Ozone is a highly reactive gas that is a pervasive air pollutant at ground level. It is a major component of urban smog, forming when emissions from mobile and industrial sources interact with sunlight. Assessing the risk of adverse health effects from such exposures is difficult because only limited data are available on the actual ozone concentrations that people experience. Under the HEI ozone sampler program, three studies were designed to advance the development and testing of personal ozone samplers. The studies were conducted by Dr. Hackney and colleagues at Rancho Los Amigos Medical Center (Part I), Dr. Koutrakis and colleagues at the Harvard School of Public Health (Part II), and Dr. Yanagisawa from the Harvard School of Public Health (Part III).