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. Drs. Kenneth Donaldson and coworkers at the Institute of Occupational Medicine examined whether exposure to ozone activates white blood cells to release substances that can damage lung tissue. The investigators exposed rats to either air alone or air containing different concentrations of ozone for seven hours each day for one, two, or four days. Several indicators of inflammation were measured following the final exposure. First, they determined the numbers and types of white blood cells in the wash fluid. Next, they evaluated the potential for these cells to release oxygen free radicals (reactive molecules that can damage lung cells), their capacity to break down fibronectin (a lung structural protein), and their ability to damage epithelial cells in tissue culture.