What are the three standard haze indexes of yellow, orange and red?

Level 1 (extremely heavy pollution) means that the regional air quality index is above 500 for 24 consecutive hours.

Level 2 (severe pollution) means that the regional air quality index is in the range of 301 to 500 (inclusive) for 48 consecutive hours.

Level 3 (severe pollution) means that the regional air quality index is in the range of 201 to 300 (inclusive) for 72 consecutive hours.

We need to take precautions when the haze weather is severe. We can install a fresh air system at home. My home has a five-degree and five-constant fresh air system, which is very comfortable even in hazy days