## 2 Materials and Methods
### Bioaccessible Concentration  
The procedure adopted for carrying out the UBM tests was the one reported in the International Standard ISO 17924 [6]. The soil fraction used in the UBM method was the one with a diameter lower than 250 \(\upmu\)m, since this portion of soil is believed to adhere to human hands and become ingested during a hand-to-mouth activity [3].  
The 4 extraction fluids, i.e., saliva, gastric, duodenal, and bile solutions were prepared the day before performing the tests according to the specific composition and indications reported in the above-mentioned standard. Each one of these fluids was prepared by mixing a 500 mL solution with inorganics to a 500 mL solution with organic and solid constituents, with a composition reported in detail in the ISO 17924 Standard [6]. Briefly, the UBM includes 2 sequential extraction steps, corresponding to the gastric and gastrointestinal phases. In the first step, the gastric and gastrointestinal samples were prepared by adding 0.6 g of soil to 9 mL of saliva and 13.5 mL of gastric fluid; then, after checking that the pH of the solution was within the range 1.2 \(\pm\) 0.05, the samples were incubated in an end-over-end shaker at 37 \({}^{\circ}\)C for 1 h. At the end of this stage, we verified that the pH of each suspension (both gastric and gastrointestinal samples) was lower than 1.5. Afterwards, the gastric samples were centrifuged at 4500\(\times\)\(g\) for 15 min, and the supernatants were collected for analysis by ICP-OES. The gastrointestinal samples were mixed with 27 mL of simulated duodenal fluid and 9 mL of simulated bile fluid, and after checking the pH to be in the range 6.3 \(\pm\) 0.5, we incubated them in an end-over-end rotator at 37 \({}^{\circ}\)C for 4 h. At the end of this phase, after controlling the pH value (that should be 6.3 \(\pm\) 0.5), we centrifuged the suspensions at 4500\(\times\)\(g\) for 15 min and analyzed the solutions by ICP-OES.  
The bioaccessibility tests performed adopting the SBET were carried out in agreement with the USEPA standard operating procedure [17]. It is worth noting that, differently from the UBM method, following the recommendation for the assessment of incidental ingestion of soil developed by USEPA [42], the soil used in the SBET method was the fraction with diameter lower than 150 \(\upmu\)m, which, on the basis of studies published in the last years, represent the dominant fraction for dermally adhered soil (e.g., [43]). The extraction fluid was represented by a 0.4 M glycine solution whose pH was adjusted to 1.5 by adding HCl. Then, 1 g of the soil sample was put in contact with 100 mL of the extraction fluid and rotated end-over-end at 37 \({}^{\circ}\)C at 30 rpm for 1 h. At the end of the experiment, we checked that the final pH of the solution was in the range 1.5 \(\pm\) 0.5. Afterwards, the soil-liquid solution was allowed to settle the solid particles, and then the liquid was filtered through a 0.45 \(\upmu\)m cellulose acetate disk before analysis by ICP-OES.  
All reagents used for performing characterization analysis and bioaccessibility tests were of analytical grade and were purchased by Sigma-Aldrich (St. Louis, MO, USA).