"Perspectives 3 is the third of a series produced by HEI to describe and interpret results from HEI and other research bearing on important and timely issues for a broad audience interested in environmental health. Perspectives 3 focuses on the health effects of ambient ultrafine particles (UFPs) and was developed under the guidance of a special HEI Review Panel. It examines the contribution of motor vehicles within the broader context of the multiple sources of ambient UFPs, explores the evidence from experimental studies in animals and in humans, as well as observational epidemiologic studies of people exposed to UFPs in the environment. It also identifies some of the broader lessons about both the specific health effects associated with exposures to UFPs and possible directions for future studies that could enhance our understanding of emissions, exposures, and effects of UFPs."--Publisher's website.