system
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human
ELI5: Why do canaries show symptoms of poisonous gas before humans do, in the context of canaries in coal mines?

In elementary school, I was told by the teacher that canaries have smaller lungs than humans, making them more susceptible to poisonous gas but that answer never sat well with me because smaller lungs should take in less total poison dose. Also birds are smaller than humans so the poison concentration should be fairly similar on a per weight basis. The only way I could see the bird showing symptoms first is if it has a disproportionately increased respiratory and metabolic demands compared to humans, resulting in faster accumulation of poisonous gas or that poisonous gas of equivalent dose is more potent towards canaries than to humans?
gpt
Canaries have small lungs, rapid breathing, and fast metabolism. The end result is that they go through a disproportionate amount of air for their size.

Carbon monoxide also isn't "poisonous" so much as it disrupts your circulatory system, there isn't a fine level and lethal level like Hydrogen Sulfide or other poisonous gasses.  CO preferentially bonds to hemoglobin preventing oxygen from bonding to that site the next time the blood passes through the lungs.  A long exposure to a low level of CO can slowly build it up in your blood stream, the faster you go through air the faster it will build up, then you'll become confused and tired.  If you're a canary that means you stop chirping well before you die.

Canaries in coal mines were really just early CO detectors that stopped chirping when there was gas.  The body weight doesn't actually matter because its not really a specific level that causes problems, a low level and long time or high level and short time can both cause similar issues(confusion, drowsiness, and eventually death)