| Variable/ categories | category 2 (n = 27) | category 1 (n = 21) | category 3 (n = 19) | F | P |
| z-score (SiO2) | −1.012 ± 0.382 | 0.268 ± 0.396 | 1.142 ± 0.583 | 132.335 | 0.000*** |
| z-score (PbO) | 1.015 ± 0.544 | −0.214 ± 0.336 | −1.206 ± 0.19 | 170.114 | 0.000*** |