| N | Contamination Degree | Contamination Index (Cd) | |
| Short Use | Long Use | ||
| 1 | Safe | <0.0 | <0.0 |
| 2 | Low | ≤1 | ≤0.5 |
| 3 | Medium | >1 - <3 | >0.5 - <1.5 |
| 4 | High | ≤3 | ≤1.5 |