INTRODUCTION: This study investigated whether the timing of activation affects the utility of an emergency underwater rebreather unit (RBU) when submerged in cold water.METHOD: On two successive occasions, 16 male UK Royal Marines were submerged in stirred water at 12.2°C for up to 78 s. The subjects were lowered (taking 18 s) into the water in a seated position and were instructed to take a large breath in, activate the unit, breath-hold for as long as possible, exhale into the unit, and breathe normally to and from the unit for the remainder of submersion. On one occasion the subjects were instructed to activate the RBU when the water reached chest height (Condition-1) and, on the other, prior to the feet entering the water (Condition-2). Measurements were made of the duration of breath-hold, rebreathing and submersion, exhaled oxygen and carbon dioxide concentrations, skin temperature, and heart rate.RESULTS: In 16 of the 32 submersions, the breath-hold was released before the subject became fully submerged and in 8 submersions the subject requested early withdrawal from the water. Mean (SD) breath-hold duration was 14.0 (13.8) s and the duration of rebreathing was 45.9 (21.9) s. The duration of breath-hold once completely submerged was longer in Condition-1 (9.1 s) than Condition-2 (4.1 s).CONCLUSIONS: The study indicates the RBU should be activated just before the mouth becomes submerged rather than before entering the water, and that the RBU will prolong underwater stay time, thereby increasing survival prospects.House CM, Shaw AM, Roiz de Sa DG. Rebreather unit to prolong underwater survival time. Aerosp Med Hum Perform. 2015; 86(12):1028–1033.