PM2.5 is a scale for measuring real-time air quality. Its values vary from 0 to 500. 

The PM2.5 indication is connected to your Philips Air Purifier's air quality light ring/bar to inform you in real time about the quality of the ambient air.

The different colours indicate the following: 

* Blue: good 

* Blue-Violet: fair 

* Red-Purple: unhealthy 

* Red: very unhealthy