from Rideski:
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Yes, but just not sure why anyone would invent a heater that does not work above a certain altitude, or that is not certified to be safe while sleeping. Cheaper materials and avoiding litigation is what I am guessing. Read quite a while and never found the answer.
It has something to do with air pressure and possible propane leak and fire .
Nothing that can use up all the oxygen is safe for sleeping either,so you need some kind of ventilation.
And of course CO can kill you also,and a CO alarm is cheap