It might be scare mongering, it might not be bad for you but perhaps it explains why it sometimes it feels so Claustrophobic and airless. Particularly the period between shutting the doors and pushback/engine start. That can be really dreadful if there is any delay.
There can be major benefits in modifying gas levels to the range of 3-5% oxygen and 3-10% carbon dioxide. These gas levels slow respiration (calming effect)
I'm sorry but feeling smothered doesn't have a calming effect on me (most people?).
Colin
(A glider pilot)