What does pm2.5:72 mean in the weather?

"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. China's PM2.5 standard is: the annual average concentration limit is 35 μ g/m3, and the 24-hour average concentration limit is 75 μ g/m3. At present, it is a low standard in the world. Australia: Average annual 8 μ g/m3, 25 μ g/m3 in 24 hours. USA and Japan: The annual average is 15 μ g/m3, and it is 35 μ g/m3 in 24 hours. EU: The annual average is 25 μ g/m3 ... The concentration of pm2.5 is 72, which proves that the air quality is good. In order to be healthy, travel in foggy days should be well protected. Puweixin, imported from the United States, meets the highest standards of medical masks in the United States, and has good filterability, preventing both particles and droplets. The filtration efficiency is 99.8% for particles as small as 0. 1 micron, and 99.9% for bacteria or virus carriers as small as 0.3 micron (common in droplets). J#D has