What's the problem with using this type of heater for heating a small one bedroom loft?

Your place won't be an oven in 2 minutes. Once you turn it off, the room will cool down faster then you expect. The fumes from the direct propane heat will give you a headache eventually, Ever heard of Carbon Monoxide poisoning?And when used on a site, windows are usally opened. I can just hear the news reports now.. what is your real name for future reference??