 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 , cost , and other factors are also important . 
