## 4 Discussion

Table 5: Comparison of the median metal levels (ppb) in toenail samples according to seafood and mollusk consumption in the 8 h prior to sample donation. Statistically significant results are highlighted in bold. should be taken into account, or alternatively, speciation should be performed to separate organic (mainly dietary) from inorganic (mainly occupational) forms. When interested in relevant long-term or cumulated exposure, the role of occupation as a source of exposure using toenails as biomarker has been assessed in several studies. Concerning occupational exposures, there is some evidence of increased toenail As concentrations in relation to residing in specific industrial areas and with exposure in the workplace, which appears to affect mainly workers in mines or smelting operations [46; 47; 48; 49]. For the other metals, data were obtained from a few studies that were associated with increased toenail Al, Cr and Cu levels in galvanizers [50]; with Co and Cu in workers of a phosphate fertilizer plant [51]; with Fe in miners [52]; with Cd, Ni and Pb in welders [53]; and with Hg in workers in chemical processing plants that use mercury [54]. In contrast, toenail Cd, Cr, Ni, Pb [46; 47] and Hg levels [47] were not higher among miners, nor were toenail Al levels higher in workers from a fertilizer plant [51]. Furthermore, no association with Co [50; 51; 52] or V [50; 52] was found, and there were contradictory results for Mo and Zn [51; 52]. Our results reveal how SMFc influences the levels of accumulated metals in non-exposed workers, who do not receive a relevant source of exposure. Even for those workers who suffer occupational exposure, diet is important for some metals, such as arsenic, cobalt or mercury in urine, but for the vast majority of metals, it is insignificant, especially in toenail samples. Metals in urine samples are likely to vary over time because of the variable exposure driven by changes in diet [55; 56; 57; 58]. These investigations have examined the temporal variability in urinary levels of As, Co, Hg and Li, with some differences between the results due mainly to differences in diet, principally fish, seafood and drinking water consumption. However, the results showed, as our study did, that participants with higher levels of arsenic and mercury in urine consumed fish more frequently. On the other hand, some studies have estimated exposure through diet with the corresponding metal levels in toenails. The most studied element was Se, with some reports showing a direct relationship between toenail levels and global Se intake [59; 60; 61; 62; 63] and other studies reporting no association [64; 65; 66; 67; 68]. For the other three metals, most of the reports did not show a good correlation between dietary intake and toenail levels. For non-essential elements, several authors have supported a positive association between Hg in toenails and seafood, mollusk and fish intake in the general population [69; 70; 71; 72], particularly tuna and saltwater fish [73]; tuna, dark-meat fish and other fish [74]; whale or shark meats [75]; and shellfish, dark-meat fish and tuna in men [76]. With regard to other metals, a cross-sectional study performed in suburban areas in Pakistan found that toenail As and Co were positively associated with SMFc frequency [77]. On the other hand, some studies have evaluated how SMFc affects workers who have occupational exposure to metals. Research developed on Chinese metal-exposed workers revealed the statistically significant relationship between SMFc and As/Hg level in urine [78; 79]. Similarly, seafood consumption has been reported to influence the levels of arsenic among steel and iron smelting in China [36] and a fertilizer factory in Italy [37]. Another study evaluated the association between cadmium exposure and seafood consumption in cadmium-exposed workers in China [80]. The results showed that workers who consumed shellfish most frequently had the highest levels of cadmium in urine despite being highly occupationally exposed to Cd [80].