' Oxygen deprivation tents or “altitude tents” are risky at best. '
But I see their use and that of other athletic oxygen deprivation gear.
In all fairness, our swim team used to do freestyle "hypoxic" drills. For example, we would breathe every other swim stroke for a pool length, then every 3rd stroke for a length, then every 4th stroke, ... until we failed. Once we failed we would fall back to breathing every other stroke. Nobody, to the best of my knowledge, ever passed out but one could feel the hit to one's brain when the O2 level got too low (or the CO2 too high, or whatever).
Those drills were uncomfortable, to say the least, and I was never convinced that they did much good. OTOH they did help me to relax in holding my breath under circumstances where breathing was not possible (e.g., in turns and underwater pushoffs where you might not breathe for nearly a pool length, and they trained me not to panic b/c I needed a breath. I suppose that, thanks to such drills, I might be a little harder to waterboard than the usual suspects, but I chose a less exciting line of work!8-))