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. Harkema and Morgan and their colleagues at Lovelace Biomedical and Environmental Research Institute used a video recording technique to measure the speed of mucous flow in different regions of the nasal cavities of rats exposed to 0, 0.12, 0.50, or 1.0 ppm ozone for six hours per day, five days per week, for 20 months. The investigators used specific stains and a technique called image analysis to determine the effect of ozone exposure on mucous content, and light and electron microscopy to study cellular changes in the epithelial cell layer.