O.35 ppm O3 exposure induces hyperresponsiveness on 24-h reexposure to 0.20 ppm O3.

Pulmonary function hyperresponsiveness, defined as enhanced response on reexposure to O3, compared with initial O3 exposure, has been previously noted in consecutive day exposures to high ambient O3 concentrations (i.e., 0.32-0.42 ppm). Effects of consecutive-day exposure to lower O3 concentrations (0.20-0.25 ppm) have yielded equivocal results. To examine the occurrence of hyperresponsiveness at two levels of O3 exposure, 15 aerobically trained males completed seven 1-h exposures of continuous exercise at work rates eliciting a mean minute ventilation of 60 1/min. Three sets of consecutive-day exposures, involving day 1/day 2 exposures to 0.20/0.20 ppm O3, 0.35/0.20 ppm O3, and 0.35/0.35 ppm O3, were randomly delivered via an obligatory mouthpiece inhalation system. A filtered-air exposure was randomly placed 24 h before one of the three sets. Treatment effects were assessed by standard pulmonary function tests, exercise ventilatory pattern (i.e., respiratory frequency, f; and tidal volume, VT) changes and subjective symptom (SS) response. Initial O3 exposures to 0.35 and 0.20 ppm had a statistically significant effect, compared with filtered air, on all measurements. On reexposure to 0.35 ppm O3 24 h after an initial 0.35 ppm O3 exposure, significant hyperresponsiveness was demonstrated for forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), f, VT, and total SS score. Exposure to 0.20 ppm O3 24 h after 0.35 ppm O3 exposure, however, resulted in significantly enhanced responses (compared with initial 0.20 ppm O3 exposure) only for FEV1, f, and VT.(ABSTRACT TRUNCATED AT 250 WORDS)