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. The study of the effects of long-term ozone exposure on lung collagen, described in this report, was one of eight studies in a Collaborative Project supported by the NTP and the HEI. The others included studies of lung biochemistry, structure, and function, and one study of nasal structure and function. Dr.