Recent toxicological evidence suggests that environmental exposures to ultrafine particles (i.e. those smaller than 0.1 μm in diameter) may pose health risks. In the past two decades international research groups have begun to systematically investigate UFP levels in differing geographic areas and microenvironments. However, only a few studies have investigated UFP concentrations in school classrooms, and no prior studies published in English have reported UFP concentrations inside urban living environments in mainland China. In this presentation particle number concentration time-series data is presented for six San Francisco Bay Area school classrooms and four Beijing high-rise apartments. Particular attention is given to quantifying occupant UFP exposures, and identifying the sources, occupant behaviors and building characteristics that most influence those exposures. The overall goal of the presented work is to contribute to a characterization of the UFP exposure of children within their school classrooms and residents within Chinese high-rise apartments, with emphasis on conditions within schools in urban areas of Northern California and apartment buildings in Beijing, China.