Grade

P_i^①

Pollution level

P_N^②

Pollution level

1

$P_i<1$

clean

$P_N<0.7$

clean

2

$1\le P_i<2$

light pollution

$0.7\le P_N<1$

moderately clean

3

$2\le P_i<3$

medium pollution

$1\le P_N<2$

slightly polluted

4

$P_i\ge 4$

heavily polluted

$2\le P_N<3$

moderately polluted

5

-

-

$P_N\ge 3$

heavily polluted