The oxygen required for keeping those flames lits will be drawn down to the basement through the vents. Otherwise, the flames would eventuually go out. I don't think you have anything to worry about. If you've trained down there before and havent felt dizzy from lack of oxygenn, I'm sure it's fine. Humans require an atmosphere of 21 percent O2. Any less than that, and you'll notice it very quickly.
I'd be more woried about carbon monoxide and radon levels if the ventilation sucks, though I wouldnt really be that worried at all.