> In other words, the air on the plane was quite a bit cleaner when there were ashtrays next to the seats.
That doesn't follow from what you wrote.
Anyway, it is easy to look at actual research on cabin air quality and see your claim is false.
"Typical levels of SHS-RSP on aircraft violated current (PM(2.5)) federal air quality standards approximately threefold for flight attendants, and exceeded SHS irritation thresholds by 10 to 100 times. From cotinine dosimetry, SHS exposure of typical flight attendants in aircraft cabins is estimated to have been >6-fold that of the average US worker and approximately 14-fold that of the average person. Thus, ventilation systems massively failed to control SHS air pollution in aircraft cabins."