What does pm2.5 mean?

PM2.5 refers to particulate matter in the atmosphere with a diameter less than or equal to 2.5 microns, also known as particulate matter that can enter the lungs. Its diameter is less than 1/20 the thickness of a human hair. Although PM2.5 is only a small component of the earth's atmosphere, it has an important impact on air quality and visibility. Compared with coarser atmospheric particles, PM2.5 has a small particle size, is rich in toxic and harmful substances, stays in the atmosphere for a long time, and is transported over long distances. Therefore, it has a greater impact on human health and the quality of the atmospheric environment.