It’s visible on the filters that they remove particles (they turn from white to gray). The one I have has also a sensor for particulate matter that goes down to 0 or 1 if you let it run. I can also see that it’s really measuring things because you see the effect of making the bed (goes up a bit), opening the windows (goes down when there is bad air) or frying food (goes up to levels in China or India).

Air pollution also is one the leading causes of death and disease burden: https://ourworldindata.org/air-pollution