###### Abstract
While most urinary markers reflect recent metal exposure, there are some exceptions like those for Cd and Sn, reflecting medium/long-term exposure to these metals [31; 32]. If metal exposures vary over time (e.g., due to fluctuations in diet, changes in the environment, occupational exposure and implementation of regulatory policies), biomarkers like nails or hair that reflect integrated exposures over longer durations may be preferable. Nails and hair are good biomarkers of exposure to As, organic Hg, U and essential metals such as Mn and Se [21; 27; 28]. However, for other metals, such as Ba, Be, Cd, Li, Mo, Ni, Pb, Sr, Tl, V, W or Zn, there is not yet an established biomarker that reliably reflects environmental exposure [21; 27; 28]. Furthermore, the use of toenails as metal exposure biomarkers has increased over time because of their indubitable logistical advantages for large epidemiological studies and because they are supposed to represent longer-term exposures compared with other biomarkers, which is an essential feature to study the involvement of certain agents in chronic diseases, but there is still excessive heterogeneity among the studies and analytical methods [21; 27; 28]. Although toenails share many characteristics with fingernails, they have specific advantages that make them more desirable as a biomarker of exposure. Toenail growth rates are slightly slower than fingernail growth rates; thus, toenails can reflect exposures that occurred further in the past. In addition, toenails are less frequently externally contaminated, although in any case, both toenails and fingernails need to be pretreated before being analyzed [33]. Fish and seafood consumption can influence the levels of some metals, interfering with the biomonitoring in the environmental or occupational setting. In the general population, the most common or massive accumulation of some metals occurs specifically through the consumption of fish and shellfish [34]. The United States Food and Drug Administration(1993) noted that fish and other shellfish represent 90% of total arsenic exposure [35]. Moreover, 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]. Thus, arsenic and mercury in nails (which reflect a longer exposure window) and urine (recent exposure) are good examples of biological markers [38; 39]. Nevertheless, the impact of seafood and fish for cadmium and lead have mixed results in the scientific literature [38; 40; 41], and little information is available for other carcinogenic metals like beryllium, chromium and nickel. For instance, while a recent small randomized controlled intervention trial identified fish consumption as a positive driver for UE of Cd and Pb [42], a cohort study of Spanish children from a heavily industrialized area showed a correlation between seafood and lead but not with cadmium [43]. Urinary levels of cadmium in Korean adult women from agricultural areas was associated with smoking but not with fish consumption [44]. We assessed the impact on short-term (urine) and long-term (toenails) metal biomarkers' levels of seafood, mollusk and fish consumption (SMFc) before urine and toenail sample donation in a group of workers occupationally exposed to metals.