better to switch back to the less narcotic gas if practicable, and adjust the decompression schedule to suit. this problem can be aggravated by the possibility of inert gas counterdiffusion, which is most likely to affect the inner ear, and can usually be avoided by a better selection of gas mixtures and switching depths. prevention narcosis 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 nitrogen narcosis 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 narcosis becomes more severe as depth increases, a diver keeping to shallower depths can avoid serious narcosis. most recreational training agencies will only certify entry level divers to depths of 18 to 20 m (60 to 70 ft), and at these depths narcosis does not present a significant risk. further training is normally required for certification up to 30 m (100 ft) on air, and this training should include a discussion of narcosis, its effects, and management. some diver training agencies offer specialized training to prepare recreational divers to go to depths of 40 m (130 ft), often consisting of further theory and some practice in deep dives under close supervision. scuba organizations that train for diving beyond recreational depths, may exclude diving with gases that cause too much narcosis at depth in the average diver (such as the typical widely used nitrox mixtures used for most recreational diving), and strongly encourage the use of other breathing gas mixes containing helium in place of some or all of the nitrogen in air – such as trimix and heliox – because helium has no narcotic effect. the use of these gases is considered to be technical diving and requires further training and certification. while the individual diver cannot predict exactly at what depth the onset of narcosis will occur on a given day, the first symptoms of narcosis 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 narcosis, although severe narcosis 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 narcosis 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 nitrogen 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 nitrogen 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 nitrogen, 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 narcosis is only one factor influencing the choice of gas mixture; the risks of decompression sickness and oxygen toxicity, work of breathing, 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 nitrogen narcosis more likely. experts recommend total abstinence from alcohol for at least 12 hours before diving, and longer for other drugs. prognosis and epidemiology narcosis is potentially one of the most dangerous conditions to affect the scuba diver below about 30 m (100 ft). except for occasional amnesia of events at depth, the effects of narcosis are entirely removed on ascent and therefore pose no problem in themselves, even for repeated, chronic or acute exposure. nevertheless, the severity of narcosis is unpredictable and it can be fatal while diving, as the result of illogical behavior in a dangerous environment. tests have shown that all divers are affected by nitrogen narcosis, though some experience lesser effects than others. even though it is possible that some divers can manage better than others because of learning to cope with the subjective impairment, the underlying behavioral effects remain. these effects are particularly dangerous because a diver may feel they are not experiencing narcosis, yet still be affected by it. history both meyer and overton discovered that the narcotic potency of an anesthetic can generally be predicted from its solubility in oil. minimum alveolar concentration is an inverse indicator of anaesthetic potency.