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 nose is the first line of defense against inhaled pathogens, dusts, and irritant gases; thus, changes induced by ozone in the normal functions of the nose could result in an increased susceptibility to respiratory infections and other diseases. In one of eight studies in the NTP/HEI Collaborative Ozone Project, Drs.