Yeah, we sell purifiers from time to time, so we got a high quality measure tool as well.
Just give you an idea on the PM2.5 readings:
-rooftop outside 12F; 163
-sleeping room (purifier location): 5-7
-rest of apartment after 10 min utilisation: 30-35
-public staircase: 72
-opposite identical apartment (no purifier): 85
All measured within 5 min an hour ago.
Have been staying for 15 yrs on the other side, history repeats