To the Editor: The recent article by Gerstenberger et al. [1], reported the mercury (Hg) concentrations, but did not report the selenium (Se) contents of the tuna. Since methylmercury is, by biochemical definition, a highly specific irreversible inhibitor of Se-dependent enzymes [2], it is necessary to measure both Hg and Se to accurately assess risks associated with Hg exposures. The brain and neuroendocrine tissues depend upon Se-dependent enzymes to prevent and reverse oxidative damage that would otherwise accompany their high rates of oxygen consumption. Because the affinity of Hg for Se is approximately a million times higher than its affinity for sulfur [3], the Seenzymes are uniquely vulnerable to inhibition by high methylHg exposures. However, rather than impairing Se enzymes, eating ocean fish such as tuna that are known to provide far more Se than Hg actually enhances nutritional Se status, explaining why ocean fish consumption has repeatedly been shown to prevent, rather than contribute to Hg toxicity [2]. In contrast, consumption of meats of predatory whales and sharks that are known to contain far more Hg than Se has been shown to be hazardous. Current U.S. Environmental Protection Agency (U.S. EPA) risk criteria are based on a study of human Hg exposures that arose predominantly from eating pilot whale meats. Since pilot whale meats are virtually unique in having exceedingly high Hg to Se molar ratios (5:1), as well as Hg contents in excess of 3 ppm [4], this is clearly a food to avoid. In contrast, ocean fish are Se-rich and have Hg to Se molar ratios that are typically on the order of 1:5, which may explain why ocean fish consumption has been shown to counteract Hg toxicity from whale meat consumption [5]. Although blood Hg and blood Hg to Se molar ratios in children exposed in utero are highly correlated [6], relationships between Hg exposures and risk are most accurately predicted from the perspective of Hg to Se molar ratios. Therefore, the selenium-health benefit value (Se-HBV) is preferable since it incorporates both the relative and the absolute amounts of Se intake and Hg exposure in a single criterion. Health benefits are directly associated with positive Se-HBVs, whereas adverse health effects are directly proportional to negative Se-HBVs [2]. Since methylmercury is an enzyme inhibitor, its chemical reactions are clearly second order. Therefore, attempting to