Ozone:The 0.4 ppm ozone exposures will be conducted in an ozone exposure chamber. Each subject will be exposed to ozone for 2 hours. During exposures, subjects will perform four 15 minute bouts of moderate exercise (minute ventilation or VE = 30 40 L/min) on a treadmill, each separated by 15 minutes of seated rest. Minute ventilation is measured for 2-3 minutes after about 4 minutes of exercise during the first exercise period, and again at about 12 minutes of exercise. It is then measured at about 12 minutes into each exercise period.