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Comment by bdonlan | Hacker News Reader

When you hold your breath, oxygenated air remains in your lungs. This allows you to maintain consciousness for potentially a minute or two.

In a decompression event, the air is sucked out of your lungs. The air that remains has a low O2 partial pressure, and so oxygen can leak from your bloodstream into your lungs. As such the body's normal mechanisms for storing oxygen simply don't work.

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As I read your comment I suddenly decided to blow all the air I could from my lungs and see how long I could go without a breath. I lasted 10 seconds.

It's probably worse than that. When you blow out all the air from your lungs, what's left is still at atmospheric pressure. In a decompression event the pressure drops very quickly unless you plug your nose or something. A sibling comment has more concrete numbers https://en.wikipedia.org/wiki/Time_of_useful_consciousness

When learning to scuba dive, they caution you not to ever hold your breath - not when at a stable depth, not when descending, and most certainly not when ascending - because the pressure differential can cause serious injuries. (This goes to the point of "if you run out of air at the bottom and need to make an emergency ascent, hum for the entire trip up to make sure you're breathing out slowly")

Now, you have larger pressure differentials when scuba diving - every 10m of depth is an atmosphere of pressure - but the half atmosphere that you get from 10k vs 40k feet is probably still enough to cause some serious issues. (10k feet is ~0.69 atm, while 40k is ~0.19)

I lasted 30s, and even then I was compelled to breathe in, without using any “useful consciousness”.

Male, 32 years, healthy, medium sporty.

This doesn't really tell you much since the compulsion to breathe is caused by rising CO2 levels, not lack of oxygen. If you breathe out, the remaining air quickly saturates.

I lasted 20 and it gave me an intense headache that‘s still there after a minute. I would probably black out after 30/40.

> In a decompression event, the air is sucked out of your lungs

Correction, sir. That's "blown out."

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