Do historic studies of fish consumers support the widely accepted LOEL for methylmercury in adults

Lack of demonstrated neurotoxicity among fish-eating populations has been cited by the World Health Organization and others as supporting an adult LOEL (lowest observed effect level) of > or = 200 ppb blood mercury equivalent. We reviewed 13 published evaluations of neurologic status as related to tissue mercury or methylmercury concentration in long-term fish-eating populations. Review criteria included study size, design (sampling strategy, characterization of exposure, characterization of outcome) and data analysis. No case of classic Minimata disease is described in the 13 studies. Also, no study clearly shows a threshold tissue level above which any form of neurologic dysfunction occurs. A study by Valciukas et al. shows no evidence of neurologic impairment in groups with blood mercury of 10-20 ppb. Studies by Spitzer et al. and McKeown-Eyssen and Reudy suggest correlation of neurologic dysfunction with rising blood mercury concentrations in the 60-120 ppb range. While other studies show no effects associated with rising mercury dose, numbers are small, participant self-selection likely and the clinical assessment limited. Together the 13 studies describe neurologic examinations in approximately 50 fish eaters having a blood mercury equivalent above 200 ppb; of these subjects, neurologic dysfunction consistent with methylmercury exposure was found in as few as six and as many as 15. We conclude that the oft-cited LOEL for (methyl)mercury of 200 ppb in blood is not supported by these studies.