Design of Study. Subjects were exposed to target levels of either filtered air, 0.300 ppm O3, 300 ug/m3 DE, or combined DE (300 ug/m3) and O3 (0.300 ppm) on Day 1 for 2 hr. On Day 2 subjects were exposed to 0. 300 ppm O3 for 2 hr. During exposures on Days 1 and 2 subjects performed four 30 min cycles of rest/exercise at ~ 50 L/min. Day 3 was a follow-up (F/U) day for measurements and no pollutant exposures occurred.