The pm2.5 blown out by the north wind in Changsha overnight is 18, which should basically be the base of the big environment.

When the daily average concentration of PM2.5 reaches 150 μ g/m3, the AQI reaches 200; When the daily average concentration of PM2.5 reaches 250μ g/m3, the AQI reaches 300; When the daily average concentration of PM2.5 reaches 500 μ g/m3, the corresponding AQI index reaches 500. The air pollution index is 0-50, the air quality level is Grade I, and the air quality status is excellent. 5 1 ~ 100 Grade II is good, 10 1 ~ 150 Grade III is light pollution, 15 1 ~ 200 Grade IV is moderate pollution, and 20/kloc-0. When the value of pm2.5 reaches 200, outdoor activities should be reduced as much as possible, because pm2.5 will do harm to people's respiratory system and cardiovascular system. Puweixin Tiantong, imported from the United States, prevents smog, and goes out to check the weather conditions for protection. Protecting the environment is everyone's responsibility. In the first half of 20 12, it was stipulated that air quality index (AQI) should replace the original air pollution index (API). AQI*** is divided into six grades, from excellent grade I, good grade II, light pollution grade III and moderate pollution grade IV to heavy pollution grade V and heavy pollution grade VI.