I´m living in the middle of a city, and PM2.5 levels get very high at times. We are way above the yearly 5ug/m3 recommendation. So I just bought a lot of air purifiers (just HEPA, everything else commercially available is useless) and they get switched on if the levels get high (via HomeAssistant). I have a device in the garden and another inside, so I am monitoring the difference on a regular basis. I optimize (manually, no fancy data crunching) for low noise produced by the filters and low <2-3ug/m3 PM2.5 values indoors. I don´t have forced ventilation, so I need to know how much do I open the windows to get humidity and VOCs down, while not letting in too much smog. Sounds more complicated than it really is.