Couldn't say. If I had to guess, a lot of ventilation happens through walls which act as a filter. Even window screens likely do. Regardless of reason, my AQI

Couldn't say. If I had to guess, a lot of ventilation happens through walls which act as a filter. Even window screens likely do. Regardless of reason, my AQI will regularly be 4-13 indoors when it is 38-54 outside, even if I have windows open. Note that this only covers PM 2.5, and not other stuff such as VOCs etc.