INTRODUCTION Trace metals have been studied by numerous investigators both in ambient air and in human tissues. While there is evidence that exposure to particulate pollutants can cause deleterious health effects (Goldsmith and Friberg@ 1977)@ there has been little effort to relate the tissue concentrations of trace metals to sources of inhalation exposure. In this study relationships were determined between the concentrations of specific trace metals in aerosols and the concentrations of trace metals in human tissues. To adequately determine an index exposure of particulate pollutants in air@ a knowledge of the size distribution and concentration of the various components is necessary A review of the literature in Chapter II reveals that short sampling periods and small samples are characteristic of many currently available size selective samplers. The use of such samplers for determining size-mass distributions for suspended particulates or trace metals is generally unsatisfactory when long-term averages are desired. To overcome the limitations of available samplers and obtain information about the particle size spectrum of total suspended particulates and trace metals in the urban aerosol@ a size selective particle sampler has been developed for continuous sampling of the urban aerosol over periods ranging from hours to weeks. The system used a parallel array of four two-stage samplers with cyclone first stage collectors. Undersize particles which pass through the cyclones are captured on glass fiber filters. To ensure representative sampling@ an inlet system was designed to obtain equal intake velocities and an electronic feedback control system was developed to maintain constant flow through the cyclone within 2% over the entire sampling period. These and other innovations permitted mass balances to be obtained