It seems to me that some posters have limited experience with gas appliances. While it is wise to be aware of the dangers of CO2, use of a camping/backpacking stove or lantern for emergency cooking and illumination is really not as dangerous as some warnings make out. Unless you are in a small space or a room specifically designed to be air-tight, the CO2 put off by a single gas lamp and, intermittently, by camping/backpacking stove poses no more risk than using a residential cooking appliance that uses natural or propane gas.

As for wood-burning fireplaces, wood-burning stoves, coal stoves, even vented gas fireplaces and heaters, these things are all designed to exhaust fumes outside. Many people, including myself, have spent weeks at hunting/vacation cabins without electricity, relying only on wood-burning and gas appliances for heat, light and cooking. Our ancestors lived with these conditions 365-days a year.

I wouldn't use an unvented gas (or any other) appliance for heat without cracking a window. I wouldn't use dirtier fuels such as charcoal or any liquid fuels indoors, both because of safety (spilled, burning fuel) as well as fumes. And the only thing I'd trust in a tent is a well-protected candle.

We've had as many as six adults stay in a 12x16', 1 room cabin for several days, with the wood stove going continuously, cooking most of our meals on an RV-size propane stove/oven combo, a Zodi propane unit for warm water, and Coleman propane lanterns for light. The cabin is fairly tight (to keep out the critters), but we never had any issues related to CO2, (cramped space and various bodily odors were another story).