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Comment by jsjohnst | Hacker News Reader

Considering that Trimix and Heliox used in deep commercial dives can have the majority (Aka >50%) of the breathable “air” be helium, your theory that there’s really any risk of danger to people seems a bit unwarranted to me.

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Helium isn't toxic but it displaces oxygen. Heliox still contains enough oxygen to breathe (at sea level it's 21% oxygen just like normal atmosphere, and when diving the partial pressure of oxygen is the same as oxygen in normal atmosphere). But if you release helium into a room full of air, patients with already-compromised respiratory systems from injury or from respiratory diseases or from opioid painkillers could be at risk for brain damage or death if they were borderline already and not being monitored closely enough.

The author's cavalier "I bet the nurse’s voices were higher pitched that day!" was incredibly inappropriate given the potential danger.

Hospital rooms aren't hermetically sealed. Helium dissipates so quickly that this isn't a realistic safety concern.

> Helium isn't toxic but it displaces oxygen.

While technically correct in theory, in any real world environment Helium will not displace Oxygen unless you are dumping orders of magnitude more helium than in this scenario.

> Heliox still contains enough oxygen to breathe (at sea level it's 21% oxygen just like normal atmosphere, and when diving the partial pressure of oxygen is the same as oxygen in normal atmosphere).

There’s so much wrong with this (and the rest of your post) I can’t think of a polite way to respond, so I’ll just wish you a good day.

I’ll admit I haven’t gotten that far in my diving certs yet, but isn’t the point of Heliox so that you can have less than 21% O2 at high depths to avoid narcosis and a fatal ppO2?

Kind of, a fatal ppO2. But in a hospital room you have the other kind of fatal ppO2, the kind where you pass out and need an O2 mask.

To simplify it a bit.

The point is to keep partial pressure of oxygen below that of maximum safe limit of 1.2 to 1.6. For example, 100% oxygen at surface is 1.0 and thus safe but will very fast reach maximum when you increase the pressure just by a few meters. Regular air with 21% oxygen will start to be a problem at 50 meters, so divers that need to go deeper need to use mixes that has less than 21% oxygen. When a mix is less than 18% oxygen they become hypoxic and cannot safely be used at shallow depth.

So the point is not to get "the same as oxygen in normal atmosphere". It is to keep the partial pressure of oxygen within safe limits, usually between 0.18 pp02 and 1.4 pp02 depending on a multitude of factors and safety margins.

The only point I was trying to convey with that statement was that there might be much less oxygen by volume in deep-diving heliox but the partial pressure of oxygen is still maintained so that cellular respiration can continue normally. Was that the only thing you thought was outrageously wrong with my comment?

I am not jsjohnst, but the only additional nitpick is that Heliox is any blend that is exclusively helium and oxygen and the reason why heliox is more commonly used for deep dive is that helium has less resistance compared to nitrogen, meaning that pushing compressed air through ones respiratory system takes a bit less effort. Helium in heliox is thus used to displace the nitrogen, with the oxygen ratio being adjusted based on health, depth, bottom depth, purpose, decompression, surface interval and so on.

Helium rises. Not slowly. It really rises very quickly. Patients are in beds, horizontal, many also on oxygen feeds. The nurses are standing. Their voices would indead become higher in pitch long before patients were impacted.

If thier voices were rising, id suspect helium but would be worried that some reaction released hydrogen. Same squeeky voices but a serious fire hazard.

I remember these constant problems with helium tanks in rockets. It's way harder to keep helium in place.

You need ~0.2 Bar of oxygen partial pressure. The purpose of Trimix/Heliox is to keep oxygen partial pressure under 1 Bar while also reducing nitrogen to prevent narcosis. You can't breathe the 200m mix at sea level pressure.

> You can't breathe the 200m mix at sea level pressure.

What makes you think you can’t (other than it being hypoxic)? You most certainly can breath Heliox, in fact, it has been used medically[0] for almost 100 years, far longer than used for commercial diving.

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

Heliox doesn’t denote a specific concentration of helium and oxygen; even just within diving the mix varies depending on the depth it is to be used at.

Correct! A common ratio for deep diving is 12/88, but 21/79, 30/70 and 40/60 are also used at less depth.

In fact, Heliox would usually not be used for dives to 200m due to HPNS risk. Usually it would be Trimix for that.

Either way, GP is mostly wrong on the points they made.

Also, the oxygen partial pressure you need to stay under is 1.6 Bar for oxygen toxicity. Huge difference between 1.0 and 1.6, depending on oxygen percentage, the difference could be a >50% reduction in MOD.

Thank you for spelling "breathe" correctly. It seems these days I see it spelled wrong more than I see it spelled correctly.

The only risk is that the O2 levels drop too low from a massive release of He...and while I'll 100% agree this is a very minor risk, I'd still rather hospitals play it safe and install O2 deficiency monitors.

The article I linked mentioned that O2 monitors are legally required for hospitals in NYC. That regulation might have been created because in 2000 someone working on an MRI machine died from a Nitrogen leak (it's also used for cooling and like He, the only risk is that it displaces O2). It's likely this person wouldn't have died if an O2 monitor was in place that sounded an alarm fast enough for the victims to leave the area. http://www.nydailynews.com/archives/news/nitrogen-gas-leak-k...

edit: googling a bit more found some interesting references on dealing with "oxygen-displacing gases". O2 monitors are on the long list recommendations/requirements. One thing I found interesting is that at one laboratory part initial assessment involves a controlled release of the maximum amount of gas that will be stored and then measuring the drop in O2.

https://www.ors.od.nih.gov/sr/dohs/Documents/ProtocolOxygenM...

http://www-group.slac.stanford.edu/esh/eshmanual/references/...

It is the main reason you have to be careful with working with liquid nitrogen in closed rooms: the danger of displacing too much oxygen. While our breathing air consists of 80% nitrogen, when you evaporate large amounts of liquid nitrogen (like 10 liters in an instant) in a closed room, there is the danger of displacing oxygen to a dangerous level. Working with smaller amounts (like 1 liter) is quite safe though.

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