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

Thx for this! Checking pics of my readings, A very empty flight crossing the Atlantic had 759; a packed one had 2,227. Still, not as bad as on some trains where I got almost 2,600 😬 If CO2 meter is not so reliable as a proxy in a plane, is there another option? Another meter? Cut the reading by 1/2?