Inert gas narcosis refers to a clinical syndrome characterized by impairment of intellectual and neuromuscular performance and changes in mood and behaviour. It is produced by an increased partial pressure of some inert gasses. In compressed air exposure, these changes, which have been observed for more than 100 years, are caused by nitrogen. The effects are progressive with increasing depth but not with increasing time at the same depth. The word ‘inert’ indicates that these gases exert their effect without undergoing metabolic change in the body, rather than inert gas in the biophysical sense.
Similar effects have been described with other metabolically inactive gases such as the rare gases (neon, argon, krypton, xenon), hydrogen and the anaesthetic gases, although at different partial pressures. Xenon is ‘anaesthetic’ at sea level and is used in some parts of the world as an anaesthetic agent. No narcotic effect of helium has been directly demonstrated at currently attainable pressures, although some narcotic properties have been postulated.
The ‘inert’ gas in compressed air is nitrogen, and its effects are also called nitrogen narcosis, depth intoxication, ‘narks’ and rapture of the deep (I’ivresse des grandes profondeurs), the term coined by Cousteau. The narcosis, although highly variable, places a depth limit to safe diving with compressed air at approximately 40 to 50 metres. Effective work at greater depth requires the substitution of a less narcotic respiratory diluent such as helium or hydrogen.
Many ‘unexplained’ scuba deaths may have been associated with nitrogen narcosis.