The U.S. Environmental Protection Agency (EPA) carries out experiments in which volunteer participants agree to be intentionally exposed by inhalation to specific pollutants at restricted concentrations over short periods to obtain important information about the effects of outdoor air pollution on human health. A new report by the National Academies of Sciences, Engineering, and Medicine finds these studies are warranted and recommends that they continue under two conditions: when they provide additional knowledge that informs policy decisions and regulation of pollutants that cannot be obtained by other means, and when it is reasonably predictable that the risks for study participants will not exceed biomarker or physiologic responses that are of short duration and reversible.