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Say I'm alone in a sealed space without any air coming in. About big would the space need to be for me to survive 12 hours?
Anon what are you planning? Don't. I'm calling the police.
I'm writing a short story where a character is trapped underground and has to contemplate murdering another character in order to preserve more oxygen for himself.
In that case the oxygen will be the less of a problem. The air is 21% oxygen, but a CO2 concentration of 1% will already make you drowsy.
Got it. Can you give me a ballpark estimate of how big a volume of air you'd need to survive for 12 hours without dying from excessive CO2?

Of course it'd vary from person to person, but I'm just after a ballpark estimate. I appreciate your time.
Is it only 2 people down there?
Yes. The character's thought process is, "It could take them 12 hours or more for us to be rescued. That means my odds of survival would go way up if the other person in the space with me were dead." So I'm looking for a volume that would allow one person (not two) to plausibly survive for 12 hours.
What percentage causes which effect is listed in Wikipedia.
According to a quick Google search, a human exhales 500 Liters of CO2 per day, so do 2 people in 12 hours. Let your character do the maths. What percentage is still ok? 500/(percentage) gives you the volume. If you say that it shouldn't exceed 10%, the cave can't be more than 500/0.1 Liters = 5000 Liters = 5 cubic meters. If your character draws the line at 3%, he must kill the other person if the cave is smaller than 500/0.03 Liters ~ 17 m^3
Thank you for your help.
It is proportional to the number of calories you burn, almost directly proportional with very slight variations based on how much hydrogen gets burned in your calories vs carbon.

It's roughly 1kg of CO2 per day for the average human. Also note that you produce more water in your piss, sweat, shit and breath than you take in because the hydrogen you burn from your food get's turned into metabolic water. The kangaroo mouse actually drinks nothing and gets all it's moisture from the dry food it eats and metabolic water.
Does a dead body produce CO2?

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