Here’s the thing: blood in a healthy human is almost always at near full oxygen saturation, even when heavily exercising. IIRC something like 98% saturated pretty much at all times. Breathing faster and heart beating faster? That’s your body’s way of keeping your blood maximally oxygenated. You can test this easily with a $20 pulse oxymeter that you can find online or at the pharmacy. Put it on, run like hell, and watch how your blood oxygen doesn’t really drop. If it does, call your doctor because it’s very likely something’s wrong.

And if the blood leaving your lungs breathing regular air is already very near saturation, that means it doesn’t have the ability to pick up much more oxygen even if it’s available! You only absorb about a quarter of the available oxygen in regular air in a regular breath to begin with, adding more oxygen is pretty much useless and will just get exhaled! We specifically have re-breathers for diving and space suits because we can’t absorb all the oxygen in one breath, and you don’t want to waste any to exhalation when your supply is highly limited! The reason your muscles run anaerobically when exercising is the bottleneck in delivering oxygen from blood to muscles, not a bottleneck in gas absorption in the lungs because the oxygen concentration is too low!

Humans evolved with 20% oxygen in the air, that’s the optimal concentration for us and the most we can take advantage of! The only people this is useful for is people who have trouble absorbing oxygen, but those people should be getting recommendations from their doctor and not buying random “health” products. Or, maybe if you’re a dinosaur and you evolved under something like 60% air oxygen, which was actually why animals were so big back then. /rant