Duration of Enhanced Responsiveness upon Re-Exposure to Ozone

Abstract

It has been repeatedly observed that ozone (O₃) re-exposure within 24 h elicits enhanced pulmonary function responses. However, there are only limited observations concerning re-exposure to O₃ at intervals between 24 h and several days. The present study was designed to assess the effects of re-exposure to 0.35 ppm O₃ at intervals of 24, 48, 72, and 120 h. Forty young adult male subjects were assigned randomly to one of four groups in ascending order of time to re-exposure (groups 1–4). Each exercised on a bicycle ergometer for 60 min at a workload that elicited a mean ventilation of 60 l/min on three occasions: protocol 1 (P1), filtered air (FA); protocol 2 (P2), 0.35 ppm O₃; and protocol 3 (P3), 0.35 ppm O₃. In addition to standard pulmonary function measures, specific airway resistance (S_Raw); exercise ventilatory pattern, i.e., respiratory frequency (f_R) and tidal volume (V_T); and subjective symptoms (SS) were assessed. Statistical analysis revealed significant differences (p < .05) for all groups between the FA (P1) responses and those for the two O₃ exposures (P2, P3) for forced vital capacity (FVC), forced expiratory volume in 1 s (FEV_1.0), S_Raw, f_R, V_T, and SS. When the two O₃ exposures (P2 and P3) were compared, only group 1 (24 h) responses were statistically significant upon re-exposure: FEV_1.0, −16.1 vs. −30.4% (p < .003); S_Raw, 20.5 vs. 34.5% (p < .05); f_R, 44.2 vs. 65.3% (p < .001); and SS (p < .015). Analysis of O₃ re-exposure data for group 2 (48 h) indicated a trend toward significantly enhanced responsiveness: FEV_1.0, −14.4 vs. −20.6% (p < .12); S_Raw, 13.4 vs. 23.7% (p < .10); f_R, 40.4 vs. 53.2% (p < .10); V_T, −24.2 vs. −31.7% (p <.10); and SS (p > .10). Statistical analyses for groups 3–4 (72–120 h intervening) revealed no significant differences upon re-exposure to O₃. Our observations demonstrated that, for the total O₃ inhaled dose used in this study, the enhanced pulmonary function responsiveness noted upon re-exposure to O₃ within 24 h may persist for up to 48 h in some subjects, but is absent 72 h after initial O₃ exposure.