I vaguely recalled the story of Minamata Bay in Japan. It is a classic environmental disaster story. And it involves methylmercury. Much of the data in regard to methylmercury impacts comes from that unfortunate event. I refreshed my memory by going back and looking at the Agency for Toxic Substances and Disease Registry’s documentation of health effects of mercury.

Industrial mercury waste was disposed of in the Minamata Bay during the 1950s and 60s. It formed methylmercury in the bay and was ingested by fish which were then eaten by people in the area. Over a hundred people there died from methylmercury poisoning and just as tragically women exposed to methylmercury during pregnancy gave birth to children with neurologic disorders. In fact methylmercury poisoning has been given the name Minamata disease.

Methylmercury bioaccumulates. What this means in a simple fashion is that methylmercury accumulates in animal tissue. It is not readily expelled from the body of an organism like a fish or a human. It all starts at the bottom of the food chain with organisms that feed in the sediments of a water body where methylmercury forms and accumulates. Those organisms are then eaten by small fish, which are in turn eaten by larger fish, which are then caught and eaten by people.

There were a couple of cases of methylmercury poisoning after Minamata, but Minamata is really when there became widespread concern about methylmercury in the environment and especially in fish. Everyone has probably heard pregnant women should limit their consumption of some types of fish because of the mercury levels in them.

There has been a good bit of research on low level methylmercury exposure. The studies have concentrated on populations that eat a lot of fish. Many of these groups of people live on islands. The studies focus on children. In the cases of Minamata and other methylmercury poisonings, children born to women exposed to methylmercury were sometimes blind, deaf, paralyzed or had impaired mental development. Researchers were looking into whether there is developmental impairment to children born from women who have diets that include a lot of fish. All sorts of tests exist to see if children are developing in a normal manner and the researchers used these. They analyzed the mother’s hair for mercury. Then they tested the children.

I’m no toxicologist so I referred to the National Academy of Sciences report in 2000 (National Research Council Committee of the Toxicological Effects of Methylmercury), on what they thought about the research. In one study, when mothers had higher mercury in their hair (13 – 15 parts per million), there was a link to lower scores on IQ and other tests of mental functions in their children. The other studies seemed a mixed bag to me, but the National Academy of Science report concluded that several of the studies conducted by researchers showed increases of abnormal findings on standardized neurological exams.

There were also animal studies cited in the National Academy of Science report. They found similar effects on neurological development in animals as there are in humans when exposed to methylmercury.  Monkey and mice studies showed Minamata disease symptoms at pretty low daily doses.

I don’t mean to scare anyone about methylmercury in fish. The FDA has an advisory for mothers and young children. They recommend not eating swordfish, tilefish, king mackerel or shark. Shark? Who eats shark? Must be someone. Anyway the FDA says two average meals of fish a week are fine (about 12 ounces) of fish low in mercury like shrimp, light canned tuna, salmon, pollock, and catfish. Here’s a link to FDA’s Mercury Levels in Fish and Shellfish (http://www.fda.gov/food/foodborneillnesscontaminants/metals/ucm115644.htm).