I've been using the Coleman sportcat heater at 9,000ft plus elevation for years. I don't sleep in a tent. I have a topper on my truck and sleep in the back. The first year I used the sportcat I took a carbon monoxide detector along to make sure everything was safe. It never went off and I didn't die so I assumed the air quality was ok.
I don't think the propane is combusted. Even though you light it initially I think it reacts with whatever is in the honeycomb of the heater to make an exothermic reaction and that's what makes the heat. Apparently carbon monoxide is not a byproduct of this reaction.
I do keep the side windows of my topper cracked slightly though, just to keep the air circulating. I know using the heater has improved the quality of my sleep, 'cause it sure gets cold up there! I used to sleep in my hunting clothes and completely bury myself in my sleeping bag. Now I sleep in my underwear with my head uncovered.
My two cents worth.
