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Comment by refurb | Hacker News Reader

Partial pressures.

You can breath at 35,000 ft without a pressured suit, but go much higher and you can't.

At sea level, you have 760 mmHg of air pressure. Oxygen is 21% of the air mixture, so you have a partial O2 pressure of 160 mmHg.

At 35,000 ft, air pressure is 179 mmHg [1], so if you breath 100% pure oxygen, you're getting the same amount of oxygen you'd get at sea level.

Go up to 50,000 ft and the air pressure is only 83 mmHg, so even breathing 100% oxygen, you're only getting 50% of the oxygen you'd get at sea level.

[1] http://www.engineeringtoolbox.com/air-altitude-pressure-d_46...

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The OP said you needed a pressure suit, and that an O2 canister wouldn't help. Since the pressure of the O2 cannister will be down regulated to 1ATM, I can breath 1ATM air, no matter where I am, including 35,000 feet.

Is there, perhaps, some reason why I can't? Lungs exploding because of the 1ATM pressure differential, for instance? It seems that 1ATM is not that much, but perhaps it is... That's what I'm curious about.

You can breath pressurized air directly from an air mask without wearing a pressurized suit. Many military aircraft use this system (the air mask is strapped tightly to the face to maintain pressure).

I'm sure at some point you need a pressurized suit to make breathing possible, but it likely has more to do with preventing decompression sickness and protecting against the cold (at very low pressure the moisture from your skin will rapidly boil, cooling off your skin).

I found this. Towards the end, it starts talking about positive pressure respiration at high altitude. http://webs.lanset.com/aeolusaero/Articles/A_Brief_History_o...

That's a great link. Here is another http://www.nasa.gov/centers/dryden/pdf/87912main_H-1093.pdf both of which say that if you have an air tight mask correctly valved you can breath at 40,000' on stored oxygen. (in the NASA case they were interested in getting between really high altitude and then down to 40K' in the event of a loss of cabin pressure)

The summary then would be that you aren't going to survive in a wheel well of an air craft above 35,000 without carrying both oxygen and a delivery system that is capable of maintaining an adequate partial pressure in your lungs.

Now when I visited NASA AMES during one of their many open houses we got to look at one of their U2 research planes and either I or one of my kids asked the pilot why they wore a 'spacesuit' and their response was "Because oxygen masks don't work above 35,000 feet." which, more accurately it would seem its harder and harder to make a working system as you go above 35,000'.

Very cool discussion btw, clearly its a very nuanced kind of thing.

Sea level is not the minimum oxygen levels tolerable by your body. Why, just a month or two ago I climbed to 14,000ft. Made me dizzy and nauseous , but the partial pressure was somewhere around 80mmHg.

Unless you hadn't showered for a long time prior, you were probably nauseated, not nauseous.

Nauseous has at least two meanings, one of which is causing nausea. The other is being inclined to vomit.

The OED attests that silverstorm's usage is the older one, predating yours by 8 years (1604 vs 1612).

Regardless (and I feel like such a broken record saying this), meaning in language is determined by consensus, not dictum, and at least for me (a 21 year old who grew up in the United States), "nauseous" can have no meaning but "sickened". "Nauseating" is what I would use for your meaning.

This is true and something I should have mentioned.

Humans can function to varying degrees at lower partial pressures of O2 than you experience at sea level (obviously, since people live at altitudes of up to 12K ft).

If you have not adapted to high altitude, you will experience cognitive decline at altitudes much higher than 10K ft, although they can be imperceptible. You can also go up to 30K ft, lose consciousness, but experience no long term harm as long as it's not for extended periods of time.

There have been glider pilots who have been caught in massive up drafts, climbed to 30K ft or higher, lost consciousness, then regained consciousness when they descended and safely landed their planes.

http://www.smh.com.au/news/national/dead-luck-ewas-flight-of...

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