Interesting subject. The comment about the camp stove needs just one addtional comment: Do NOT use it in the shelter with the door closed. It wouldn't take long for the carbon monoxide and oxygen depletion to kill you.
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Also, I'd think that some kind of a carbon dioxide scrubber might be necessary even in the short term if you have 6 people in a 6x8x10 space(you didn't state the length of the shelter so I used a WAG) that's only 80 cubic feet per person. It won't take very long for the air to get stale unless your vents have fans to exchange the air. If for some reason the vents get obstructed by debris the situation becomes critical quickly. You'd have about 5 hours of usable air
if everyone is being still.
Discovery Health "How much oxygen does a person consume in a day?"
Regards,
Hobie