How Deep Can Humans Go? | Hacker News Reader

How Deep Can Humans Go?

mcgill.ca

139 pointsbookofjoe104 comments

The limits for other types of diving are interesting, too.

All of the different gases you use for SCUBA diving have a maximum operating depth. At high pressures, various gases become toxic or cause side effects. Oxygen has a limit somewhere in the 1.2-1.6 bar range, and it becomes toxic. For deep dives, you need a lower percentage of oxygen in the mixture. Nitrogen causes nitrogen narcosis at higher pressures, so for deep dives, you need a lower percentage of nitrogen in the mixture. You need to add something else if you want to lower both nitrogen and oxygen, and that’s helium. At high enough pressures, helium causes high-pressure nervous syndrome, a.k.a. helium tremors.

That’s normally the limit. Pretty much every gas is bad for you at these pressures. There is an experimental alternative, which is to use hydrogen gas for the deepest dives, but people are understandably cautious about breathing a mixture of oxygen and hydrogen.

In principle liquid breathing is still an option. It's not great, but certainly better than oxygen + hydrogen.

Have there been any human trials of total liquid breathing? Hydreliox breathing has been tested a few times, and has the advantage that it's fairly compatible with existing technology (compressed air bottles, gas regulators, etc.).

Yeah, it's hydrogen, hazardous to mix and handle, but so is acetylene, and that can explode all by itself if it's under too much pressure. Mess up the mix and you die, but if you mess up the mix for any breathing gas, you're pretty effed (the wrong N2/O2 mix can cause any of hypoxia, narcosis, seizures at various depths).

Not many people have done a pure hydrogen/oxygen dive (vs. hydrogen/helium/oxygen), but from one who has on a rebreather:

> “The first cautious sip of hydrogen just to activate the ADV was satisfying,” [Harris] said. Gas density was not subjectively improved, but Harris noticed an obvious benefit—the HPNS-induced hand tremors he typically experienced after 180 meters/590 feet disappeared. [1]

[1]: https://indepthmag.com/n1-the-inside-story-of-the-first-ever...

> Have there been any human trials of total liquid breathing?

It's used on a somewhat regular basis in clinical settings (e.g. liquid ventilation). I'm not aware of any diving experiments, though. There might have been some classified military tests, but nothing official AFAIK.

Liquid ventilation has been used with some success as a salvage therapy for critically ill patients, especially preterm neonates. However, it carries serious risks of its own. And it requires an external mechanical pump to circulate the liquid; the human diaphragm isn't strong enough to move the liquid in and out.

https://doi.org/10.5001%2Fomj.2011.02

I think liquid breathing for human divers is going to remain science fiction forever. There just aren't any circumstances where it would make sense to do that instead of using an atmospheric diving suit (or an ROV).

In the realm of science fiction that has science... Timemaster by Robert L. Forward (who writes science papers with a plot - and that's a good thing).

The main character wanted to get out to the asteroid belt in an accelerated time frame (not relatively accelerated but rather faster than the current slow methods)

> "Sorry, sir," said Mary, the perky nose on her image twitching under her large glasses as she thought. "I'd be glad to turn over all the cable we've been making for the Mars rotovator, but by the time Bull's division could turn it into a cable catapult, you'd be there using the existing system."

> "Can we push the gee limit higher?" asked Randy.

> "Well... yes..." admitted Bull. "We have small express pods we use to send emergency cargo. We can accelerate those at ten times the normal three-gee launch acceleration and get them up to three hundred kilometers a second."

> "How soon would that get me there?" asked Randy.

> Bull, his fingers too large to operate a cuff-comp, pulled an hp pseudocray out of his shirt pocket, and did a short calculation. "Three weeks," he said. "But those capsules accelerate at thirty gees! You'd be squashed flat!"

> Randy paused for a while as he thought. "I've read about deep-sea divers who survived at high pressures by breathing an oxygen-carrying liquid," he said. "If I floated in a tank of that, I could handle thirty gees easily."

> Tony, Mary, and Bull each thought for a while; then all three nodded, although reluctantly.

> "If I were you, I'd check with some medical and diving experts first," said Tony. "It could be hard on your lungs."

---

Granted, that's science fiction and there are some real problems with that approach too...

https://en.wikipedia.org/wiki/Liquid_breathing#Space_travel

> Acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15g to 20g. Extending acceleration protection beyond 20g requires filling the lungs with fluid of density similar to water. An astronaut totally immersed in liquid, with liquid inside all body cavities, will feel little effect from extreme G forces because the forces on a liquid are distributed equally, and in all directions simultaneously. Effects will still be felt because of density differences between different body tissues, so an upper acceleration limit still exists. However, it can likely be higher than hundreds of G.

> Liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application.

