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While there is a growing body of data on the health effects of particulate matter, there is little information available from areas that experience frequent dust storms, such as Spokane, WA. As part of a three-year study to investigate the health effects associated with exposure to atmospheric aerosols, ambient particulate matter in Spokane is being characterized according to particle size and chemical composition. In this report, particulate matter concentrations measured using continuous tapered-element oscillating microbalances for three size ranges are discussed. Particles with aerodynamic diameters less than 10 micrometers (PM10), less than 2.5 micrometers (PM2.5), and less than 1.0 micrometer (PM1.0) were measured at a residential site; PM10 and PM2.5 were measured at an industrial site. Based upon 1.5 years of data, PM10 was found, on average, to consist of approximately 40% PM2.5 at the industrial site and approximately 50% at the residential site, with higher fractions (up to 50% at the industrial site and up to 80% at the residential site) observed in the late fall or early winter (October-November), At the residential site, PM2.5 was comprised of approximately 80% particles 1.0 micrometer and smaller. Only one windblown dust storm occurred during the sampling period, on July 24, 1994, during which both PM10 and PM2.5 concentrations were enhanced.