Recent reports have shown that people with asthma who have a particular gene, known as the GSTM1 null gene, are more susceptible to the effect of air pollutants. The purpose of this research study is to learn if volunteers who have asthma and have a GSTM1 null gene have increased response (change in lung function and increase in lung cells collected from sputum) compared to volunteers with asthma who have the GSTM1 sufficient gene when challenged with 0.4 ppm ozone during intermittent exercise. The principal purpose of this study is to identify hyper-responsive, responsive and non-responsive groups of human subjects with mild asthma based on their airway neutrophilic response to ozone exposure, and to perform analyses on DNA from airway cells to explore possible differences in genetic profiles between the three groups. An additional pilot aim is to compare expression of a small number of specific genes of interest in a subset of ozone-responsive and ozone-non-responsive subjects with mild asthma.