9

I used to think this question was a bit ridiculous. The volume of air that we take in while breathing may be high but the lungs only consume 25% of the oxygen for any breath and the rest is returned. Closing the windows while sleeping in a room or sleeping in a closed tent does not affect one's breathing. (At least, it never did for me and I have never heard of such a case.)

However, when I tried my new bivouac sack (measures 2.1 m long, 66 cm wide, 50 cm high) last weekend there was a point during the night where I experienced some unrest. I could not sleep (which is not that unusual when it is cold) but also my breathing became very heavy and shallow. More surprisingly, when I opened the bivouac sack's door, the breathing relaxed immediately. I did some background research on this and from this calculation, it seems that humans consume one cubic meter of air in half a day. So if we break this down to a quarter cubic meter for 6 hours of sleep, it might come close to the volume of air that was available in the sack. (But then again I still can't believe it.)

In the end, I left the door a bit opened for the whole night. However, I do not think this is solution will always work.

Now I wonder, was what I experienced mainly psychological or are oxygen shortages while sleeping in a bivouac a real thing?

Additional background that may or may not inform answers to the question: The temperature was about 0 °C (32 °F), the air was humid (rather foggy, close to a river) and the elevation was at 500 meters above sea level. I went to stay in the bivouac at 7 pm, the breath shortages occurred, I think, around midnight.

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    Just an FYI for your research until someone answers: the gas that matters when discussing respiration like this is CO2, not oxygen. CO2 is what controls your body's breathing systems. – whatsisname Feb 23 '17 at 7:53
  • So you are saying, it is not about depleting oxygen but about accumulating CO2 in the bivouac? – eigenvector Feb 23 '17 at 7:58
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    yes, that is the idea – whatsisname Feb 23 '17 at 8:00
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    Alpkit found this to be a real issue. alpkit.com/develop/bivvy-bag-development – vclaw Feb 23 '17 at 11:28
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    Incidentally, on oxygen content: the air we breath is 21% oxygen, and the air we exhale is 16% oxygen. So the difference is 5 percentage points, not 5%. It's actually more like 25%, going from 21 to 16. – Pete Becker Feb 23 '17 at 13:22
7

If you put a plastic back over your head, you are going to suffocate. A good bivvy bag should be designed such that suffocation is not an issue. However, breathing into a bivvy bag is a terrible idea because the vapour in your breath will condense and make you wet.

Make this question a non issue by not breathing into your bivvy bag. Leave an opening. Check the manufacturers recommendation on your product. I bet it gives the same advise.

Andy Kirkpatrick has talked about a time when he was bivvying in Patagonia in a special bag made by Rab Carrington. The bag had a 'snorkel' of sorts to allow for breathing. During sleep Andy had a suffocation experience because the snorkel had collapsed. Sorry i dont have a link to this.

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I would not have guessed you could deplete O₂ or saturate CO₂ but evidence that you can.

You compared to a tent. A typical backpacking / climbing tent design is different. The walls and ceiling are breathable with a rain-fly over the top. Even a 4 season tent is breathable.

without rainfly with rainfly

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