*2.3. Statistical Analysis*

All data were analysed by using the statistical package SPSS 25.0. Values with a *Z*-score above 2.5 and under −2.5 were considered as outliers. In addition, a visual inspection of the boxplot was also conducted to verify these outliers. Non-detected levels of metals were assumed as to be one-half of the respective LOD (ND = 1⁄2 LOD). Only trace elements with values 70% (or more) above the LOD were considered for further statistical evaluation. To assess the distribution of the values, the Kolmogorov-Smirnov test was used. Correlations between metal concentrations were performed employing the Spearman correlation coefficient. The Student's t-test was used to compare differences of metal levels between boys and girls. In turn, ANOVA and subsequent T3 Dunnett's post-hoc tests were employed to assess differences between groups according to the zones of residence and the significance in the temporal evolution. The level of statistical significance was established at *p* < 0.05.
