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. When inhaled, ozone can cause cough, shortness of breath, and transient changes in breathing patterns; however the health significance of these effects is unknown. Dr. David Thomassen and coworkers examined the ability of ozone to alter the structure and growth characteristics of epithelial cells from rat tracheas in ways consistent with precancerous changes. The frequency of such alterations was counted in rat tracheal epithelial cells exposed in culture under various conditions: a single 40-minute exposure to ozone (0.7 or 10 ppm), multiple ozone exposures (0.7 ppm, twice weekly for five weeks), or exposure to ozone (0.7 ppm) either before or after treatment with the known chemical carcinogen MNNG. Such alterations were also counted in tracheal epithelial cells removed from rats exposed to ozone (0.12, 0.5 or 1 ppm) for six hours per day, five days per week, for one to four weeks.