@QueSyrahSyrah
Planes. Don't. Recirculate. Air.
Actually they do recirculate some (well most modern jet airliners do)..here's my take on it.
Air circulates around the cabin - on each lap some of it gets dumped overboard... to replace that loss there's a continuous top of fresh air from outside, traditionally provided from the front stages of the engines but on some newer types it comes from dedicated compressors.
The portion of air that does not get dumped and is recirculated goes through HEPA filters before being fed back the conditioning system and then into the cabin again...( I think that's been mentioned more than once today...)
End result is the whole volume of cabin air gets replaced multiple times an hour,
From the passengers POV they are either breathing newly introduced fresh air or air that has been through the filters. They certainly shouldn't be breathing unfiltered recirculated air.
FWIW in addition there's lot of clever work done on how the air flows through the aircraft to reduce things like, err...the travel of odours around the cabin, and coincidentally to some extent that reduces the likely hood of any other contaminants spreading.
TBH the air in an airliner cabin is probably better managed and certainly better filtered air than you'll find on most buses, trains, cars and in places like airport terminals etc.
HTH.