Authors' Response

We thank the editor for the opportunity to respond to Drs. Grahame and Hidy's comments. The results of this study were published as part of the Proceedings of the 10th International Inhalation Symposium sponsored by the Fraunhofer Institute Toxikologie und Experimentelle Medizin in 2006, and are part of a series of investigations in various stages of publication. The Multiple City Ambient PM Study (MAPS) project is a collaboration between the U.S. Environmental Protection Agency (EPA) PM (Particulate Matter) Centers and the U.S. EPA, analogous to the previously reported EPA Center-sponsored Source Apportionment Workshop (Thurston et al., 2005). While this paper was the first to be published, numerous others are in preparation, and the source apportionment manuscript by Duvall and colleagues has now been accepted for publication in this same journal (Duvall et al., 2007. The overall goal of the project is to collect at designated locations in the United States size-fractionated particles that might differ in source contributions, and to provide a common set of extracted particles for investigators to use in bioassays already established in their laboratories. A central group collected and extracted the particles to ensure consistency, and provided the samples and chemical analyses to all participating investigators. No restrictions on study design for assays were made, and investigators were free to link chemical composition and health effects using methods of their choosing. Given the simple and straightforward design (three particle sizes collected over a month in five U.S. cities), we are perplexed by Drs. Grahame and Hidy's lengthy and often misconstrued comments. Regarding the latter, the article clearly states particle collection dates and cutoff sizes; the particles were extracted with water (not dilute acid) for use in the bioassays; and PM recovery values were given for mass concentrations and not elemental recovery. Drs. Grahame and Hidy state that we used a “nonstandard method for collecting particles,” but there is in fact no standard methodology of collecting particles for bioassay. This technique (developed by aerosol physicists) is currently in wide use internationally to provide adequate quantities of ambient PM for broad toxicity testing. The thrust of the effort is to make progress on the point made by Grahame and Hidy, namely, understanding which PM components, size attributes, and sources are responsible for the observed health outcomes. Our ongoing studies have greatly expanded chemical characterization of size-segregated ambient PM, including collocated Teflon and quartz filters for more complete analysis of trace elements and organic and elemental carbon. As stated by Grahame and Hidy, these kinds of studies require close collaboration among toxicologists, atmospheric chemists, and biostatisticians to provide quality data regarding the pollutant sources and attributing effects appropriately. We anticipate that this preliminary study will help refine the science for both the approaches in sampling methodologies and subsequent toxicity assessment of PM.