Tiongs will always be tiongs~

No filter also can register a drop in air quality readings.
XM AP uses a Sharp IR LED module. Actually it's quite ok. I suspect Xiaomi's implementation and reading of the voltage output from the Sharp module might not be done that nicely?
Last year in the 2015 Haze and El Nino thread + Air Purifier thread, I engaged in a healthy debate regarding this vs our usual Laser particle counters with another EDMWer.
He was extremely adamant in that the Xiaomi AP's air quality function is very good. Of course, he has the right to his opinion.
Granted, the laser particle counters are not very accurate (without reduction of intake air to 35% RH, it can never be, not even the 5-figures MetOne BAMs).
But they are very sensitive and gives you very high resolution in determining even very slight changes in the air quality. It's all relative....