Hmmm. Boarding a flight for an urgent work trip. Thought I might look at the amount of recycled air inside the plane. Steadily increased from 600ppm to 1300ppm as the door closed. Being on the ground is the worst air.... supposedly. Let’s see. 
Backed away from gate (disconnected from APU.
Taxiing.
At cruise altitude now for about 20 minutes. Bouncing around at 1450-1550ppm.
Caveat to all of this. BOS-LAX. I am fortunate that I am sitting in business class without a neighbor (my requirement of flying during a pandemic). But I am also in a little box of my own. Little encased cabin which rises above my head height when seated.
(A few hours back.... airports empty again)
I was interested in the recycled fraction. The air coming straight from the nozzle is a mix of outside air and direct from HEPA filter air. I saw 1200-1256 at 1 minute intervals over 5 minutes (my arm got tired after that).
Time out from the science.... this is not your ordinary plane food! Damn! Yes, that is a lobster roll!
Mid flight.
Dropping as we descend
Spiked again on landing
Then the jet bridge
Jet bridge, CO2 dropped very quickly.
The next phase : car service from LAX to hotel. Barriers in car, two windows open, air on flow through. Still higher than I would like.
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