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. Prolonged ozone exposure may injure respiratory tissue, leading to the development or exacerbation of chronic lung diseases such as fibrosis or emphysema. An excess of connective tissue can lead to fibrosis and changes in connective tissue are believed to be an underlying cause of emphysema. Dr. William Parks\' study of the effects of ozone exposure on the mRNAs that code for connective tissue proteins was one of eight studies in the NTP/HEI Collaborative Ozone Project, and one of three in which researchers examined the effects of ozone on rat lung connective tissue. Dr. Parks used molecular biology techniques to compare the level of mRNAs that code for the connective tissue proteins collagen and elastin in lungs from rats exposed to clean air with the level in rats exposed to 1.0 ppm ozone for 20 months. Tissue samples from a separate study of rats exposed for two months to the same ozone levels also were available for comparison.