[2][3] The effect is consistently greater for gases with a higher lipid solubility, and although the mechanism of this phenomenon is still not fully clear, there is good evidence that the two properties are mechanistically related.

Thus narcosis while diving in open water rarely develops into a serious problem as long as the divers are aware of its symptoms, and are able to ascend to manage it.

To dive at greater depths, as narcosis and oxygen toxicity become critical risk factors, gas mixtures such as trimix or heliox are used.

These mixtures prevent or reduce narcosis by replacing some or all of the inert fraction of the breathing gas with non-narcotic helium.

[10] The most dangerous aspects of narcosis are the impairment of judgement, multi-tasking and coordination, and the loss of decision-making ability and focus.

[15] Reported signs and symptoms are summarized against typical depths in meters and feet of sea water in the following table, closely adapted from Deeper into Diving by Lippman and Mitchell:[11] The cause of narcosis is related to the increased solubility of gases in body tissues, as a result of the elevated pressures at depth (Henry's law).

[7][22] Significant impairment due to narcosis is an increasing risk below depths of about 30 m (100 ft), corresponding to an ambient pressure of about 4 bar (400 kPa).

[15] When breathing air at depths of 90 m (300 ft) – an ambient pressure of about 10 bar (1,000 kPa) – narcosis in most divers leads to hallucinations, loss of memory, and unconsciousness.

[25] Narcosis has been compared with altitude sickness regarding its variability of onset (though not its symptoms); its effects depend on many factors, with variations between individuals.

Thermal cold, stress, heavy work, fatigue, and carbon dioxide retention all increase the risk and severity of narcosis.

An indirect physical effect – such as a change in membrane volume – would therefore be needed to affect the ligand-gated ion channels of nerve cells.

[33] Trudell et al. have suggested non-chemical binding due to the attractive van der Waals force between proteins and inert gases.

[17][18][22] More recently, specific types of chemically gated receptors in nerve cells have been identified as being involved with anesthesia and narcosis.

[8] The management of inert gas narcosis is usually simply to ascend to shallower depths, where much of the effect disappears within minutes.

In the event of complications or other conditions being present, ascending remains the correct initial response unless it would violate decompression obligations.

[39] Deep dives should be made only after a gradual work-up to test the individual diver's sensitivity to increasing depths, taking note of reactions.

[45] The National Oceanic and Atmospheric Administration (NOAA) Diving Manual now states that oxygen and nitrogen should be considered equally narcotic.

[46] Standard tables, based on relative lipid solubilities, list conversion factors for narcotic effect of other gases.

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.

[49] Because of similar and additive effects, divers should avoid sedating medications and drugs, such as cannabis and alcohol before any dive.

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.

"[53][54] Junod suggested that narcosis resulted from pressure causing increased blood flow and hence stimulating nerve centers.

[56] The first report of anesthetic potency being related to lipid solubility was published by Hans H. Meyer in 1899, entitled Zur Theorie der Alkoholnarkose.

[61] The (NOAA) Diving Manual was revised to recommend treating oxygen as if it were as narcotic as nitrogen, following research by Christian J. Lambertsen et al. in 1977 and 1978,[62] but this hypothesis has been challenged by more recent work.

It also found that the onset of narcosis follows a short period of raised alertness during descent, and some of the effects persist for at least 30 minutes after the dive.