We're not at the point where we can launch at hundreds of Gs yet (and I'm not sure I would want them to be centripetal Gs initially either - that sounds very not fun) ... but when we get to the point were we want to launch a person into space in the direction of another planet at 10 or 20 or 30 Gs of initial acceleration, this is likely something to be revisited.

I would be concerned with sudden high intensity jerks, and with handling rotational acceleration (causing shear stress between different solid/liquid structures). Chiefly with the inertia and buoyancy of the brain and possible concussion/impact/torsion against the skull and spinal cord caused by those jerks.

Armchair speculation from an avid scifi reader... I wouldn't be surprised if at 30 g's we start to see mass destruction of capillaries as blood pools in loops and kinks and then bursts. Which is extra bad when some of those are inside your brain.

Colonel Stapp did short duration 46g in 1954 and survived.

Is that the time on the rocket sled when his eyeballs filled with blood? Nasty. He was very brave/totally crazy.

I am curious what a earth to space launch rotovator that is capable of doing 30g would have for its overall acceleration profile.

Robert Forward (the author of Timemaster) was a physicist and I would do a check of his anatomy related topics before getting into one... but I would be confident of his math.

He wasn't unfamiliar with the technology being used.

https://en.wikipedia.org/wiki/Robert_L._Forward

> He then went to work at the research labs of Hughes Aircraft, where he continued his research on gravity measurement and received 18 patents. He took early retirement in 1987, to focus on his fiction writing and consulting for such clients as NASA and the U.S. Air Force. In 1994, he co-founded the company Tethers Unlimited, Inc. with Robert P. Hoyt, where he served as chief scientist and chairman until 2002.

https://en.wikipedia.org/wiki/Tethers_Unlimited,_Inc.

> Founded in 1994 by Robert P. Hoyt and Robert L. Forward, Tethers Unlimited began developing products based on space tether technologies, including concepts for removal of space debris and momentum exchange tethers for launching payloads into higher orbits. TUI has since broadened its suite of technologies to include power, propulsion, actuation, and communications systems for small satellites, robotic technologies for on-orbit fabrication and assembly, optical fiber winding and deployment, software defined radio communications, and 3D printed radiation shielding.

...

And so, on his word, I'd suspect that it would not be unreasonable to do a fluid breathing 30g ... given the right fluid breathing setup. The rotovator setups are looking at significant fractions of a day for earth capture to space release (6 or 12 hours). I don't think there would be human significant jerk, snap, crackle, and pops either. Though again, I'd be curious to see that acceleration profile (not that I'd ever be doing it myself).

Office Of Naval Research did a human trial https://youtu.be/TIGCdA2YLyY?si=z_7RK06wLO7T926D&t=409

IIRC he got pneumonia due to difficulties removing the fluid from his lungs afterwards.

That first bit, where you breathe in the fluid, must be horrendous. No thanks!

Since we are talking about interactions with the nervous system, how do we go from the pressure at which these gasses were delivered to the alveoli to their behavior in the blood?

I understand the practice GP is describing pretty well but when I try to simulate what is going on in my mind, it's not like these gasses are in gaseous form once they are in the blood. Are we just talking about the concentration that are reached because the pressure affects the diffusion gradient?

You've got exactly the right image in mind: higher partial pressure of a gas in the lungs corresponds to a higher dissolved concentration of that species (H2, He, N2, O2, whatever) in the blood and tissue.

> people are understandably cautious about breathing a mixture of oxygen and hydrogen.

That would be one heck of a case of indigestion. Maybe they can pre-mix it with the pink stuff. Maybe that explains the pink tint to the fluids in The Abyss??

As an aside with the rat scene ... that wasn't special effects (though did have cuts).

https://filmschoolrejects.com/the-abyss-breathing-fluid/

> The rat demonstration scene in The Abyss was unsimulated and used real-life oxygenated breathing fluid. Ed Harris, however, was tasked with pretending to breathe in his water-filled helmet. This was especially challenging during underwater shots, of which there were many.

> ...

> Supposedly, the only purpose for the cuts in the sequence was to avoid showing the rats defecating from panic. In total, the crew shot the scene five times with five different rats. Aftercare included holding the rats upside down to drain their lungs of fluid coupled with a vet visit. None of these efforts assuaged the concerns of the English censors, who cut the scene for UK distribution.

https://www.imdb.com/title/tt0096754/trivia/?item=tr0738209

> Fluid breathing is a reality. Five rats were used for five different takes, all of whom survived and were given antibiotic shots by a vet. The rat that actually appeared in the film died of natural causes a few weeks before the film opened. According to James Cameron, the scene with the rat had to be edited out of the UK movie version because "the Royal Veterinarian felt that it was painful for the rat". James Cameron repeatedly assures that the rats used for this take didn't suffer any harm.

---

Note that in https://en.wikipedia.org/wiki/Liquid_breathing the "Diving" section is under proposed uses. The human uses have been very limited to specific medical situations.

