数值范围
评价级别
1 ≤ X 1 . X 2 ≤ 2
Ⅰ类
2 < X 1 . X 2 ≤ 3
Ⅱ类
3 < X 1 . X 2 ≤ 4
Ⅲ类
4 < X 1 . X 2 ≤ 5
Ⅳ类
5 < X 1 . X 2 ≤ 6
Ⅴ类
6 < X 1 . X 2 ≤ 7
劣Ⅴ类,但不黑臭
X 1 . X 2 > 7
劣Ⅴ类,并黑臭