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Carbon dioxide is heavier than air, so it settles into cellars and pits and waits there.

Chemistry Facts, sourced to PubChem, National Library of Medicine.

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It is about half again as dense as air, which matters because the gas has no smell and does not trigger the alarm that makes you gasp. That alarm is set off by carbon dioxide building up in the blood, not by a shortage of oxygen, so an atmosphere that has displaced the oxygen with something else is more dangerous rather than less. Brewery cellars, silage pits and volcanic hollows have all killed people this way.

The claim comes from PubChem, National Library of Medicine.

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