Methyl Mercury Exposure at Niigata, Japan: Results of Neurological Examinations of 103 Adults
Authors: Kimio Maruyama, Takashi Yorifuji, Toshihide Tsuda, Tomoko Sekikawa, Hiroto Nakadaira, Hisashi Saito, June 2012
Large-scale poisonings caused by methyl mercury (MeHg) have occurred in Japan (Minamata in the 1950s and Niigata in the 1960s) and Iraq (in the 1970s). The current WHO neurological risk standard for adult exposure (hair level: 50 µg/g) was based partly om evidence from Niigata which did not consider any cases who were diagnosed later and/or exposed to low level of MeHg (hair mercury level less than 50 µg/g).
Methods. Early in the Niigata epidemic in June 1965 there were two extensive surveys. From these two surveys, we examined 103 adults with hair mercury measurement who consulted two medical institutions. We compared the prevalence and the distribution of neurological signs related to MeHg poisoning between exposure categories.
Result. We found 48 subjects with neurological signs related to MeHg who had hair mercury concentration less than 50 µg/g. Among the neurological signs, sensory disturbance of the bilateral distal extremities was observed more frequently, followed by disequilibrium, hearing impairment, and ataxia, in groups with hair MeHg concentration both below 50 µg/g and over 50 µg/g.
Conclusion. The present study suggests the possibility that exposure to MeHg at levels below the current WHO limits could cause neurologic signs, in particular, sensory disturbance.