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Comment by ziddoap | Hacker News Reader

>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).

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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.

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