text: Oxygen toxicity Diving with and handling nitrox raise 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  to ensure a maximum ppO2 of no more than . 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. Many dive shops, dive operators, and gas blenders (individuals trained to blend gases) require the diver to present a nitrox certification card before selling nitrox to divers.
  Some training agencies, such as PADI and 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 .  The deeper depth, called the "contingency depth", is reached when the partial pressure reaches . 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).