The instructions of my Coleman heater says to have a window or tent vent open but I haven't died yet from keeping my window closed. It has a high and low setting and I only use the low setting. My bedroom is 8.5' x 11.5' in length and width, my ceiling slopes from 7' to 6' gradually over the 11.5' distance. This will be my 3rd cold season using it. I also have a carbon monoxide detector on my bedroom wall, it's never beeped yet. I'm alright on the safety side.
I keep 2 carbon monoxide detectors and 2 smoke detectors in my living room where I have a wood burning heater. My wood burning heater is a closed unit, pretty safe, I sometimes use the "fireplace" term interchangeably even though there is a difference.
This is my "fireplace" >