<unk>  while deep diving is prevented by filling dive cylinders with a gas mixture containing helium. Helium is stored in brown cylinders. The most straightforward way to avoid<unk>  is for a diver to limit the depth of dives. The other main preventive measure is properly informed selection/choice of which gas to use for the particular dive under consideration. Since<unk>  becomes more severe as depth increases, a diver keeping to shallower depths can avoid serious<unk> . Most recreational dive schools will only certify basic divers to depths of , and at these depths<unk>  does not present a significant risk. Further training is normally required for certification up to on air, and this training should include a discussion of<unk> , its effects, and management. Some diver training agencies offer specialized training to prepare recreational divers to go to depths of , often consisting of further theory and some practice in deep dives under close supervision. Scuba organizations that train for diving beyond recreational depths, may forbid diving with gases that cause too much<unk>  at depth in the average diver (such as the typical widely used nitrox mixtures suitable for most recreational diving), and strongly encourage the use of other breathing gas mixes containing helium in place of some or all of the<unk>  in air – such as trimix and heliox – because helium has no narcotic effect. The use of these gases forms part of technical diving and requires further training and certification. While the individual diver cannot predict exactly at what depth the onset of<unk>  will occur on a given day, the first symptoms of<unk>  for any given diver are often more predictable and personal. For example, one diver may have trouble with eye focus (close accommodation for middle-aged divers), another may experience feelings of euphoria, and another feelings of claustrophobia. Some divers report that they have hearing changes, and that the sound their exhaled bubbles make becomes different. Specialist training may help divers to identify these personal onset signs, which may then be used as a signal to ascend to avoid the<unk> , although severe<unk>  may interfere with the judgement necessary to take preventive action. Deep dives should be made only after a gradual training to test the individual diver's sensitivity to increasing depths, with careful supervision and logging of reactions. Scientific evidence does not show that a diver can train to overcome any measure of<unk>  at a given depth or become tolerant of it. Equivalent narcotic depth (END) is a commonly used way of expressing the narcotic effect of different breathing gases. The National Oceanic and Atmospheric Administration (NOAA) Diving Manual now states that oxygen and<unk>  should be considered equally narcotic. Standard tables, based on relative lipid solubilities, list conversion factors for narcotic effect of other gases. For example, hydrogen at a given pressure has a narcotic effect equivalent to<unk>  at 0.55 times that pressure, so in principle it should be usable at more than twice the depth. Argon, however, has 2.33 times the narcotic effect of<unk> , and is a poor choice as a breathing gas for diving (it is used as a drysuit inflation gas, owing to its low thermal conductivity). Some gases have other dangerous effects when breathed at pressure; for example, high-pressure oxygen can lead to oxygen toxicity. Although helium is the least intoxicating of the breathing gases, at greater depths it can cause high pressure nervous syndrome, a still mysterious but apparently unrelated phenomenon. Inert gas<unk>  is only one factor influencing the choice of gas mixture; the risks of decompression sickness and oxygen toxicity, cost, and other factors are also important. Because of similar and additive effects, divers should avoid sedating medications and drugs, such as cannabis and alcohol before any dive. A hangover, combined with the reduced physical capacity that goes with it, makes<unk>  more likely. Experts recommend total abstinence from alcohol for at least 12 hours before diving, and longer for other drugs.
Nitrogen narcosis
Malignant hyperthermia
Anoxia
Barbiturate overdose
Histotoxic hypoxia
Pseudohypoxia
Hypoxia 
Hypothermia