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February 29[edit]

Nitrogen narcosis[edit]

Why one still gets nitrogen narcosis if per Breathing gas, "most breathing gases [...] are a mixture of oxygen and one or more inert gases", ie similar or identical to air? The narcosis article says high pressure is involved, but I still don't understand why when the breathing mix is basically air and not a pure nitrogen. 212.180.235.46 (talk) 21:44, 29 February 2020 (UTC)[reply]

I think the main difference is solubility. Nitrogen dissolves in blood at high pressure; helium not so much. So a breathing gas made with helium replacing some or all of the nitrogen reduces the risk of nitrogen narcosis (and also of the bends, a separate issue but also caused by nitrogen). --Trovatore (talk) 21:48, 29 February 2020 (UTC)[reply]

Ok, would this still be an issue if you lower nitrogen percentage in the mixture? Instead of air-like 78% of nitrogen, say 10% nitrogen and 90% oxygen? Or 5%? 212.180.235.46 (talk) 00:01, 1 March 2020 (UTC)[reply]

Not sure of the exact numbers, but at high pressures even oxygen can become toxic. --Khajidha (talk) 00:11, 1 March 2020 (UTC)[reply]

Oxygen is somewhat toxic in higher-than-normal concentrations even at normal atmospheric pressure, as well as a fire hazard. See oxygen toxicity. (The astronauts who died in the Apollo 1 launch-pad fire were breathing pure oxygen at slightly above normal atmospheric pressure, because that was the only way the test they were performing could be done in an Apollo capsule, but were safe from toxicity because at such pressures the damage to the body is slow to develop.) --69.159.8.46 (talk) 03:57, 1 March 2020 (UTC)[reply]

Oxygen by itself is not a fire hazard, it depends on what else is there. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:37, 1 March 2020 (UTC)[reply]

Yes, where "what else" is "anything that will burn". --69.159.8.46 (talk) 19:31, 2 March 2020 (UTC)[reply]

From skimming the article, it appears that it is the partial pressure of the N₂ that determines whether it has a narcotic effect (and how severe). So yes, decreasing the proportion of N₂ in the air mix will reduce the effect (while going deeper, and therefore increasing the pressure, will increase it). Iapetus (talk) 20:12, 1 March 2020 (UTC)[reply]

"Inert" gas narcosis a function of the gas' narcotic potential and partial pressure, not concentration. It certainly is possible to reduce nitrogen narcosis by replacing nitrogen with something less narcotic although this would typically be helium, not oxygen. 2A01:E34:EF5E:4640:4CB5:BD3F:EC15:C20A (talk) 18:53, 1 March 2020 (UTC)[reply]

The Breathing gas#Oxygen article discusses the risk of oxygen toxicity. The article specifically mentions

The fraction of the oxygen determines the greatest depth at which the mixture can safely be used to avoid oxygen toxicity. This depth is called the maximum operating depth.[1][3][6][9]

and

The maximum safe P_O₂ in a breathing gas depends on exposure time, the level of exercise and the security of the breathing equipment being used. It is typically between 100 kPa (1 bar) and 160 kPa (1.6 bar); for dives of less than three hours it is commonly considered to be 140 kPa (1.4 bar), although the U.S. Navy has been known to authorize dives with a P_O₂ of as much as 180 kPa (1.8 bar).[1][2][3][6][9] At high PO2 or longer exposures, the diver risks oxygen toxicity which may result in a seizure.[1][2] Each breathing gas has a maximum operating depth that is determined by its oxygen content.[1][2][3][6][9]

and

where the oxygen content exceeds atmospheric levels, generally to a level where there is some measurable physiological effect over long term use, and sometimes requiring special procedures for handling due to increased fire hazard. The associated risks are oxygen toxicity at depth and fire, particularly in the breathing apparatus.[citation needed]

Our Gas blending for scuba diving also discusses oxygen toxicity although mostly just says similar stuff.

We even have a Maximum operating depth article which mentions

This limit is based on risk of central nervous system oxygen toxicity, and is somewhat arbitrary, and varies depending on the diver training agency or Code of Practice, the level of underwater exertion planned and the planned duration of the dive, but is normally in the range of 1.2 to 1.6 bar.[1]

In case it's unclear from these, Enriched Air Nitrox and Trimix (breathing gas)#Hyperoxic trimix are used in certain circumstances. But obviously just increasing the oxygen concentration does not work for many cases.

BTW, you might also be interested in Trimix (breathing gas)#Advantages of keeping some nitrogen in the mix, High-pressure nervous syndrome, Trimix (breathing gas)#Disadvantages of helium in the mix, Compression arthralgia and Decompression sickness. The effect of using helium on the risk of decompression sickness, is different from what I had understood until now, and I wonder if User:Trovatore may also share some confusion in the area. Likewise, I wasn't aware of the other reasons besides cost why you may not want to just use heliox.

BTW, despite that one citation needed tag, our articles look in decent shape albeit I haven't checked the quality of the references and do not know about the subject. I.E. I'm just assuming from the referencing and writing it's not utter nonsense.

One example, I noticed our article mentions

Helium dissolves into tissues (this is called on-gassing) more rapidly than nitrogen as the ambient pressure is increased. A consequence of the higher loading in some tissues is that many decompression algorithms require deeper decompression stops than a similar decompression dive using air, and helium is more likely to come out of solution and cause decompression sickness following a fast ascent.[9]

Yet I came across [1] which makes me think there may be some dispute over what is necessary and why. But this is also very recent, and I have no idea how well accepted it is.

Nil Einne (talk) 15:50, 2 March 2020 (UTC)[reply]

The classic "decompression accident" is related to off-gassing the gas dissolved in the diver's body and, for this purpose, only inert gases are considered since the oxygen is assumed to have been metabolized.

For "inert" gas narcosis, oxygen is often, but not always, taken into account (narcotic potential is predicted by the meyer-overton law which suggests that oxygen should be more narcotic than nitrogen at the same partial pressure). So, binary mixes like nitrox are sometimes considered either less nacotic than, or identically narcotic to, air (The principal advantage of nitrox is to reduce nitrogen loading but at the cost of an increased oxygen stress).

Binary mixes like triox are preferred at depths between 30m and 40m where the narcotic potential of nitrox may be considered excessive. In principle it would be possible to extend the use of triox to greater depths by using lower and lower oxygen fractions, but the cost of the helium becomes prohibitive so trimix is generally preferred.

There are two forms of oxygen toxicity, the one which concerns divers being related to the integral of oxygen partial pressure with respect to time.

At sufficient depth, a breathing mix with a safe oxygen partial pressure would be dangerously hypoxic (I've used mixes with only 4% oxygen), in which case one or more separate "travel" gases are required for the initial shallow portions of the dive.

37.175.103.191 (talk) 17:34, 2 March 2020 (UTC)[reply]