I am using a cheap Xiaomi (there's not much choice where I live), and a few PMS5003 sensors scattered around the apartment. It works fine for 25 m²-something ro

I am using a cheap Xiaomi (there's not much choice where I live), and a few PMS5003 sensors scattered around the apartment. It works fine for 25 m²-something room when it's relatively clean outside (around 100 µg/m³ or less), but you really have to push it (and suffer the resulting noise) when pollution outside goes above 300 µg/m³ or so. It's been able to keep indoor air around 20 µg/m³ with outside levels going above 800 µg/m³, but I have to use ear plugs because the noise is impossible to tolerate for more than half an hour (and it's pretty difficult to sleep next to). If you let the heavily polluted air in (a few hundred µg/m³), then close the window and set the purifier to maximum power, it cleans the room in ~20-25 minutes.