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 |