The air inside an airplane is actually arguably more harmful today than it was in the immediate aftermath of the short-haul smoking ban. Due to smoking on planes they actually had proper air filtration while in the modern day there are no federal regulations on air filtration for planes and what air carriers elect to employ is very minimal.
Throughout a flight carbon oxides, aldehydes, and other harmful particulates are known to build up in the air up to levels known to pose immediate harm to those subjected to it.