A new report from the Health Effects Institute (HEI), "Understanding the Health Effects of Ambient Ultrafine Particles" concludes that while there have been a growing number of laboratory and field studies of the effects of ultrafine particles (UFPs), “toxicologic studies in animals, controlled human exposure studies, and epidemiologic studies to date have not provided consistent findings on the effects of exposures to ambient levels of UFPs, particularly in human populations. The current evidence does not support a conclusion that exposures to UFPs alone can account in substantial ways for the adverse effects that have been associated with other ambient pollutants such as PM2.5.”