I know people on tumblr looove stories of underwater cave diving, but I haven't seen anyone talk about nitrogen narcosis aka "raptures of the deep"

basically when you want to get your advanced scuba certification (allowing you to go more than 60 feet deep) you have to undergo a very specific test: your instructor takes you down past the 60+ foot threshold, and she brings a little underwater white board with her.

she writes a very basic math problem on that board. 6 + 15. she shows it to you, and you have to solve it.

if you can solve it, you're good. that is the hardest part of the test.

because here's what happens: there is a subset of people, and we have no real idea why this happens only to them, who lose their minds at depth. they're not dying, they're not running out of oxygen, they just completely lose their sense of identity when deep in the sea.

a woman on a dive my instructor led once vanished during the course of the excursion. they were diving near this dropoff point, beyond which the depth exceeded 60 feet and he'd told them not to go down that way. the instructor made his way over to look for her and found a guy sitting at the edge of the dropoff (an underwater cliff situation) just staring down into the dark. the guy is okay, but he's at the threshold, spacing out, and mentally difficult to reach. they try to communicate, and finally the guy just points down into the dark, knowing he can't go down there, but he saw the woman go.

instructor is deep water certified and he goes down. he shines his light into the dark, down onto the seafloor which is at 90 feet below the surface. he sees the woman, her arms locked to her sides, moving like a fish, swimming furiously in circles in the pitch black.

she is hard to catch but he stops her and checks her remaining oxygen: she is almost out, on account of swimming a marathon for absolutely no reason. he is able to drag her back up, get her to a stable depth to decompress, and bring her to the surface safely.

when their masks are off and he finally asks her what happened, and why was she swimming like that, she says she fully, 100% believed she was a mermaid, had always been a mermaid, and something was hunting her in the dark 👍

temperedknight

It's not "a subset of people" (though it does affect different people at different times and depths) and we know perfectly well why - this is gas narcosis. At deeper depths, gas in your tank compresses, making it more concentrated. This makes it toxic (I don't know the chemistry, I'm just an Open Water certified diver, which is like having a basic driver's license; nothing to write home about) in a similar way to alcohol, in that it will severely impair your judgement, alter your personality, remove your inhibitions, etc.

Gas narcosis is more common with higher blends of oxygen (standard dive air is 22/78 o2/n2; nitrox is 30/70). There's one boon - gas narcosis usually fades if you ascend enough that the gas decompresses.

Gas narc is only one way diving can murder you in a very grim way. It is why you NEVER dive alone and never go to a depth you aren't trained for. There's a horrid video of a guy trying to do a "bounce" dive (down very deep and then back up as fast as possible; this is very stupid and is usually only done by morons to set depth records) without proper equipment or training. His family is lucky they got anything back at all - the footage shows him so narc'd out of his mind he can't remember how his buoyancy control device works (it's literally one button).

catboybiologist

There's a couple of details slightly off in this explanation! Most notably, it's conflating three different issues of gas under pressure in your body that are actually separate phenomena with separate considerations and ways to handle. These are decompression illness, nitrogen narcosis, and oxygen toxicity.

Before I go into these, please please please understand that I am not a dive instructor, a physiologist specializing in this, or medical professional. I'm a recreational diver that happens to be a molecular biologist, so I might have a bit more fun fact type info on this, but treat it as that: fun facts. Get certified through PADI or a similar organization to actually get trained.

I'm also using a lot of sloppy calculations with lots of approximations and rounding, so those are probably off by quite a bit, but I hope they get the core points across.

With that out of the way, let's go into those three different phenomena.

