Toxicity reference values and tissue residue criteria for protecting avian wildlife exposed to methylmercury in China

Mercury (Hg) is a globally distributed environmental contaminant with both natural and anthropogenic sources. Of the forms and oxidation states of Hg, the organic form, methylmercury (MeHg), is the most biologically available and the most toxic (Scheuhammer et al. 2007). MeHg can be neurotoxic, embryotoxic, and can impair physiological function, particularly by disrupting endocrines (Tan et al. 2009) and altering reproductive behavior (Frederick and Jayasena 2010). Because MeHg can be bioaccumulated and biomagnified through the food web, diet is the major pathway by which vertebrates are exposed (Liu et al. 2008). Species occupying the higher trophic levels in aquatic systems are considered to be at the greatest exposure risk, particularly the birds at trophic levels 4 or 5. Although concentrations of Hg may exist in surface water at or near only historical background levels, the concentrations of Hg that exist in wildlife are higher (Liu et al. 2008). Chronic dietary exposure to relatively small, environmentally relevant concentrations of MeHg is sufficient to be accumulated by tissues to concentrations that impair reproduction of birds (Frederick and Jayasena 2010).