I’m speaking out of my depth here but I believe exposure related disease is based on the particle density over time. Particles can clog the respiratory system, reducing blood oxygen and introducing free radicals. To a large extent the body is able to remove the debris, but if there is too much buildup due to prolonged exposure to high AQI then it can be difficult to recover without intervention.

I was at a chain drug store the other day and saw a display for canned oxygen, regular and Peppermint flavor!?! It seemed like another sign of the apocalypse to me, but maybe just a gimmick or stopgap for terrible air quality events.