Comment: Probably because your stove & oven are vented

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In reply to comment: Hmmm, I have a question... (see in situ)

Probably because your stove & oven are vented

so the carbon monoxide that is released goes out the outside vent. When you just leave the burners on or the oven door open for a long period of time it is released out into the room. A gas furnace has a venting system usually through the roof and unless your home is tightly sealed it usually does not build up to a dangerous level. But even so, it is suggested and in some places mandated that a carbon monoxide detector be installed. Hope this helps.