The UK is a nation of animal lovers.

Although I have seen the Abyss a few times in the UK and it had the rat scene. Maybe they added it back in?

I did not know about the rat breathing thing! I wonder if this was ever trialled on rats/mice in a more scientific environment prior to the movie, or do we as a species have the claim of a big scientific breakthrough in underwater breathing being done as a stunt for a movie.

Humans have apparently been testing this stuff since the '50s and experiments on rats breathing in perfluorocarbon were done in the '60s, and humans in the '70s, and that'd be where James Cameron got the idea.

https://www.todayifoundout.com/index.php/2021/08/can-humans-...

> people are understandably cautious about breathing a mixture of oxygen and hydrogen

Wimps.

> In addition to the challenges posed by these high pressures, FDs have no access to oxygen (O2) as they are underwater in a hypoxic (no O2) environment.

Am I missing something, or is this the most unnecessary statement you could possibly make in an article about free diving? I think most readers know you can't breathe underwater.

And to add to the confusion, there is actually "hypoxic water" which is water containing no oxygen and is harmful to marine life.

It can be quite dramatic. Fish can die almost instantly if they traverse the boundary.

Apparently the author is fresh out of B.Sc. She may not have much practice with writing pieces like this.

(Daniela is a recent B.Sc. graduate from the program of Physiology at McGill. She is very passionate about understanding the human body and how we can all individually adapt our daily lifestyles to improve its functioning.

Part of the OSS mandate is to foster science communication and critical thinking in our students and the public. We hope you enjoy these pieces from our Student Contributors and welcome any feedback you may have!)

Another, more disturbing, possibility is that she could be right to include it for some of today's lay audience. I am seeing staggering levels of ignorance about chemistry in media here. Latest example being a radio anchor who had obviously never encountered the word cadmium in her life and made a very awkward (and failed) attempt to sound her way through it.

It feels like it might have been written by AI. The cadence of the text is really unnatural.

On further consideration, I think it's reasonable to remind people that free diving doesn't involve oxygen tanks, as the article had mainly been talking about only about the difficulty of dealing with pressure up to that point. It's a weird sentence, but makes more sense when read like "... and on top of being crushed by the water pressure, they don't have oxygen tanks either".

As a freediver, the author is not correct that “freedivers often hyperventilate.” Not hyperventilating before diving is one of the most important safety rules in freediving. It drops your blood CO2 so you don’t feel like you need to breathe, but doesn’t actually increase your O2- leading to losing track of how long you’ve been down, and blacking out. Instead, you just learn to handle the feeling of high CO2, but use that feeling as a timer to tell you when you’ve been down too long.

Perhaps they are confusing hyperventilation (rapid breathing in and out) with gulping extra air (without exhaling)?

Don’t they also discus that with the frog breathing technique?

I highly recommend the film “The Big Blue” by Luc Besson. One of the best freediving movies based on a fictional story about Jacques Mayol and Enzo Maiorca.

https://en.wikipedia.org/wiki/The_Big_Blue

Trigger warning: suicide is touched upon in the movie (in a bit more romanticized version than the real life events, if I remember correctly)

IIRC the plot is a bit thin, but the visuals are stunning.

I've had nitrogen narcosis while scuba diving. It manifested itself as underwater kleptomania, with me trying to take various worthless tat off the sunken trawler we were diving on. I remember unscrewing light bulbs and trying to remove some manky old curtains, to take as 'prizes'. It seemed quite logical at the time. Rather concerning for my dive buddy though!

I’ve had shallow water blackout - and let me tell you it feels great - as if you hyperventilate before diving, you can suppress the urge to breathe after fighting it a little while - and that period between suppressing the urge to breathe and your brain just going lights out is unbelievably tranquil.

As a kid I used to dive into the pool at an angle and spin round really fast at the bottom of the dive. The caused a small 'high' (from blood rushing into the brain?). It was probably quite dangerous!

If you're interested in freediving or want to know more about it, and you like reading, James Nestor (maybe more well known for his book "Breath" about... well, breathing through your nose) wrote a book called "Deep" that is quite readable and I found informative (had no prior interest in freediving and haven't thought about it much since the book, but it was still worth reading!)

I've found so many good book recommendations on HN, so thought I'd drop that here!

This is one of those things that, were an alien to encounter the stats sheet for humans, they would go, "holy crap humans do that?" Ability to lift crazy heavy things when there's enough adrenaline rushing through your system is pretty wild too. Yay humans!

I am told that <https://old.reddit.com/r/HFY/> may be for you.

Have seen at least one other of these articles lately, on the theoretical limits of some human physical capacity. Here's another, on sprinting:

https://www.washingtonpost.com/sports/olympics/interactive/2...

