*2.1. Sample Collection*

Between June and September of 2017, hair samples were obtained from 94 schoolchildren (44 boys and 50 girls aged 10–13 years). About 2–3 cm of hair was cut from an area close to the occipital region of the scalp. Only naturally coloured hair was selected in order to avoid biased results. The participants were classified in 3 groups according to the area of residence: (1) urban (named as *downtown*), (2) close to an important chemical/petrochemical complex (named as *CH*/*PCH complex*) and (3) near a large oil refinery, the HWI and a municipal incinerator (named as *refinery*/*HWI*/*MI*) (Figure 1). A consent form according to the declaration of Helsinki was signed by the tutors of each participant. The study protocol, 07/2017, was reviewed and approved by the Ethical Committee for Human Studies of the Pere Virgili Health Research Institute (IISPV), Reus/Tarragona, Spain in March 30, 2017 [22,23].

**Figure 1.** Sampling area.

#### *2.2. Pre-treatment and Chemical Analysis*

Hair samples were washed with 1% Triton X-100 (E. Merck, Darmstadt, Germany) in an ultrasounds bath for 20 minutes in order to remove external contamination. Then, the soap was removed with distilled water and samples were 3 times-washed with Milli-Q water. Around 150 mg of sample were placed in hermetic Teflon bombs and digested with 2 mL of 65% nitric acid (Suprapur, E. Merck, Darmstadt, Germany) for 12 hours at 110 ◦C. Samples were then filtered, diluted to 10 mL of Milli-Q water and stored at −20 ◦C.

The concentrations of As, Be, Cd, Cr, Hg, Mn, Ni, Pb, Sn, Tl and V in hair samples were analysed according to previous studies conducted in our laboratory [6,31–33] using inductively coupled plasma-mass spectrometry (ICP-MS, Perkin Elmer Elan 6000). Each sample was tested by duplicate, being *Lobster Hepatopancreas* employed as quality control (*NRC Canada*, *TORT-2*) every 5 samples. Blanks used during the digestion were also run every 5 samples. The limits of detection (LODs) were 0.03 μg/g for Be, Cd, Pb and Tl; 0.07 μg/g for As, Cr, Mn, Ni, Sn and V; and 0.13 μg/g for Hg.
