RATIONALE:: OBJECTIVES:: METHODS:: MEASUREMENTS AND MAIN RESULTS:: Subjects experienced a significantly greater (P = 0.008) change in FEV1 (± SE) immediately after exposure to 0.06 ppm ozone compared with CA (-1.71 ± 0.50% vs. -0.002 ± 0.46%). The decrement in FVC was also greater (P = 0.02) after ozone versus CA (-2.32 ± 0.41% vs. -1.13 ± 0.34%). Similarly, changes in %PMN were greater after ozone (54.0 ± 4.6%) than CA (38.3 ± 3.7%) exposure (P < 0.001). Symptom scores were not different between ozone versus CA. There were no significant differences in changes in FEV1, FVC, and %PMN between subjects with GSTM1-positive and GSTM1-null genotypes. CONCLUSIONS