OI

ON/%

Evaluation grade

Pollution level

OI < 0.05

ON < 0.033

I

Cleaning

0.05 ≤ OI < 0.20

0.033 ≤ ON < 0.066

II

Mild pollution

0.20 ≤ OI < 0.50

0.066 ≤ ON < 0.133

III

Moderate pollution

0.50 ≤ OI

0.133 ≤ ON

IV

Severe pollution