breath then don’t snorkel. If you can run a mile without stopping there is zero problem. You can tolerate a little hypoxia and hypercapnea (slightly low oxygen and high carbon dioxide). If you can’t walk up two flights of stairs without stopping or having chest pain or shortness of breath which stops you, then don’t do it.

There are mountains of research that would support that.

I guarantee the guy in this article struggled with going up 2 flights of stairs without feeling very short of breath. I’d say he’s pushing 260lbs at least and likely very unfit.