Looks like charcoal inside a bucket in a tent. If they're "feeling" dizzy, there's no way the CO level is high enough. It still can kill them if the CO level is low, but they'll have to endure all the unwanted, undesireable symptoms brought on by the CO. A adequately high enough level would be able to render someone unconscious within seconds, avoiding all the unpleasant symptoms from the CO.
I hope they don't suffer too much and that they reach the peace they deserve.