In the Chinese study of 907 children from four areas in Guizhou Province, the degree of exposure to fluoride was clearly linked to IQ test scores:
Type of area Fluoride concentration Number of children Mean IQ
Non-fluorosis <
Slight fluorosis 0.8 ppm 227 89.7 ± 2.7
Medium fluorosis 2.5 ppm 224 79.7 ± 12.7
Severe fluorosis 3.2 ppm 230 80.3 ± 12.9