Smart Air tested the particle counter inside the Xiaomi Mi2 air purifier. Data shows that the built-in particle counter does not give an accurate measurement of air quality. The air quality monitor consistently under-measured the air quality in a room, when compared to other more accurate air quality monitors.
This means the Xiaomi air purifier shows you a lower pollution level than the actual pollution level. This is dangerous, because you may think the air in your room is clean, whereas it is not.