> Reeeealy? How long would you like to sit in a room with a natural gas furnace whose exhaust is not vented outside?

That could be used to any furnace. There are byproducts in burning anything, that need to be filtered lest they start displacing the oxygen in the room at a fatal rate.

I wouldn't want to start a wood fire in my house unless the vent was open.

My natural gas furnace doesn't vent to the outside, and neither does my stove. Natural gas has orders of magnitude less pollution than coal.

Has no vent is not the same as does not vent. Try the same in an air tight bank vault and you will quickly notice the difference. Natural gas cooking stoves don't need vents because most kitchens are far from air tight. But, ocationally you see some nut try and seal off a room with plastic which then goes very poorly for them.

It's also worth pointing out that the amount of gas burnt on a stove is a good deal less than that burnt by a furnace, too. A typical burner is 7K BTUs, while a typical furnace is 80K BTUs.