Got this heater for Christmas to use for winter tailgating. It throws out a good amount of heat to huddle up against. However, it does indicate that the unit is safe to use indoors. I happen to be 
a health and safety instrumentation technician (primarily gas detection) and brought one of my detectors home with me. I tested the heater in an old uninsulated shed with the doors open. Within 3 minutes the CO2 levels spiked to 3000ppm and the O2 had dropped by .3ppm. I would not suggest using this indoors