Oxygen is assumed equivalent in narcotic effect to nitrogen for this purpose by some authorities and certification agencies.
[2] In contrast, other authorities and agencies consider oxygen to be non-narcotic, and group it with helium and other potential non-narcotic components,[3] or less narcotic, and group it with gases like hydrogen, which has a narcotic effect estimated at 55% of nitrogen based on lipid solubility.
There has been difficulty in identifying a reliable method of objectively measuring gas narcosis, but quantitative electroencephalography (EEG) has produced interesting results.
[3][7] Some studies have shown a decrease in CFFF during air-breathing dives at 4 bar (30 msw), but have not detected a change with partial pressure of pure oxygen within the breathable range.
A functional connectivity metric based on the so-called mutual information analysis has been developed, and summarized using the global efficiency network measure.
[3] Although carbon dioxide (CO2) is known to be more narcotic than nitrogen – a rise in end-tidal alveolar partial pressure of CO2 of 10 millimetres of mercury (13 mbar) caused an impairment of both mental and psychomotor functions of approximately 10% –[5][2] the effects of carbon dioxide retention are not considered in these calculations, as the concentration of CO2 in the supplied breathing gas is normally low, and the alveolar concentration is mostly affected by diver exertion and ventilation issues, and indirectly by work of breathing due to equipment and gas density effects.
The potential causes can be split into four groups: insufficient ventilation, excessive dead space, increased metabolic carbon dioxide production,[10] and high carbon dioxide content of the breathing gas, usually only a problem with rebreathers.
One agency, GUE, prescribes the gas mixtures their members are allowed to use, but even that requirement and membership of the organisation is ultimately the choice of the diver.
Some early (1978) experimental results suggest that, at raised partial pressures, nitrogen, oxygen and carcon dioxide have narcotic properties, and that the mechanism of CO2 narcosis differs fundamentally from that of N2 and O2 narcosis,[5] and more recent work suggests a significant difference between N2 an O2 mechanisms.
[6] Other components of breathing gases for diving may include hydrogen, neon, and argon, all of which are known or thought to be narcotic to some extent.
Since in the absence of conclusive evidence, oxygen may or may not be considered narcotic, there are two ways to calculate END depending on which opinion is followed.