What standard does the pollution index reach to start the emergency response of heavy pollution weather?

According to the classification method in the Technical Regulations of Ambient Air Quality Index (AQI) issued by the Ministry of Environmental Protection, the air quality index (AQI) is between 20 1-300, which is a serious pollution; 30 1-500 is a serious pollution. According to the air quality forecast, and considering the degree and duration of air pollution, the heavy air pollution is divided into four warning levels, which are four, three, two and one from light to heavy, marked with blue, yellow, orange and red respectively, and the first warning level (red) is the highest level.

(1) early warning level 4 (blue): severe pollution is predicted in the future 1 day;

(2) Warning Level 3 (yellow): It is predicted that serious pollution will occur in the future 1 day or last for 3 days;

(3) Early warning level II (orange): It is predicted that severe pollution or serious pollution will occur alternately in the next three days;

(4) Early warning level 1 (red): It is predicted that serious pollution will occur in the next three days.