PM is an abbreviation for particulate matter. The number next to PM generally refers to the diameter of the particle pollutant measured in micrometers. According to the EPA, 'PM 2.5 pollutants consist of fine inhalable particles that are generally 2.5 micrometers and smaller'. It is ideal to have the lowest level of particulate matter in the air around you as they can pose the greatest health risks.*
The monitor on the air purifier will illuminate different light colors based on the PM 2.5 level detected in the room in accordance to the EPA standards for PM 2.5 exposure.
Blue color: Good air quality
Green color: Moderate air quality
Yellow color: Bad air quality
Red color: Very bad air quality
* https://www.epa.gov/pm-pollution/particulate-matter-pm-basics