**3. Results**

The concentrations of the 11 analysed elements in 94 samples of hair from schoolchildren living in Tarragona County are shown in Table 1. Arsenic, Be and Tl were not detected, whereas Cd and V were only detected in 37 and 23 samples, respectively. In previous campaigns of the monitoring program, traces of Cd and V were only found in very few samples of hair [6,31–33]. Although the information relative to these elements is shown, they are excluded from the statistical analysis.


**Table 1.** Metal concentrations (μg/g) in hair of 94 school children living in Tarragona County (2017).

ND = not detected, LOD = limit of detection. Data are given as mean ± SD.

Lead presented the highest mean concentration (1.44 μg/g), ranging from undetected values to 11.9 μg/g. Mercury also presented relatively high levels, with a mean and a range of 0.73 and 0.13–2.70 μg/g, respectively. The mean concentrations of Sn, Mn and Cr were 0.41, 0.30 and 0.14 μg/g, respectively. Finally, Cd and V were detected only in a few samples. Both metals exhibited the lowest concentrations (0.04 and 0.07 μg/g, respectively). When evaluating the correlation between metal levels (Table 2), a weak but significant positive correlation, was found between Mn and Ni, Pb (*p* < 0.01) and Sn (*p* < 0.05) and also between Ni and both Cr and Sn (*p* < 0.01).

**Table 2.** Correlations between metal concentrations in hair of 94 schoolchildren living in Tarragona County (2017).


Spearman correlation coefficient (r) is shown. \* and \*\* means significant differences at *p* < 0.05 and *p* < 0.01 (bilateral), respectively.

When metal concentrations were assessed according to the sex of the schoolchildren, significant differences were found in the levels of Hg, Ni, Pb and Sn (Figure 2). Thus, girls exhibited significant lower levels of Hg (0.61 μg/g vs. 0.88 μg/g, (*p* < 0.05)) and Pb (1.03 μg/g vs. 1.89 μg/g, *p* < 0.05) and significant higher concentrations of Ni (0.69 μg/g vs. 0.36 μg/g, *p* < 0.01) and Sn (0.54 μg/g vs. 0.28 μg/g, *p* < 0.001) than boys. Finally, Cd, Cr and Mn did not show significant differences between girls and boys.

**Figure 2.** Metal concentrations (μg/g) in hair of schoolboys (*n* = 44) and girls (*n* = 50) living in Tarragona County (2017). Data are given as mean ± SD. Statistics: non-parametric t-test. Significant differences between both groups at: \**p* < 0.05, \*\**p* < 0.01 and \*\*\**p* < 0.001.

Mercury was identified as the only element showing significant differences in the levels in hair according to the specific zones of residence (Table 3). Significantly higher concentrations of Hg were found in the children living in Tarragona downtown with respect to those living in the remaining two evaluated areas (*p* < 0.01), both of them with industrial characteristics.


**Table 3.** Metal concentrations (μg/g) in hair of school children living in Tarragona County according to the specific zones of residence (2017).

ND = not detected. Data are given as mean ± SD. Statistics: ANOVA and T3 Dunnett's post-hoc test. Data not showing a common superscript (a, b, c) indicate significant differences between zones (downtown, CH/PCH complex and refinery/HWI/MI) at *p* < 0.05.

The temporal trends of metal concentrations in hair of schoolchildren are summarized in Table 4. The results of the current study are compared with those of previous surveys, including the baseline study (1998) and the intermediate campaigns (2002, 2007 and 2012). A significant decrease of Cd in recent years was noted when compared to the baseline results (*p* < 0.001). Chromium levels significantly increased in 2007 and 2012 in comparison to previous years (*p* < 0.001) but the concentration of this element notably decreased in 2017 (*p* < 0.001), being this reduction significant in contrast to the previous surveys. Despite Ni concentrations were significantly lower in 2002 (*p* < 0.001), they have remained nearly constant through time. Furthermore, although Pb concentrations decreased significantly after the baseline survey (*p* < 0.001), in 2017 they raised again, being significantly higher than those observed in 2007 and 2012 (*p* < 0.01). Tin showed significantly lower concentrations in the period 2002–2012 (*p* < 0.001), while current values are comparable to those found in 1998. Mercury was the only element which did not exhibit significant differences between surveys. Finally, V was found at relatively low concentrations, being detected in only 23 samples in 2017 but contrasting with data from previous surveys, where it was never detected.


**Table 4.** Metal concentrations (μg/g) in hair of school children living in Tarragona County obtained in the studies conducted in 1998, 2002, 2007, 2012 and 2017.

ND= not detected. Data are given as mean ± SD. Statistics: ANOVA and T3 Dunnett's post-hoc test. Data not showing a common superscript (a, b, c) indicate significant differences between years (1998, 2002, 2007, 2012 and 2017) at *p* < 0.05.
