HEPA filters, it doesn’t refresh the oxygen so it would be very dangerous to ~only~ filter air in a sealed space Aircraft air is said to be 50/50 in flight (50% fresh air/50% stale air HEPA filtered) however there are additional risks of close seating & filtration off at boarding/disembarkation

so if air is 50/50 in flight the CO2 reading will “read” the stale air without knowing it has been filtered, so the C02 reading doesn’t pick up the risk reduction that comes with filtration