Do not confused CO (carbon monoxide) with CO2 (carbon dioxide) CO is way more dangerous than CO2. It blocks the red blood cells in the blood and no oxygen is transported through the body. The amount required in the air to do this is very small.
It is usually generated if the burn is inefficient. So if the room is enclosed the flames uses up (depleted) the oxygen, this results in a poor burn generating CO. Wrong jets could also cause the same issue along with a few others.
That's why we normally have a chimney/flue, the CO just vents harmlessly into the atmosphere. There are deaths every winter from CO poisoning. Good ventilation reduces the risks I said earlier, you fall asleep and simply don't wake up.
Obviously there is the other issue of a gas leak,,,,bang.
More here,
https://www.health.harvard.edu/a_to_z/c ... ing-a-to-z
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