A few follow-up questions that occur to me: will an understanding of theoretical limits make the actual record breaking seem more routine and less interesting? And will many records be broken beyond the theoretical limit, making these calculations seem rather crude and short-sighted?

There was a youtube video I came across a few weeks ago which took Usain Bolt's world record and did a bio-mechanical analysis to see how far off he was from his absolute theoretical best. It was... surprisingly close. I think they had him at like an 8.9s 100m if he did everything perfectly.

For those who don't know the current records well, from Wikipedia[0]:

The men's world record is 9.58 seconds, set by Jamaica's Usain Bolt in 2009, while the women's world record is 10.49 seconds, set by American Florence Griffith-Joyner in 1988[a].

[0] https://en.wikipedia.org/wiki/100_metres [a] is a footnote on the wikipedia page, discussing the validity of the record due to wind speed measurement concerns; read the page and footnote if you care about that level of detail

Why should we believe this theoretical approach is more accurate than the 30 meters believed to have been the limit for diving?

It will be interesting to see what records the Enhanced Games (where athletes are allowed to take [more/different] performance enhancing drugs openly) throw up when they are held.

You'd need to persuade someone already at a pretty elite level to take part though. Maybe someone who just did their last Olympics?

They may do better than unenhanced humans (a fair number of eastern bloc records from the pre-testing era still stand today, especially in women's throwing events), but they're not going to exceed any hard biomechanical limits. No drug can grow more type II fibers or change the firing rate of the nerves.

The one extreme option out there is surgery to change the attachment point of the tendon to the bone, generating more torque from the same contractile force. Some possibly acropyphal rumors claim lifters in nations with very unscrupulous doctors may have done that when the opportunity presented itself because of an incidental muscle tear.

I agree in that they won't make bones tougher or tendons more elastic. They maybe able to add more TypeII fibres though if they grow additional muscle?

The other big thing on the horizon is gene doping, such as with Myostatin in Bully Whippets.

>Of these, the no-limits category presents FDs with the greatest challenge as they descend using a loaded sleigh and ascend using an air balloon.

I thought that ascending too quickly could lead to issues (if I remember House correctly, "the bends")? Or does that only occur at greater depths than a freediver would reach? This seems even more dangerous than normal freediving, which seems pretty dangerous as-is. (I know nothing about diving).

The bends are caused by accumulated nitrogen dissolving in tissue\blood. No breathing, means no added nitrogen, so no build up. Also it is a somewhat slow process, no breathing means they aren't under long enough for it to develop even if they had extra nitrogen.

It is a problem caused by Scuba, and the fact that Scuba does what it is supposed to, extend the time you can spend underwater, is what causes the bends.

Not quite. They do have the nitrogen in the air they brought down with them in their lungs. Repeated freediving can cause the bends.

https://www.tdisdi.com/pfi-diver-news/freediving-and-dcs/ has some tables for freediving surface intervals

> It is a problem caused by Scuba, and the fact that Scuba does what it is supposed to, extend the time you can spend underwater, is what causes the bends.

To expand on this a bit (pun intended), freediving after SCUBA is extremely dangerous, and this even includes shallow stuff like snorkeling. SCUBA adds dissolved gas to your blood, but once it's there decompression from any source puts you at risk for the bends. Repeated dives will have additional safety limits imposed due to the residual gas, which is managed by a complicated series of dive tables or a modern dive computer.

https://www.naui.org/resources/dive-tables-review/

To further expand upon this, quickly ascending to high altitude (flying airplane or balloon) after diving is also dangerous, for the very same reason. Pilots are trained to wait out at least 24 hours after a dive, or at least 12 hours if the dive didn't require decompression steps.

To further expound on why snorkeling and freediving after scuba can be dangerous, it is helpful to understand that the greatest gas compression occurs in the first 10 meters underwater (Boyle’s law). Scuba divers are trained to ascend slowly (~10m/minute) in order to off-gas expanding nitrogen (and other gasses) and prevent bubbles from forming in tissue (which causes decompression illness). After scuba diving the human body still has a much higher residual nitrogen saturation, so even fairly shallow (less than 10m) rapid descents/ascents in the water column can cause bubbles to form.

I was told that I shouldn't go scuba diving the day before an early-morning flight, because if the plane decompresses, I could get the bends while everyone else just gets a little hypoxic and uses their handy drop-down O2 supply.

There is also an increased risk of decompression illness simply because passenger aircraft typically maintain a lower atmospheric cabin pressure (US federal limits pressure equivalent to 8,000 ft elevation). Going from a high pressure environment (underwater) to a low pressure environment (8,000 ft equivalent elevation in aircraft cabin) too quickly can also cause the bends. Dive computers use algorithms to calculate when it is safe to fly post-dive.

