There are many factors of air pollutants. At present, China's environmental protection departments adopt the index of PM 10 when monitoring air pollutants. Its definition is to monitor the concentration of dust or floating dust with air equivalent diameter of 10μm in ambient air. From this, we know that PM2.5 refers to the concentration of dust or floating dust with a diameter less than or equal to 2.5μm in the ambient air.

PM2.5 refers to particles with a diameter less than or equal to 2.5 microns in the atmosphere, also known as particles that can enter the lungs. Its diameter is less than 1/20 of the thickness of human hair. Although PM2.5 is only a small part of the earth's atmospheric composition, it has an important impact on air quality and visibility. Compared with atmospheric coarse particles, PM2.5 is small in particle size, rich in toxic and harmful substances, and stays in the atmosphere.