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Furnaces use methane or natural gas for fuel. The chief danger here is explosion. Natural gas is odorless, but they add a rotten egg chemical to it to alert you to the danger of explosion.
Whenever you have combustion, you will produce mostly carbon dioxide (CO2) but also some carbon monoxide (CO). Carbon monoxide is particularly dangerous because it is toxic at much lower levels than CO2 . DC was in no danger from either of these if his funace was out, because they are not produced except from combustion.
DC, if your pilot light goes out frequently, it could be a sign of backdraft which does lead to eleveated levels of CO in your residence.
Any gas, methane, CO2, or even N2 can be dangerous if the concentration is high enough, because it prevents you from getting O2.
If you have a gas furnance, you should get a CO and methane detector to protect your self from these risks.
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