I'm really surprised by that. Looking at some charts, depending on the dive you're probably spending 30 minutes or less going from 2atm to 1atm, but now you need several hours or even a day before it's safe to go to .75atm.

There are wide safety margins in diving. Most dive trips involve multiple dives per day (starting with deepest dive first, with subsequent dives shallower). The times I’ve gone deeper or same depth on subsequent dives my computer usually puts me into deco fairly quickly. At some point subsequent dives aren’t feasible (typically no more than 4-5/day, less is prudent). I think it is with this in mind that the recommendation against flying is made, along with an abundance of caution due to limited experimental evidence (Navy dive tables were developed based on human subjects, so the effects are better understood). It looks like there are some civilian studies done, but more is needed on flying.

https://dan.org/health-medicine/health-resources/diseases-co...

We'd spend more time, but it's difficult to loiter 10m underwater. The residual gas from a dive takes about a day to release, and this impacts both follow-up dives and flights. Treatment options are different as well: an airlift (or just diving again and surfacing slower) vs diverting a whole flight.

Yes, there is a slight risk even without a loss of airplane cabin pressure. You can find the consensus recommendations for flying after diving here.

https://dan.org/health-medicine/health-resource/health-safet...

The dive shop on my last vacation made me stop 24 hours before my flight home.

In the case of Herbert Nitsch's accident, he went so deep that even the short bottom time was sufficient to cause DCS on the ascent with just the standard air he was carrying. N2 diffusion is a function of both time spent at depth and the partial pressure of N2 from depth itself, and in his case, time was short but depth was extraordinary. In hindsight, his attempt would have qualified for a deco stop, had he had the lung capacity to do one on the way up.

Indeed, if you're doing extreme free diving, it would make more sense to breathe a more depth-compatible (nitrogen-replaced) mix at the surface before diving, no?

Then your lungs (and blood) would be full of a safer gas mix at depth.

I'm not sure if any of the deeper gas mixes are safe to breathe at surface level though? I think so?

Yes, some "technical" freedivers do pre-breathe nitrox or pure oxygen at the surface before descending.

https://www.deeperblue.com/technical-freediving-are-breathho...

It does happen, but usually it's not a problem if they don't do it too often. The amount of nitrogen in a single breath of air is pretty small and their time at depth is very short. Repeated dives can increase the amount of dissolved nitrogen though. This can be a problem especially if they have a heart defect called Patent Foramen Ovale which allows bubbles to bypass the lungs directly into arteries heading to the brain.

For instance, for a 140ft dive, the no decompression limit for SCUBA (The amount of time you can stay at that depth without making stops at shallower depths to remove nitrogen) is 5 minutes. The typical no limits free dive is 2 minutes.

They do experience a mild case of the bends on every dive but never notice it because it's so minor.

https://www.mayoclinic.org/diseases-conditions/patent-forame...

https://divetables.com.au/dive-tables/

They're not breathing in compressed air at depth (which could then expand in their blood upon surfacing), just what they brought in from the surface.

It's not breathing compressed air that causes the problem, but rather that the gasses in your lungs are pressurized at all + rapidly lowering pressure on your entire body on ascent.

In freediving, the air in your lungs is still pressurized equally at depth to what it would be with scuba, so the same thing happens.

In scuba, you do replace the amount of problematic gas in your lungs as you breathe, which potentially leads to higher body saturation (as you're not just exhausting what's in one single lungful). But that's more about gas interchange at all than pressure.

If you spend long enough at depth, even without breathing, you run into the same problem.

Yeah, the "no-limits" with weights/bags is very dangerous. After a 240m/800ft free-dive, Herbert Nitsch had severe decompression sickness and needed a hyperbaric chamber[1]. I thought AIDA stopped tracking WRs for it because of the absurd risk, but all I can find is that they stopped doing competitions for it, though the article is dated 2011[2]. At least if you swim yourself down, you have time to listen to your body.

Note that these depths are absolutely insane for anything an ambient SCUBA diver would do, even for many technical saturation divers. If you want an actual breathing gas mix for that depth, using a mix of hydrogen, oxygen, and helium[3] may be the safest way (too much helium can cause HPNS).

[1]: https://web.archive.org/web/20121105083822/http://www.deeper...

[2]: https://gulfnews.com/uae/a-limit-on-no-limits-freediving-1.8...

[3]: https://en.wikipedia.org/wiki/Hydreliox

Until reading the first article, I was unaware that one could get decompression sickness from free diving. The Wikipedia article on this dive also says that Nitsch passed out from nitrogen narcosis (and specifically not oxygen starvation) towards the end of the ascent, something else that I was unaware of being a risk (not that I have any intention of trying deep free diving.)

https://en.wikipedia.org/wiki/Herbert_Nitsch#Later_attempt_a...

