CO₂ isn't toxic the same way CO is. Mostly, with CO₂, the problem is a lack of O₂.

Carbon monoxide fools your body into believing it's oxygen and binds with hemoglobin and doesn't un-bind easily. Because it's not oxygen, nothing will accept delivery. You breathe too much CO₂, and all you have to do is breath good air. You might get a headache, but you won't die. If someone has CO poisoning, air alone won't help. (You can get somebody with CO poisoning outside in good air, and they can still die later.) A hospital might.

The most important thing in most cases is a cell phone. That's only my opinion, but not getting stranded alone, with no one knowing where you are, seems to be important. A space blanket or something for warmth is a good thing to have on board.