Decompression Illness or DCI refers to a broad set of symptoms associated with dissolved nitrogen decompressing in your blood and other tissues. Breathable air for SCUBA is compressed in a tank for storage. As you breath it, it decompresses to the current surrounding pressure, which increases by 1 atmosphere per 30ft/10m, plus 1 atm from the atmosphere itself. This gas is small in volume under this pressure, and can be essentially "forced" into your tissues by this pressure. But, since it's not occupying a significant amount of volume, it doesn't do anything- until you start to go up. Nitrogen leaving your tissue is significantly slower than it entering your tissues, so it'll expand, and sometimes form a gas out of solution while inside your body. This is… not good. Accumulated nitrogen in your joints will cause the most iconic symptom of DCI- "the bends", in which your joints are in extreme pain that is partially alleviated by bending them. While not really DCI, related to this is lung overexpansion- if you don't exhale enough during an ascent, your lungs can decompress and overexpand, causing damage or rupture. This is why ascents while diving are carefully controlled, and why dive computers are essential pieces of equipment- they monitor your cumulative time at various depths over the course of your dive, and tell you how long you can stay at those depths without risking yourself during the ascent.

Nitrogen Narcosis is much more poorly understood, and is the subject of a reblog I made of this top level post. As you said, the whole mermaid thing casts doubt on the story- the sensation is similar to being drunk. Best as we know, it's caused by dissolved nitrogen in your brain tissue. Unlike DCI, this affects you while you're at depth, and is quickly alleviated by rising in the water a little bit and waiting. It can affect anyone, but the depth and conditions that it affects people under vary based on the usual collection of random physiological factors. It's fairly common to be mildly narced at deeper depths, and having awareness of when that's happening to you and gently ascending a short distance to allow it to pass is part of deep diver training.

Oxygen Toxicity is where nitrox considerations come in. Unlike nitrogen, oxygen is metabolically processed by your body, and therefore doesn't accumulate in your tissues over time. So we don't have to worry about that, right? Well, no. For reasons that are again, poorly understood, oxygen is toxic when the partial pressure in your body is above ~1.6 atm. Practically, this is usally limited to 1.4atm. At this point, seizures, loss of motor function, and loss of conciousness can occur- generally, this is to be avoided at all costs. What is partial pressure? It's the percentage of the gas pressure on you that is exerted by that particular gas- eg, right now, you're probably breathing regular air at 21% oxygen, at 1 atm of pressure, so the partial pressure of oxygen in your body is 0.21 atm. The recreational dive limit of 130ft/40m only takes you to about 5atm of total pressure. Breathing standard air, that only means a partial oxygen pressure of 1 atm, well below dangerous levels. Sometimes, however, SCUBA is done with Nitrox. Nitrox is alternately mixed gas in your air tank that contains an atypical ratio of oxygen and nitrogen. Nitrox isn't a set percentage of oxygen, but rather can refer to any alternate ratio of oxygen and nitrogen in the tank you're using. Selecting the appropriate percentage for your dive is a key component of diving on nitrox, because of oxygen toxicity. Functionally, the benefit of nitrox is that it increases your dive time for two reasons- one, you breathe less and consume air slower because you're getting more oxygen per breath, and two, you avoid the previous two issues with nitrogen (mostly DCI, though). But, it also puts you in a binary situation- you now have a hard line that you can't cross, usually shallower than just on air. Within that boundary, you'll have a longer, better dive, but you can't cross it. Figuring out this line is simply- the partial pressure of oxygen is just the proportion of oxygen times the total pressure you're under, and pressure increases by 1 atm per 10m. So essentially, (max depth in meters) = 1.4atm/[ppO2*{(depth/10m)+1}]. If you're using a 35% oxygen mix, that works out to 30 meters, or around 100 feet.

It's a little pedantic, but I hope this was interesting! Physiology under weird conditions like diving is surprisingly poorly understood, but it's always fun to talk about what we do know!

A Mini Guide to RSS Feeds

peter parker is spider-man????

Things do not have to be "useful" to have value.

Whoohoo!

TAG's contract is up in 2024. If you're showing up for the WGA strike and the SAG-AFTRA strike, please remember to show up for them too. They're a tiny guild, they'll probably be striking, and they'll need all the help they can get!