With regard to the hydreliox mix, I see it has a scant 0.8% oxygen, and needs to be less than 5% to avoid having an explosive mixture. Even the latter figure, I think, would lead to hypoxia in most people at atmospheric pressure, so I imagine these ratios are adjusted with pressure, and probably that these mixtures are only used in saturation diving.

Don't believe everything you read. While an extremely high PPN2 can cause unconsciousness due to narcosis, it wouldn't be possible to definitively identify that as the cause. There could have been something else going on physiologically.

Hydreliox has only been used in a few experimental or scientific dives, mostly with saturation procedures but at least once by cave divers. (It's possible that militaries could have done some secret dives as well.)

https://indepthmag.com/hydrogen-dreamin/

> While an extremely high PPN2 can cause unconsciousness due to narcosis, it wouldn't be possible to definitively identify that as the cause.

In a later post, I wondered (without any particular reason to think it might be so) whether the decompression sickness Nitsch experienced was manifest in neural tissue, and now I am wondering whether this 'nitrogen narcosis' was an early instance of the strokes which occurred during his treatment for that decompression sickness.

you're confusing something. as your thread sibling explains you cannot get the bends if you're not breathing. he got the bends from aborting the descent, requesting oxygen at that point from a scuba diving regulator and then ascending too fast (while breathing). the too fast ascent of course was medically correlated with why he aborted the free dive.

There wasn't any aborting of the dive, it went to plan going down and back up, until he blacked out during ascent at 100 m ("I lost consciousness due to narcosis (a first in free diving)"). The rescue divers surfaced him as he was unconscious, and then he asked for oxygen (presumably 100%) to do some (emergency) decompression at 10 m for 20 min.

Here's a statement from the guy:

https://www.deeperblue.com/herbert-nitsch-talks-about-his-fa...

Red Bull—naturally—also produced a whole movie about the thing:

https://www.youtube.com/watch?v=lcrz2YNBAw8

i stand corrected. so, one can get dcs even from free diving. i guess it makes sense considering the extreme depth and pressure. given that 80% of his last breath is nitrogen eventually enough will diffuse into the tissue and accumulate to cause dcs.

>...you cannot get the bends if you're not breathing.

Whether you are breathing is not fundamentally the issue - what matters is what gases you have in your lungs, under what pressure, and for how long. If you have nitrogen in your lungs, it will diffuse into your bloodstream, and thence to tissues, until the partial pressures are equalized. This is not normally a problem for free divers - even most deep free divers - because the combination of time and pressure are not great enough, but it seems that Nitsch might have crossed a line here.

I tried doing a back-of-envelope calculation to see if this seemed plausible. No-decompression limits in dive tables for air breathing bottom out at 5 minutes at about 60 m, and while the depth cutoff is presumably set by nitrogen narcosis and/or oxygen toxicity, it is clear that nitrogen absorption is quickly reaching the bends-risk level (for comparison, this dive reached a depth of 253m and lasted about 4 1/2 minutes.)

There are several factors to be taken into consideration. Firstly, Nitsch's lungs would have been considerably collapsed at depth, and so presumably less-effective at gas transfer. secondly, dive tables are conservative. On the other hand, though, they are based on the tissues in which the diffusion is slowest (if it was effectively instantaneous, there would be no risk of the bends on ascent) and assume the diver is ascending slowly even when they are not taking decompression stops, so maybe in Nitsch's case, the problem arose in different, faster-diffusing tissue (neural tissue?) than that on which the tables are based.

Given all this, I'm inclined to believe Nitsch's own claim that he had a case of decompression sickness from this dive.

Great video presentation on an experimental dive to 230 meters, using hydrogen: https://www.youtube.com/watch?v=skL5EQa8DFY

"The probability of survival in such a dive is greater than zero" - 20m17s

BTW, for anyone that has never dived to any depth, you don't really feel the change in pressure much at 20m or 30m, even though you are at 3 to 4 times atmospheric pressure. Apart from your ear canals, which can very painful if you don't manage to equalize them. The air from the scuba tank is fed at ambient pressure, so your chest isn't compressed and you don't really feel it on the rest of your body.

(Former freediver here)

Down to ~27 meters, there is also nothing special when freediving, assuming that you took the fullest possible breath before diving. This is because the water pressure compresses your chest, but it is within the normal excursion limits, i.e., it would compress to the same degree if you fully exhale on dry land, so it feels like something that commonly happens.

Below that, it feels like something tries to squeeze you softly. You can try to emulate this feeling on dry land by forcefully exhaling everything that you can plus a bit more with arms down, closing the glottis, and then lifting your arms up.

I think the article intro timeline is incorrect, Wikipedia on the page for Enzo Maiorca[0] reports the record in 1956 was already 41 meters.

[0] https://en.m.wikipedia.org/wiki/Enzo_Maiorca

An enjoyable article on free diving from 2009: https://www.newyorker.com/magazine/2009/08/24/the-deepest-di...

