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. The investigators exposed guinea pigs to 1 ppm ozone for three hours on either one day or four successive days. They looked for changes in two properties of the tracheal epithelium: its permeability and its capacity to transport ions. To evaluate permeability, the investigators measured the movement of several different kinds of uncharged molecules from one side of the epithelium to the other. To assess ion transport, they measured the movement of two ions, as well as several electrical parameters that reflect ion movement.