The effects people are worried about are at 1k to 2k ppm. Even assuming CO2 is displacing O2 only, the drop in O2 levels is less than the change in about 100ft

The effects people are worried about are at 1k to 2k ppm. Even assuming CO2 is displacing O2 only, the drop in O2 levels is less than the change in about 100ft of elevation. Since oxygen only accounts for about 1/5 of the displaced gasses, you're talking about a difference in about 20 ft of elevation. Not enough to notice even for horribly out of shape people, and definitely not enough to cause the kind of effects reported.