Hypobaric hypoxemia is experienced by passengers during commercial aircraft flight. In order to assess the extent of hypoxemia and to test whether hypobaric hypoxia can be accurately estimated at sea level, the results of the normobaric hypoxia altitude simulation test (N-HAST) were compared with those of the hypobaric hypoxia altitude simulation test (H-HAST) in six normal control subjects and nine patients with chronic airflow limitation (CAL) at simulated cabin altitudes of 6,000 ft (1,829 m) and both at rest and during exercise at 8,000 ft (2,438 m). Serial arterial blood samples were drawn during the breathing of 15.1 and 16.3% inspired oxygen at sea level (N-HAST) at rest and during light exercise, and during the breathing of room air at simulated cabin altitudes (H-HAST) of 609 mm Hg (6,000 ft) and 565 mm Hg (8,000 ft) at rest and during light exercise. As measured with the H-HAST technique, the mean (+/- SD) PaO2 of the normal group fell from 96.2 +/- 6.2 mm Hg (sea level) to 70.1 +/- 6.0 mm Hg (6,000 ft), and to 61.7 +/- 1.6 mm Hg (8,000 ft at rest) and 54.8 +/- 7.1 mm Hg (8,000 ft during exercise) (p < 0.005 by analysis of variance [ANOVA]). In the CAL group, the mean (+/- SD) PaO2 fell from 75.8 +/- 8.2 mm Hg (sea level) to 57.0 +/- 6.3 mm Hg (6,000 ft), and 49.5 +/- 6.1 mm Hg (8,000 ft at rest), and 38.6 +/- 7.5 mm Hg (8,000 ft during exercise) (p < 0.005 by ANOVA).(ABSTRACT TRUNCATED AT 250 WORDS)