What does secondary response of heavy pollution mean?

The air pollution index is 5 1- 100, and the air quality grade is Grade II. The response of heavy pollution from light to heavy is four levels of early warning. The second level means that the air pollution index is 5 1- 100, and the air quality level is second level. The air quality is good. At this time, the air quality is considered acceptable, and there is no harm to public health except that a few people are sensitive to some kind of pollution.