What is the standard of air index?

Air pollution index (API-abbreviation of air pollution index) is a method to reflect and evaluate air quality. It simplifies the incomprehensible pollutant concentration into a single conceptual numerical form, which is convenient for intuitive expression of air quality and air pollution degree. The classification standard of air pollution index (API) is: Grade I, with API less than 50 and excellent air quality. At this time, the air is relatively clean, so we should take part in outdoor activities and breathe fresh air. Grade ii, API 5 1~ 100, with good air quality. At this time, there is no adverse effect on human body, and outdoor activities can be carried out normally; Grade iii, API101~150, slightly polluted; API 15 1~200, which is slightly polluted. At this time, patients with heart disease and respiratory diseases should appropriately reduce physical exertion and outdoor activities, but it has no obvious effect on healthy people; Grade 4, API 20 1~300, moderate pollution. At this time, the elderly and patients with heart and lung diseases should try to stay indoors, and healthy people should also reduce outdoor activities appropriately; Grade 5, API greater than 300, heavily polluted. At this time, except for special needs, you should try to avoid staying outdoors.