I feel vaguely nauseous for the entire day after getting off a plane, and don't feel myself again until a good night's sleep.
not surprisingly, airlines, manufacturers, and the FAA are all working together to suppress a thorough understanding.
the LA Times did a good investigatory piece at the end of last year: https://www.latimes.com/projects/toxic-chemicals-planes-covi...
When they start a jet engine, the fuel before it ignites would just blow out the exhaust. They catch it in a reservoir and pump it back into the engine to burn it off.
Well, that pump was glitching out, in some way they thought they could fix without a wrench. After about five minutes the fumes started getting sucked into the cabin, and every five minutes after they would tell us they were still working on it. What a shitshow.
The vast majority of the time the airplane is at cruising altitude, though. That air is as clean as you are going to get on planet earth.
A lot of it is oil and jet fuel. This is a known hazard, even to the flight crew.
In the industry these are referred to as "fume events" or "contaminated air quality events" (CAQE).
Flight attendants commonly use coffee grounds to mask the other, worse smells.