A 7-year, time-series, epidemiologic study is ongoing in Spokane, Washington, to examine the associations between ambient particulate constituents or sources and health outcomes such as emergency department (ED) visits for asthma or respiratory problems. One of the hypotheses being tested is that particulate toxic metals are associated with these health outcomes. Spokane is a desirable city in which to conduct this study because of its relatively high concentrations of particulate matter, low concentrations of potentially confounding air pollutants, variability of particulate sources, and presence of several potential particulate metals sources. Daily fine-and coarse-fraction particulate samples are analyzed for metals via energy-dispersive X-ray fluorescence (EDXRF) and instrumental neutron activation analysis. Particulate sources are determined using receptor modeling, including chemical mass balancing and positive matrix factorization coupled with partial source contribution function analysis. Principal component analysis has also been used to examine the influence of sources on the daily variability of the chemical composition of particulate samples. Based upon initial analyses using the EDXRF elemental analyses, statistically significant associations were observed between ED visits for asthma and increased combustion products, air stagnation, and fine particulate Zn. Although there is a significant soil particulate component, increased crustal particulate levels were not found to be associated with ED visits for asthma. Further research will clarify whether there is an association between specific health outcomes and either coarse or fine particulate metal species. Spokane is a semiarid, western city with a medium-sized urban population of approximately 200,000, and over 400,000 who live in the Spokane Valley. The area experiences periodic violations of the air quality standards for particulate matter with a mass median aerodynamic diameter less than 10 µm (PM 10 ) and is currently classified as a nonattainment area for PM 10 . There are several particulate "seasons" that lead to greatly differing particulate chemical characteristics and concentrations (10). In the winter, the Spokane Valley is prone to inversions, and in addition to vehicular exhaust, residential wood burning contributes significantly to particulate pollution. In the fall, prescribed agricultural burning of myriad acres of bluegrass seed and wheat stubble fields takes place. In the late summer, during hot, dry periods, Spokane experiences high PM 10 because of re-entrainment of dust from unpaved roads and parking lots, and during windy periods, windblown dust storms may occur (11). In contrast to many urban areas in the Eastern United States, the sulfur dioxide (SO 2 ) concentration, the fraction of secondary PM mass (including both sulfates and nitrates), and the level of particulate acidity are relatively low in Spokane PM (12). Major point sources of particulate pollution include a municipal waste incinerator and an aluminum smelter and...