What do you think happens to the air in, say a cinema, or pub, or rock concert?
The air you're breathing now (stand by for a shock!
) may contain elements which have
been breathed before! 
Passengers travel in their millions, and crew in their tens of thousand spend their entire working lives on these aircraft which have been in service now since the very late 50s - several generations of working crew - with nothing to
definitely put a finger on, and certainly only a fractional percentage who complain about symptoms.
"Aerotoxicity" (make up a good name to give it credence) is a conspiracy theory of the same order as
the Aspartame mob.