Ethanol is the gel fuel for these units. The chemistry of ethanol is C2 H5 OH. Combustion will combine the fuel with oxygen forming 2CO2 + 3H2O Not too harmful; however if the combustion is for any reason incomplete the carbon will oxidize to CO rather than CO2 and that would be bad. I would guess that in combination with a CO monitor the units are fine, probably on the order of burning sterno (C5 H10 O3), but definitely not in tightly enclosed unventilated rooms.
I like the electric option or vented gas if you want heat. Besides, how much does this fuel cost? I saw $60/case of 24 posted on this website to an earlier inquiry that was complaining about the high cost of this stuff. Pretty pricy fire!