I used to work for a major airline- one you've heard of, but I won't sound off which one it is.
We had a heating/air conditioning unit we could hook up to the planes so we could have air conditioning at night during cleaning. All it took was one of the ground tugs to pull it up, a ground crewperson to hook it to the plane, start the diesel, flip it to heat or A/C, throw another switch and inside became comfortable.
So when the airline tells you they can't control the air quality onboard the plane, they are lying. I know, after I quit the airline a flight attendant (without her access badge) told me I was giving her "attitude" when I told her to have ground crew hook up the HVAC cart. Then the lady behind me had an asthma attack because it was so hot. Whose attitude?