Harvard researchers warn of legacy mercury in the environment

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Environmental researchers at Harvard University have published evidence that significant reductions in mercury emissions will be necessary just to stabilize current levels of the toxic element in the environment. So much mercury persists in surface reservoirs (soil, air, and water) from past pollution, going back thousands of years, that it will continue to persist in the ocean and accumulate in fish for decades to centuries, they report.

"It's easier said than done, but we're advocating for aggressive reductions, and sooner rather than later," says Helen Amos, a Ph.D. candidate in Earth and Planetary Sciences at the Harvard Graduate School of Arts and Sciences and lead author of the study, published in the journal Global Biogeochemical Cycles.

Amos is a member of the Atmospheric Chemistry Modeling Group at the Harvard School of Engineering and Applied Sciences (SEAS), where researchers have been collecting historical data on mercury emissions as far back as 2000 BC and building new environmental models of mercury cycling that capture the interactions between the atmosphere, oceans, and land.

Their model reveals that most of the mercury emitted to the environment ends up in the ocean within a few decades and remains there for centuries to millennia. These days, emissions are mainly from coal-fired power plants and artisanal gold mining. Thrown into the air, rained down onto lakes, absorbed into the soil, or carried by rivers, mercury eventually finds its way to the sea. In aquatic ecosystems, microbes convert it to methylmercury, the organic compound that accumulates in fish, finds its way to our dinner plates, and has been associated with neurological and cardiovascular damage.

"Today, more than half of mercury emissions come from Asia, but historically the U.S. and Europe were major emitters," says second senior author Daniel J. Jacob, Vasco McCoy Family Professor of Atmospheric Chemistry and Environmental Engineering at Harvard SEAS and Professor of Earth and Planetary Sciences. "We find that half of mercury pollution in the present surface ocean comes from emissions prior to 1950, and as a result the contribution from the U.S. and Europe is comparable to that from Asia."

Higher mercury levels in humans associated with increased risk for diabetes

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A new study found that higher levels of mercury exposure in young adults increased their risks for type 2 diabetes later in life by 65 percent. The study, led by Indiana University School of Public Health-Bloomington epidemiologist Ka He, is the first to establish the link between mercury and diabetes in humans.

The study paints a complicated nutritional picture because the main source of mercury in humans comes from the consumption of fish and shellfish, nearly all of which contain traces of mercury. Fish and shellfish also contain lean protein and other nutrients, such as magnesium and omega-3 polyunsaturated fatty acids, that make them important to a healthy diet.

In the study, published online early in the journal Diabetes Care, the people with the highest levels of mercury also appeared to have healthier lifestyles -- lower body mass indexes and smaller waist circumferences, more exercise -- than other study participants. They also ate more fish, which is a possible marker of healthy diet or higher social economic status. Risk factors for type 2 diabetes include being overweight.

The study, which involved 3,875 men and women, established the link between mercury levels and type 2 diabetes risk after controlling for lifestyle and other dietary factors such as magnesium and omega-3 polyunsaturated fatty acids, which could counter the effects of the mercury.

These findings, said He, point to the importance of selecting fish known to have low levels of mercury, such as shrimp, salmon and catfish, and avoiding fish with higher levels, such as swordfish and shark. FDA and EPA guidelines for fish consumption highlight this, particularly for women who are pregnant or of childbearing age and for young children.

"It is likely that the overall health impact of fish consumption may reflect the interactions of nutrients and contaminants in fish. Thus, studying any of these nutrients and contaminants such as mercury should consider confounding from other components in fish," He and the authors wrote in the study. "In the current study, the association between mercury exposure and diabetes incidence was substantially strengthened after controlling for intake of LCn-3PUFAs (omega-3) and magnesium."

The study participants were recruited from Birmingham, Ala., Oakland, Calif., Chicago and Minneapolis,and then followed for 18 years as part of the Coronary Artery Risk Development in Young Adults Study. He, chair of the Department of Epidemiology and Biostatistics in the IU School of Public Health-Bloomington, is principal investigator of the ancillary study, the CARDIA Trace Element Study, which is funded by the National Institutes of Health.
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