To the Editor In their review of in-flight medical emergencies, Dr Martin-Gill and colleagues outlined characteristics of the aircraft cabin environment, including mild hypoxia, that might precipitate or contribute to such emergencies, and provided recommendations for how to manage them medically. However, the underlying physiology that links routine cabin hypoxia to adverse medical events was not discussed. It is important to consider how physiological responses to cabin hypoxia might actually cause clinically significant effects in passengers, as this has implications for management of the most serious conditions in flight.