Table 1.

Classes of the single pollution index (SPI).

Class	SPI	Grade	Description of Soil Heavy Metal Pollution
1	≤1.0	Safety	Clean
2	1.0 < SPI ≤ 2.0	Slight pollution	Slightly clean
3	2.0 < SPI ≤ 3.0	Mild pollution	Soil pollution exceeds background, crops start to be polluted
4	3.0 < SPI ≤ 5.0	Moderate pollution	Soils and crops have been polluted moderately
5	SPI > 5.0	Severe pollution	Soils and crops have been polluted severely