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 Health Effects Institute collaborated with the NTP to provide eight HEI-funded investigators access to animals that underwent the same rigorously controlled ozone inhalation protocol and quality assurance processes along with the NTP animals. HEI funded this follow-on study to allow Dr. Harkema and his colleagues at Lovelace Biomedical and Environmental Research Institute the opportunity to employ sophisticated microscopic and quantitative techniques to examine the effects of ozone exposure on the nasal passages of F344/N rats. The tissues used in this study came from the NTP/HEI animals that had been exposed to ozone for 20 months and from the NTP animals that had continued the exposure protocol for 24 months.