Image from Scientific American

A strange disease broke the peace in Minamata Bay, a coastal area southwest of Japan blessed with rich fishing resources. In 1956, a girl was hospitalized with difficulties in walking, speaking and even eating. Soon, her sister and neighbors showed similar symptoms. The unknown disease continued to expand along the Yatsushiro Sea coast, killing more than 1,500 people over time. However, its cause remained a mystery for twelve years until researchers discovered that it was the mercury discharged by a local factory into the water that poisoned the people.

Even today, mercury in food is still a major health concern. According to the U.S. Agency for Toxic Substances and Disease Registry, mercury is ranked the third most hazardous substance to the U.S. population, which seriously harms the central nervous system. This toxic metal mainly comes from fish. Low in saturated fat and rich in omega-3 fatty acids, fish is believed to decrease our risk of stroke and enhance our metabolism while offering us low-fat energy. However, as our waters have been polluted, this seemingly ideal source of protein has also become the top source of mercury exposure.

Mercury occurs in volcanoes, forest fires and other natural sources. But human activities account for almost two thirds of the mercury emissions into the environment. “In the past 100 years, man-made emissions have caused the amount of mercury in the top 100 metres of the world’s oceans to double,” the United Nations Environmental Programme (UNEP) warns.

Ironically, after its journey through the water cycle and the food web, this man-made toxicant finally comes back to the man.

From human sources back to the human body

The major human source of mercury is stationery fossil fuel combustion that burns the heavy metal into the air. Another big but less noticed contributor is small-scale gold mining, which uses mercury to attract the gold and then burns it off into the air. In Peru, small-scale gold mining has not only teared up at least 64,000 acres of rainforest, but has also posed severe health threat to its people.

Direct waste discharges from industries and consumer products and mercury runoff from mines, chemical facilities, and contaminated soils, are also to blame for mercury release.

Once sent off into the atmosphere, mercury travels throughout the environment, in a process known as the Mercury Cycle. Most of it deposits in clouds as gases and particles. During rainfall or snowfall, a large portion falls directly onto the ocean surface. Another part gathers in the upland watersheds. It flows through streams and rivers and eventually enters the ocean. On the global setting, as the ocean transcends national boundaries, the mercury leaves its footprint worldwide. Dealing with the mercury pollution has thus become a global task.

The 2013 UNEP data shows that Asia emits about half of the global mercury while China contributes one third of the world’s total. Following are Sub-Saharan Africa and South America, which account for 16.1% and 12.5% respectively. Other contributors include Europe and North America.

As the mercury enters the ocean, some of it is further carried to inshore areas by ocean currents. Some continues its circulation between the air, land and water by volatilization and precipitation. It may be eventually removed from the system by burial in sediments or being bound to stable mineral compounds. However, before this happens, a substantial part of the mercury enters the aquatic food web. It is first converted by bacteria into methylmercury–the most toxic form of mercury that can accumulate along the food chain. Methylmercury is taken up by microscopic algae. Algae are then eaten by zooplankton. Zooplanktons are consumed by small fish, which are in turn hunted by large fish. As the food chain goes up, higher levels of methylmercury are left in the organisms.

This process is known as biomagnification. Eventually, the fish at the top of the food chain contain the highest amount of methylmercury. “Long-lived, predatory fish, such as swordfish and tuna, can have methylmercury levels as much as ten to 100 million times higher than methylmercury concentrations in the surrounding ocean water,” the Dartmouth College research reports.

Unfortunately, we humans prefer the taste of these large fish. According to the United Nations Food and Agriculture Organization, pelagic fishes, including tunas, mackerels and jacks, were the most harvested in 2010, which contain the highest methylmercury concentrations.

Image from U.S. Biodiversity Research Institute 2012

Marine shellfish, mammals and other seafood are also sources of mercury intake. Eventually, the mercury, generated and disseminated largely from human sources, returns to us through the food on our plate.

What comes after and what to do?

Methylmercury is well absorbed by our digestive tract into human body. It forms a complex that resembles a type of acid commonly found in the body and gains entry into cells. It then follows the blood circulation, distributed to all body tissues in about 30 hours. But to get rid of methylmercury, your body needs almost a year.

Studies show that the major toxic effects of methylmercury are on the developing nervous system. Victims may have seizures, hearing and visual problems, and in the worst case, die painfully. Effects on the brain include decrease in brain size, cell loss, disorganization of cells, etc. As mercury builds up over time, it can also cause problems in kidney, liver, heart and other organs.

Particularly, “women of childbearing age, pregnant and breastfeeding women, developing fetuses, and children under the age of 12 are among the most vulnerable,” according to the Dartmouth College research. Mothers can pass mercury to their babies during pregnancy or through breast milk. The U.S. Food and Drug Administration thus suggests the vulnerable group eat up to 12 ounces (two average meals) of a variety of fish that are lower in mercury each week. Shark, swordfish, king mackerel and tilefish are the ones to avoid, because their mercury concentrations are the highest. Five commonly eaten fish – shrimp, salmon, pollock, catfish and shellfish – are relatively safer.

For adults, the simplest way is to eat a variety of seafood instead of a steady diet of the high-mercury fish. The U.S. Environmental Protection Agency suggest that an adult female with a body weight of 132 pounds (60kg) eat a fish meal within 6 ounces (170 grams) a day, to avoid adverse health effects. Under this guideline, the five commonly eaten fish above as well as crab, lobster, trout and hake, can be taken within two meals per week. Carp, bass and perch are once a week while tuna should be kept once a month, according to the U.S. Biodiversity Research Institute.

But problems are never solved just by avoidance. To fundamentally deal with mercury pollution, policy makers need to control mercury emissions and assess mercury concentrations in the environment. Globally, countries also need to work together in restricting mercury sale and circulation. In early 2013, the UNEP staged the Minamata Convention for Mercury on curbing mercury emissions worldwide, indicating one further step toward global cooperation.

Ultimately, the earlier the action, the smaller the loss. In 1998, 35 scientists agreed that “when an activity raises threats of harm to the environment or human health, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.” Over the years this belief has proved its truth through historical lessons.

• The Coastal and Marine Mercury Ecosystem Research Collaborative (C-MERC), “Sources to Seafood-Mercury Pollution in the Marine Environment”, 2012
• Biodiversity Research Institute (BRI), “Mercury in the Global Environment”, 2012
Agency for Toxic Substances and Disease Registry (ATSDR). 2011. Priority List of Hazardous Substances. Atlanta.
Minamata City, “Minamata Disease-Its History and Lessons”, 2007
Bella, Nikki La et al, “Mercury in the Environment”, 2003
United Nations Environment Programme (UNEP), Global Mercury Assessment, 2013
U.S. Food and Drug Administration (FDA), Mercury Levels in Commercial Fish and Shellfish (1990-2010)
FDA, “What You Need to Know About Mercury in Fish and Shellfish”, 2004
The National Academies, “EPA’s Methylmercury Guideline Is Scientifically Justifiable For Protecting Most Americans, But Some May Be at Risk”, 2000
UNEP Minamata Convention on Mercury
“Mercury in the Body and Health Effects”
“How does mercury affect our health?”
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