I experience the same thing when I smoke certain strains, which strains they are I have no clue... Even after cardiovascular activities I REALLY notice how low the oxygen in the air seems to be.

Also when trying to meditate I can tell that the 21% of oxygen in the atmosphere is pretty low, but juuuuust high enough to keep us alive.

OSHA claims anything below 19.5% is deficient. That's cutting it pretty close!