Table 1 Classes of the single pollution index (SPI) and Nemerow composite pollution index (NCPI)
Class | SPI | Grade | NCPI | Grade |
|---|---|---|---|---|
1 |  ≤ 1.0 | Safety |  ≤ 0.7 | Safety |
2 | 1.0 < SPI ≤ 2.0 | Slight pollution | 1.0 < SPI ≤ 2.0 | Alert |
3 | 2.0 < SPI ≤ 3.0 | Mild pollution | 2.0 < SPI ≤ 3.0 | Slight pollution |
4 | 3.0 < SPI ≤ 5.0 | Moderate pollution | 3.0 < SPI ≤ 5.0 | Moderate pollution |
5 | SPI > 5.0 | Severe pollution | SPI > 5.0 | Severe pollution |