Grade division

Single pollution index

Class of pollution

I

PN ≤ 0.7

Clean (SAFE)

II

0.7 < PN ≤ 1

Relatively clean (alert limit)

III

1 < PN ≤ 2

mild concentration

IV

2 < PN ≤ 3

middle level pollution

V

PN > 3

heavy pollution