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I have often wondered about carbon monoxide accidents caused by free-standing heaters burning in a poorly ventilated room causing fatalities. I am aware that CO is explosive between 12 and 75% volume air and wonder why it doesn't eventually ignite from the flames or glow present when the CO level reaches 12%? (This never seems to happen). Or does the decreasing oxygen content in the air due to the combustion cause the fire to die out before the critical level of 12% is reached?

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    $\begingroup$ Your last sentence seems reasonable. $\endgroup$ – Mithoron Jan 29 '18 at 15:26
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    $\begingroup$ I would be much more concerned about dying in CO, it takes less that required for an explosion. If you have any" CO" in your room, you will get a headache. $\endgroup$ – blacksmith37 Jan 31 '18 at 3:39
  • $\begingroup$ In a completely unventilated room, with only CO being produced, to get 12% CO implies that you have used up 6% of the O2 (out of ~21% in the air). The remaining gas mix besides CO is no longer 'air' as understood in explosive/flammability ranges. $\endgroup$ – Jon Custer Jan 31 '18 at 19:58
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    $\begingroup$ Maybe it happens but nobody ever notices as anyone in the room died a long time earlier from CO poisoning. $\endgroup$ – matt_black Jan 31 '18 at 20:23
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Another possibility: the "carbon monoxide" formed by combustion is not pure enough CO to get to flammability (12% total CO) in air. The gas could be mostly carbon dioxide but still enough CO to poison you.

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I know from experience that carbon monoxide/air has a very low fame velocity. I think that air/carbon monoxide mixtures would make a very poor fuel air explosive.

I think however that by the time you get to the point of having sufficient carbon monoxide in a room to allow a flame front to start to travel from one side of the room to the other that unless you are wearing self containing breathing apparatus you will be dead of carbon monoxide poisoning.

Jon Custer is very right, if we consider methane as the fuel then we can write the equation.

$\ce{2CH4 + 3O2 -> 2CO + 4H2O}$

Thus to reach 11% carbon monoxide in a sealed room with a methane flame burning under very special and rich conditions to only form water and carbon monoxide. I think that all the oxygen in the room will be used up long before we reach 11% $\ce{CO}$ in the air. I think that the highest $\ce{CO}$ concentration we could get in the room would be 6.67% by volume. I think that as the oxygen level in the room will be so low that even with additional $\ce{CO}$ added to the room air then the room gas/air mixture will be unable to explode.

The only time I imagine you could get a carbon monoxide/air mixture to "explode" would be if a carbon monoxide cylinder was to leak and then an ignition source was to be added.

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