One a friend and I hooked ourselves up to continuous pulse oximetry and had a contest to get the lowest recorded oxygen level. We tried everything we could think of, from just holding our breath to end-expiratory breath holding to hyperventilating to clear O2 (I used to do some recreational free-diving) beforehand to exercising (jumping jacks)...
Neither of us could get it below 98%, and this was at a mile of elevation (UNMH in Albuquerque).
Neither of us could get it below 98%, and this was at a mile of elevation (UNMH in Albuquerque).