Nitrox - Dangers - Oxygen Toxicity

Oxygen Toxicity

Diving and handling nitrox raises a number of potentially fatal dangers due to the high partial pressure of oxygen (ppO₂). Nitrox is not a deep-diving gas mixture owing to the increased proportion of oxygen, which becomes toxic when breathed at high pressure. For example, the maximum operating depth of nitrox with 36% oxygen, a popular recreational diving mix, is 29 metres (95 ft) to ensure a maximum ppO₂ of no more than 1.4 bar (140 kPa). The exact value of the maximum allowed ppO₂ and maximum operating depth varies depending on factors such as the training agency, the type of dive, the breathing equipment and the level of surface support, with professional divers sometimes being allowed to breathe higher ppO₂ than those recommended to recreational divers.

To dive safely with nitrox, the diver must learn good buoyancy control, a vital part of scuba diving in its own right, and a disciplined approach to preparing, planning and executing a dive to ensure that the ppO₂ is known, and the maximum operating depth is not exceeded. Most dive shops, dive operators, and gas blenders require the diver to have a nitrox certification card before selling nitrox to divers.

Some training agencies, such as Technical Diving International, teach the use of two depth limits to protect against oxygen toxicity. The shallower depth is called the "maximum operating depth" and is reached when the partial pressure of oxygen in the breathing gas reaches 1.4 bar (140 kPa). The deeper depth, called the "contingency depth", is reached when the partial pressure reaches 1.6 bar (160 kPa). Diving at or beyond this level exposes the diver to a greater risk of central nervous system (CNS) oxygen toxicity. This can be extremely dangerous since its onset is often without warning and can lead to drowning, as the regulator may be spat out during convulsions, which occur in conjunction with sudden unconsciousness (general seizure induced by oxygen toxicity).

Divers trained to use nitrox may memorise the acronym VENTID-C or sometimes ConVENTID, (which stands for Vision (blurriness), Ears (ringing sound), Nausea, Twitching, Irritability, Dizziness, and Convulsions). However, evidence from non-fatal oxygen convulsions indicates that most convulsions are not preceded by any warning symptoms at all. Further, many of the suggested warning signs are also symptoms of nitrogen narcosis, and so may lead to misdiagnosis by a diver. A solution to either is to ascend to a shallower depth.