oxygen partial pressure of the breathing gas. The partial pressure of oxygen in the air or other breathing gas mixture in the lungs controls the oxygen loading of blood. A critical PO2 of in the lungs will sustain consciousness when breathing is resumed after a breath-hold dive. This is about 4% oxygen in the lungs and 45% oxygen saturation of the arterial blood. At 30 msw (4 bar), 2% by volume oxygen in the lung gas gives a PO2 of . At 10 msw (2 bar), for the same 2% oxygen, the PO2 would be , i.e. marginal. At the surface the same 2% oxygen drops to , ignoring metabolic use. Consequences The usual consequence, if the airway is not protected, is drowning. A breath-hold diver who has blacked out and has been promptly returned to the surface, will usually regain consciousness within seconds. While the diver is still unconscious underwater, they are at high risk of drowning. While unconscious the diver has lost voluntary bodily control, but still has protective reflexes that protect the airway. One of these is laryngospasm, which closes the larynx, to preventing water from entering the lungs. If the diver has reached the surface, and the divers face is kept above water, when the laryngospasm relaxes spontaneous breathing will often resume. The laryngospasm will eventually relax, and if the diver is still underwater then water will enter the airway and may reach the lungs which will cause complications if resuscitation is successful, and secondary drowning is possible. The time between loss of consciousness and death varies considerably depending on a number of factors but can be as little as two and a half minutes. If the diver's airway is protected by a full-face mask or diving helmet, the immediate risk is death by asphyxiation, which can occur within a few minutes of cessation of breathing. If the diver sinks and the pressure increases sufficiently, the gas may become capable of supporting consciousness again, but the problem of latent hypoxia remains until a higher oxygen content gas is provided. If the diver is on surface supply, a prompt switch of gases may be sufficient to restore consciousness, and this may also apply to a scuba diver if immediate and appropriate action is taken by another diver. Immediate surfacing of a hypoxic diver using underwater breathing apparatus presents the risk of decompression illness from lung barotrauma or