decompression sickness, and the risk depends on the pressure exposure history of the diver. Scope of risk Risk of latent hypoxia leading to blackout and further complications depends on the mode of diving in use. Open circuit scuba and surface supplied diving using hypoxic breathing mixtures to avoid oxygen toxicity Breathing mixtures for diving must limit partial pressure of oxygen to avoid the risk of acute oxygen toxicity, Recreational technical divers generally limit partial pressure of oxygen at the maximum planned depth of a dive to approximately 1.4 bar. When diving to depths below 57 m this requires the use of breathing gases with less than 21% oxygen. Gases with less than about 0.16 bar partial pressure of oxygen are considered insufficient to reliably maintain consciousness, so for depths below about 77 m the breathing gas which is safe to breathe at depth is not considered safe to breathe at the surface, and this effect increases with depth. At 130 m the richest gas mix acceptable would be about 10% oxygen. The depth at which this provides 0.16 bar PO2 is about 6 m, so there is a high probability of loss of consciousness if this gas is used shallower than 6 m To reduce this risk, divers will use a travel gas which is suitable for the first part of descent, and switch to bottom gas when it is convenient and safe to breathe it. On the ascent, oxygen rich gases are valuable for accelerating decompression, so there is an additional reason to switch. Unless there is insufficient overlap in safe depth range for decompression gas and bottom gas, the same gas can be used for travel and decompression, which reduces the number of cylinders that must be carried. Failure to switch gas at the required depth can lead to blackout. Surface supplied divers may be required to work at maximum depth for longer and the partial pressure of oxygen may be limited to reduce pulmonary oxygen toxicity, so the discrepancy in oxygen content when surfacing may be greater, however in this case gas switching is controlled by the surface personnel, and the diver's airway is protected by the full-face mask or helmet, and the surface personnel can monitor the status of the diver on the voice communications system, so the overall risk is reduced in comparison with scuba. Rebreather diving During ascent at a rate where oxygen addition