Two studies were conducted to ascertain the lowest ozone concentration threshold for statistically significant adverse effects on cardiopulmonary function and symptoms in male and female surrogates of the flight attendant population. Both studies simulated in-flight environmental conditions at 1,829 m mean sea level (MSL) chamber altitude. The ozone exposures were 0.2 parts per million by volume (ppmv) for 4 h in the first study and 0.3 ppmv for 3 h in the second study. Each subject was similarly exposed to air only (no ozone) on another occasion. Treadmill exercise, equivalent to workloads of on-duty flight attendants, was performed at altitude. Cardiorespiratory measurements were made during ozone exposure, whereas spirometry and symptom assessments were made immediately before and after exposure. The ozone threshold for reversible adverse effects on symptoms and spirometry function was reached by a 3-h exposure to 0.3 ppmv. The data suggest a greater symptomatic sensitivity to ozone in females.