Interesting, i find this publication knowledgeable, health matters.

Come to talk of it, some of our cooling and heating systems usually recirculate the air already in the house which may likely have a mix of dust, dirt, and pollens

True. staying indoor for long with the rotation of your heating or cooling system changed the atmosphere in the room
Sometime in my case. The fan blow heat which is not always convenient for breathing as such i navigate outside for better atmospheric oxygen