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Classes of the Nemerow composite pollution index (NCPI).
| Class | NCPI | Grade | Description of Soil Heavy Metal Pollution |
|---|---|---|---|
| 1 | ≤0.7 | Safety | Clean |
| 2 | 0.7 < NCPI ≤ 1.0 | Alert | Slightly clean |
| 3 | 1 < NCPI ≤ 2.0 | Slight pollution | Soil pollution exceeds background, crops start to be polluted |
| 4 | 2 < NCPI ≤ 3.0 | Moderate pollution | Soils and crops have been polluted moderately |
| 5 | NCPI > 3.0 | Severe pollution | Soils and crops have been polluted severely |
