May 2003, the U.S. Environmental Protection Agency's Particulate Matter Centers Program sponsored the Workshop on the Source Apportionment of PM Health Effects. The goal was to evaluate the consistency of the various source apportionment methods in assessing source contributions to daily PM_2.5 mass-mortality associations. Seven research institutions, using varying methods, participated in the estimation of source apportionments of PM_2.5 mass samples collected in Washington, DC, and Phoenix, Arizona, USA. Apportionments were evaluated for their respective associations with mortality using Poisson regressions, allowing a comparative assessment of the extent to which variations in the apportionments contributed to variability in the source-specific mortality results. The various research groups generally identified the same major source types, each with similar elemental makeups. Intergroup correlation analyses indicated that soil-, sulfate-, residual oil-, and salt-associated mass were most unambiguously identified by various methods, whereas vegetative burning and traffic were less consistent. Aggregate source-specific mortality relative risk (RR) estimate confidence intervals overlapped each other, but the sulfate-related PM_2.5 component was most consistently significant across analyses in these cities. Analyses indicated that source types were a significant predictor of RR, whereas apportionment group differences were not. Variations in the source apportionments added only some 15% to the mortality regression uncertainties. These results provide supportive evidence that existing PM_2.5 source apportionment methods can be used to derive reliable insights into the source components that contribute to PM_2.5 health effects. Key words: fine particles, health effects, mortality, particulate matter, source apportionment, sulfate, time-series, uncertainty. Environ Health Perspect 113:1768-1774 (2005) . doi:10.1289/ehp.7989 available via http://dx.doi.org/ [Online 1 September 2005]

Address correspondence to G.D. Thurston, New York University School of Medicine, Nelson Institute of Environmental Medicine, 57 Old Forge Rd., Tuxedo Park, NY 10987 USA. Telephone: (845) 731-3564. Fax: (845) 351-5472. E-mail: thurston@env.med.nyu.edu

We thank the individual researchers who participated in this workshop, often on their own time and resources. We also thank Columbia University's Arden House Conference Center in Harriman, New York, for hosting the May 2003 workshop that led to this manuscript.

The workshop was organized under the auspices of the participating U.S. Environmental Protection Agency (EPA) PM Health Effects Research Centers (grant R827351 at New York University, R827351 at the University of Washington, R827353 at Harvard University, and R927354 at the University of Rochester) . Support for the organization and administration of the workshop was also provided by the New York State Energy Research and Development Authority (grant 375-34215) . Additional support was provided by the New York University-National Institute of Environmental Health Sciences Center grant (ES00260) .

The information in this document has been subjected to review by the U.S. EPA National Health and Environmental Effects Research Laboratory and approved for publication. Approval does not signify that the contents reflect the views of the agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.

The authors declare they have no competing financial interests.

Received 31 January 2005 ; accepted 1 September 2005.