Low mercury levels damage nerves.

Jan 07, 2009

Yorifuji T, T Tsuda, S Takao, E Suzuki and M Harada. 2008. Total mercury content in hair and neurologic signs: historic data from Minamata. Epidemiology.

Synopsis by Jennifer Adibi

A reanalysis of decades-old data finds that neurologic problems caused by eating mercury-contaminated fish plagued Japanese residents 10 years after the poisoning and at lower doses than originally measured.

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Eating fish contaminated with methylmercury is a primary public health concern globally. Fish are an important part of a healthy diet, and many people around the world rely on fish as their major source of protein. But fish can be contaminated, by mercury and other toxicants.  How you weight the trade-off depends upon what you understand about the benefits and the costs.  This new result indicates the costs of mercury exposure are higher than assumed.

One of the largest methylmercury poisonings occurred during the 1950s and 1960s when Japanese living in fishing communities ate contaminated fish. Mercury is still a major health problem and so brings "renewed attention to the evidence from the incidents" of this historical event, say the authors.

By re-examining old data in a different way, researchers can sometimes gain new insights and understanding than from the originally-reported conclusions.

Almost half a century ago, people living near the Shiranui Sea in Japan ate mercury-laden fish for several years, leading to large-scale mercury poisonings. Residents of two fishing villages (Minamata and Niigata) were most affected, suffering from neurologic disorders linked to the extremely high methylmercury levels in their blood and in their nails.

The massive mercury pollution was caused when a local chemical company dumped wastewater directly into the sea, exposing fish, other wildlife and people to the dangerous metal.

Now, the authors of this new study chose a different way to examine the original hair sample data from 1960 and the nerve problems identified in 1971. They found a clear relationship between increasing mercury exposure and higher risk of one type of nerve disorder that affects the cheeks and mouth.

In another analysis, they compared the exposed population in two heavily-hit villages to a similar population in another village that was not exposed. They found that the risks of all varieties of neurologic defects were much larger and more consistent among the exposed vs. the unexposed populations. They also saw that the risks of neurologic disorders were associated with mercury concentrations as low as 10 micrograms/g in hair as opposed to the cut-off of 50 set by the World Health Organization (WHO).

The authors conclude that "long-term exposure to methylmercury at levels below the current WHO limits can cause neurologic signs, in particular, perioral sensory loss."