You don't need to fire coal to produce CO. Both natural gas and propane produce CO when combusted. In a home, a small crack in your furnace's heat exchanger is

You don't need to fire coal to produce CO. Both natural gas and propane produce CO when combusted. In a home, a small crack in your furnace's heat exchanger is all it takes. It's very common for natural gas to be used to heat homes and office spaces, at least in the US. Additionally, water heaters and dryers might use natural gas. Gas stoves do, too. You ever notice how CO alarm instructions have a blurb about not putting the alarm too close to your furnace, or too close to a gas stove?