BACKGROUND: Previous studies of acute hypoxia have largely examined different altitudes in isolation. Pilots, however, receive two exposures during in-flight hypoxic emergencies (IFHEs): the initial exposure at altitude, followed by a second mild exposure after descending and removing the breathing mask. Conventional wisdom holds that performance recovers with blood oxygen saturation and that exposure to mild hypoxia is safe. This study examined the possibility that the effects of moderate hypoxia may linger to overlap with the effects of mild hypoxia during sequential exposures such as those experienced by pilots during an IFHE.METHODS: Subjects performed a simulated flight task and secondary task while being exposed to normobaric hypoxia via the ROBD-2.RESULTS: Average error on the flight task during exposure to 3048 m (10,000 ft) was marginally worse when preceded by exposure to 7620 m (25,000 ft; 7.40 ± 3.32) than when experienced in isolation (6.42 ± 3.82). Performance on the secondary task was likewise worse when the mild exposure followed the moderate exposure (0.27 ± 0.30 lapses per minute) than when the mild exposure occurred by itself (0.19 ± 0.20 lapses per minute). Minimum Spo₂ showed a similar pattern of results (84.87 ± 4.37 vs. 86.61 ± 2.47).DISCUSSION: We believe our results are most likely due to a failure to recover from the original moderate exposure rather than an additive effect between the exposures. Even so, our findings suggest that pilot impairment following an IFHE may be worse than previously believed.Robinson FE, Horning D, Phillips JB. Preliminary study of the effects of sequential hypoxic exposures in a simulated flight task. Aerosp Med Hum Perform. 2018; 89(12):1050-1059.