Carbon Monoxide, just to be safe. I just got back from FAARRRRRR west Texas. Big Bend and Terlingua. Used my Coleman several nights. I had the tent vented enough that you could feel a breeze but the heater still raised the temperature enough to where I could sleep nekid in a 40degree sleeping bag. The only real sad part is the 1 lb canisters only last 5 ½ hours so sometime during the night it’ll get really really cold and you have to put a fresh canister in.