Modine Heater

I keep hearing that unvented propane heaters are unsafe for bedrooms because of carbon monoxide, etc., but why are they OK to use indoors, but just not bedrooms?
P.S. – I have a carbon monoxide detector that plugs into the wall.

We have a large room we are trying to heat in an adobe block barn. The room is 80 X 40 and quite drafty. Would it be safe to use a propane heater in this room for heat, the kind they use on patios for outside dining?