I wonder if there are lessons to be drawn from other marine mammals which can diver way deeper and longer than assisted humans, although they all also metabolise oxygen. How for example do cetaceans deal with the nitrogen in their bloodstream on returning to the surface?

Marine mammals have evolved adaptations that reduce nitrogen uptake, but there is some evidence to indicate that they do suffer from decompression sickness.

https://www.nature.com/scitable/blog/saltwater-science/do_wh...

evolution

I learned about shallow water hypoxia when I first took up SCUBA diving (before most of you were born :)). It occurs during ascent, for two reasons. The most obvious is that you've been holding your breath longer when you're ascending. The other is that when your body is under pressure, the partial pressure of oxygen in your lungs is higher, allowing the hemoglobin to absorb more. As you ascend and the pressure lessens, the partial pressure of oxygen decreases accordingly.

Apparently shallow water hypoxia is now a thing that life guards worry about, despite the fact that it's unlikely in a swimming pool. I swam underwater about fifty feet in my neighborhood pool, which is five feet deep, and was told by the lifeguard never to do that again. Indeed there's a sign that says "don't hold your breath underwater." Maybe you're supposed to breathe underwater?

I blame the American Red Cross, which has a history of dumb things IMO, starting with teaching the crawl stroke to non-swimmers instead of the side stroke or back stroke, both of which require far less coordination, and allow you to keep your face out of the water while swimming.

But don't get me started, or I'll tell you about "drown proofing."

A kid at my high school, who was on the swim team, died from swimming underwater. They did informal competitions to see who could swim the furthest underwater. He was practicing that by himself, and overextended himself, passing out underwater. There were other people in the pool but no one noticed until it was too late.

I'm sure that's not the only case. Between the risk of drowning, and the difficulty for a lifeguard to tell between someone holding their breath underwater vs actually drowning, I'm not surprised a lot of pools have "don't hold your breath underwater" signs.

wow, that's terrible. I was a competitive swimmer in my youth, and we did similar competitions from time to time (very rarely). I've never seen or even heard of a swimmer passing out underwater.

A more common drill we would do in practice were called "hypoxic" sets, where we would do one length of the pool breathing every 3 strokes, then the next every 5 strokes, then 7, 9 etc.. until you were going across the whole length (25 meters) without breathing. Not everyone could do it towards the longer distances without breathing, and the coaches would look out for "cheaters", but never once did anyone pass out. Maybe most swimmers, by way of the typical training and exertion in the pool, just don't develop a very good suppression of the "breath signal". I also never remember seeing anyone purposely hyperventilate so that they can stay under water longer.

A sad story. Was he hyperventilating? As the article explains, that makes chances of passing much higher.

I think you must have misunderstood your scuba diving instructor because you have that part almost entirely wrong. Scuba divers don't hold their breath while ascending. This is extremely dangerous due to lung over-expansion injuries (gas embolism).

https://dan.org/alert-diver/article/lung-expansion-injuries/

Among healthy people at sea level, hemoglobin oxygen saturation is already near 100%. You can't squeeze more oxygen into the hemoglobin regardless of inspired PPO2. What actually happens at higher PPO2 levels is that extra oxygen dissolves into the bloodstream and circulates unbound from hemoglobin. (This is why recompression chambers can be used to treat patients suffering from carbon monoxide poisoning.)

Ascending back to the surface reduces PaO2 (assuming no change in breathing gas) but this presents no risk of hypoxia since the diver would just be returning to normal levels. The only exceptions would be for a technical diver breathing a hypoxic mix (like less than about 15% O2), or using a rebreather where some sort of error or failure prevents oxygen from being injected into the loop.

No, I understood perfectly well that they were talking about coming up from a deep(ish) breath holding dive (snorkeling). But I learned this in the context of a SCUBA course, for which at least at that time the first half was snorkeling. I think that was to weed out people who weren't comfortable in the water. I realize I didn't make that clear when I wrote that.

They wrote "holding your breath _longer_", which I think just means slightly longer breaths so that you float up, complementing the floating device.

Holding your breath when ascending is the number 1 thing every scuba diver is taught not to do.

Divers may hold breaths a little longer or breath out a little earlier to make subtle changes to their buoyancy when swimming horizontally. But this is definitely now how you are trained to ascend or descend.

No, that's not how scuba divers ascend. Or at least not a safe technique, and not what is taught in certification courses.

Also a diver here. There are generally different biological mechanisms involved for “free diving blackout” vs “hypoxic blackout” (which is related to exertion while breathholding). Freedivers train to relax muscles and stay calm, so blackouts due to pressure changes are more common rather than to exertion. Swimming horizontally under water requires a lot of effort, which burns more oxygen. Combine high exertion with untrained individuals not aware of the risk, or misuse of hyperventilation (which doesn’t increase oxygen, but decreases CO2), and suppressing urge to breath, and you can also blackout.

