Why does the weather forecast say both smog and good air?

According to the unified regulations of the State Environmental Protection Bureau, China's air quality is divided into five grades. It processes a series of complex air monitoring data according to a certain method, calculates the specific air pollution index, and then determines its air quality grade.

The specific standards are as follows:1when the air pollution index reaches 0-50; Secondary from 5 1 to100; Third-level pawn101-200; Level 4 of 20 1-300; Level 5 when it is above 300. Among them, Grade 3 belongs to mild pollution, Grade 4 belongs to moderate pollution and Grade 5 belongs to severe pollution.

When the air pollution index is greater than 50 and less than or equal to 100, the air quality is good.