Hypoxia

A shortage of oxygen reaching the tissues, causing impairment and blackout with little warning; a key hazard in freediving and rebreather diving.

Hypoxia is an inadequate supply of oxygen to the body's tissues. For a diver the frightening part is how quietly it arrives: the human body has no reliable alarm for low oxygen, our urge to breathe is driven mostly by rising carbon dioxide, not falling oxygen. As the partial pressure of oxygen in the blood drops, judgment fades, vision grays out, and consciousness can switch off with almost no warning. Underwater, that blackout is usually fatal without immediate rescue.

The two communities that manage hypoxia daily are freedivers and rebreather divers. In freediving, hypoxia causes shallow water blackout, classically in the last 10 m of ascent where expanding lungs and falling ambient pressure make the oxygen partial pressure plummet, which is why freedivers never dive alone and buddies watch the final ascent. On a rebreather, a failed oxygen sensor or an incorrectly handled loop can let the oxygen in the breathing gas fall to hypoxic levels, so rebreather training is built around monitoring PO2 constantly and bailing out to open circuit at the first doubt.

Open circuit scuba divers meet hypoxia mainly through gas handling errors, such as breathing a hypoxic trimix travel gas in shallow water where its oxygen fraction cannot support consciousness. The countermeasures are procedural: analyze and label every gas, follow the gas switching plan, and never breathe from an unmarked cylinder. It is also why hypoxic mixes and deco gases live in dedicated, clearly labeled cylinders with their own regulators, so a wrong grab is hard to make in the first place.