Its not the CO2 they might put out that is dangerous, its the CO (Carbon Monoxide), and yes they do put it out. I have an 85K BTU heater of the Reddy Heater type, made by Koening I think, today it would be rated at 110K BTU for the same size unit. You must have fresh air coming in if you are using in a ordinary garage. Crack the door up a couple of inches, prop an outside man door open a couple of inches, crack open a couple of windows, but keep some ventilation in there. If you notice any kind of headache, shut it off, and get to fresh air right away. If the garage is attached to the house, use lots of ventilation to keep the CO from getting in the house.
I use K1 fuel now that I have to buy it. DON'T use diesel, it stinks! previously, for years, I used Jet A, it burns very clean and with little odor, and I didn't have tank or inlet screen problems from contaminated fuel, and this was "sump" fuel from the bottom of the truck.
The forced air heaters, aside from ruining your hearing, do not burn clean, hence the smell from unburned hydrocarbons and other stuff in the fuel, and as a result of the poor burn, produce CO. Wick type kerosene type heaters burn very clean and produce very little of CO or smell.
CO is odorless, and colorless. It binds to your red blood cells, and prevents oxygen from being absorbed into the red blood cells.. It takes hours to work out of your system, and not amount of oxygen will speed up the process.
Charles