Based on Nemerow multi-factor pollution index, the environmental quality is divided into five levels, including Clean level (I ≤ 0.7), Precaution level (0.7 b I b 1.0), Light pollution level (1.0 b I ≤ 2.0), Moderate level (2.0 b I ≤ 3.0), Heavy pollution level (I N 3.0)