Here are a few good articles: http://www.freedive.net/chapters/SWB3.html

https://www.redcross.org/take-a-class/resources/articles/sha...

It can even happen to more experienced individuals: https://www.wtkr.com/2015/11/24/navy-investigation-seals-who...

That's just 17 yards. I've done that far on a single flip turn. Did you just spend like 70 seconds under there or something? Some lifeguards are a bit weird.

I've always taken those "Don't hold your breath underwater." signs to mean like breath holding practice where you try and hold your breath as long as possible in one place. Not active swimming.

After watching pretty much every swimming event at the recent Olympics, there was definitely a lot of those turns spent underwater and not one of the swimmers seemed negatively impacted by it.

It's actually possible to swim faster underwater than at the surface due to differences in drag. In sanctioned swim meets such as the Olympics, competitors aren't allowed to go more than 15m underwater.

https://www.competitorswim.com/understanding-the-15-meter-un...

I guess the fastest stroke is the Fish Kick. Since you are kicking sideways and underwater, the pressure wave from your kick isn't being reflected from the surface or the bottom of the pool, allowing you to go way faster. It's bonkers! But I guess it's extremely difficult to pull off because of how hard it is to balance on your side while kicking underwater.

https://en.wikipedia.org/wiki/Fish_kick#:~:text=The%20fish%2....

Where I'm from everyone is taught breaststroke by friends/family just to be able to stay afloat. And if one goes to real swimming classes then they are taught crawl stroke next.

I'm somewhat confused about how the Red Cross is involved with shallow water hypoxia... could you clarify?

Also, now I'm curious about "drown proofing," if you wouldn't mind elaborating!

Sorry, I didn't see your question before. I think there's an article in wikipedia about it, but from what I recall: there were studies that said most people drown in water just over their head, not in deep water. Drown proofing was a technique where you held your breath, and sank. I'm not sure how that was supposed to work, since most people are positively buoyant, especially if they take a deep breath--something about kicking up so your shoulders came out of the water, and then you sank down at first from the weight of the part of your body above water, and then the rest of the way from momentum. When you hit the bottom, you kicked off with your body angled toward shore. Rinse and repeat, until you were in water where you could stand up.

What's wrong with that? Lake and river bottoms (and ocean beaches) can have troughs, you could easily wind up in water too deep to do this. And if you can kick enough to get your shoulders out of the water, why not just use your arms to propel yourself toward shore? It just seemed like teaching people a simple stroke (like breast, side or back--anything but crawl) would be easier and safer.

But then I fail to understand how people drown because they get caught in rip currents. Getting out of one is so easy.

The only drown proofing I can think of is what the Navy Seals do in BUDs. They tie their hands behind their back and their legs together and then pop up for air when they need it. It's a lot harder for those guys because their body fat percentage is microscopic so they sink like rocks. They would probably have to give me a couple pounds of lead to sink properly...

https://www.youtube.com/watch?v=1Pe3L5OcAEg

> there's a sign that says "don't hold your breath underwater."

What exactly do they expect you to do instead?

Exhale.

> I swam underwater about fifty feet in my neighborhood pool, which is five feet deep, and was told by the lifeguard never to do that again. Indeed there's a sign that says "don't hold your breath underwater."

That's insane and not far off from a level of conservatism that dictates "Don't get in the water, because it's dangerous."

If a kid is spending all day in the pool doing 5' breath holding underwater swims... maybe I'd be concerned?

But it's crazy to say there's a >0% but <1% risk... so we're going to ban it.

> I blame the American Red Cross, which has a history of dumb things IMO,

Recommendation by committee. Initial reqs are decent, then become increasingly unreasonable as people add "What if"s.

As the quip goes, engineering is knowing what trade-offs are worth taking, not being unwilling to accept any trade-off.

PS: Did you go to GT when they still had the mandatory requirement?

If I'm in the position of that lifeguard, I A) don't want to deal with any actual or near-drownings because I generally like people, B) am going to tell people to comply with the posted rules or I'll get fired.

If I'm the pool owner, A) I don't want my pool in the news as "the one where that kid drowned", and if it is, I want 100 witnesses where the lifeguard kept shouting at him to stop doing that or he'd be kicked out. I don't want a reputation of hiring lifeguards that don't. B) I don't want to deal with any wrongful death suits.

Sure, test your limits, but if you're going to do so, do it somewhere else. Ideally with supervision, but I won't tell you how to live your life. If you look up videos for 'static apnea', WR breath-holders have one or two safety staff in the pool with them in arms' reach.

who are all these people thinking we humans have reached our limit for diving? with each record broken, it just proves we can go beyond! let's see how far the human body can go, maybe we'll discover something new - like gills or something!

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