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. Philip Bromberg and coworkers at the University of North Carolina, Chapel Hill examined whether exposure to ozone alters properties of the airway epithelium.