Mercury: the Toxic Pollutant That Keeps Coming Back

What goes up, must come down. And up, and down again. At least that's the case with mercury; the problem with this toxic chemical is that it sticks around for a long time, cycling between the air and the soil and ocean. Eventually it winds up in the deep sea or deeply buried sediments, but that takes centuries. 

In fact, half of the mercury in the upper part of the ocean—where it accumulates in fish that humans eat—was emitted by people prior to 1950, according to a study published in May in the journal Global Biogeochemical Models. In total, 83 percent of the surface ocean's mercury ended up there from human activities. The finding has led researchers to suggest that mercury emissions be dramatically cut.

"The mercury we've emitted in the past is a really big contributor to what we see today," said Helen Amos, a doctoral student in earth and planetary sciences at Harvard University. "I strongly advocate for aggressive reductions [of mercury emissions] in the immediate decade."

If nothing is done to reduce mercury emissions, ocean levels could nearly double by 2050. That would have "serious implications" for human health, Amos said. Mercury levels in the atmosphere are nearly eight times greater than natural "background" concentrations, the study found.

Even if emissions are cut in half—a very optimistic assumption—the effects wouldn't be seen in food webs for at least a decade.

Exposure to small amounts of mercury can cause brain and nerve damage, especially in the young, years of research show. It also readily makes its way from the blood of pregnant women into fetuses, affecting brain development.

Most of the mercury emitted into the atmosphere rains down on the world's oceans, and the rest ends up on land. But its is a volatile element, and much of it evaporates and returns to the air after being deposited on land and into the ocean. In fact, a majority of the mercury (60 percent) that rains out from the air has been re-emitted from soil or water, returning like an undead pollutant that cannot be snuffed out.

The primary source of mercury emissions is small-scale (or "artisanal") gold mining, for example in Southeast Asia and South America, according to the study; a close second is emissions of mercury from burning coal. Together, the two account for about two-thirds of current mercury pollution, Amos said.

In small-scale mining, often carried out by the poor and done illegally, mercury-containing quicksilver is added to a slurry of ore. The mercury binds to any gold present and sinks to the bottom. It can then be burned off over an open flame, emitting large amounts of the pollutant directly into the atmosphere, Amos said. The practice can also poison the people doing the mining, she added.

Earlier this year, governments around the world said that they would reduce some types of mercury pollution, in an agreement known as the Minamata Convention on Mercury, named for a town in Japan where methylmercury poisoning devastated many residents in the mid-1900s. This agreement will phase out the use of some mercury-containing products and curtail some mercury pollution by 2020.

In the mean time, however, mercury levels in the atmosphere and ocean will continue to rise, Amos said. Even if emissions are cut in half—a very optimistic assumption—the effects wouldn't be seen in food webs for at least a decade, she added.

Other steps need to be taken to protected people—especially pregnant women and their developing fetuses, and newborns—from mercury, said Philippe Grandjean, a medical epidemiologist at Harvard who wasn't involved in the study. Grandjean details how pollutants like mercury are causing a "silent epidemic" of chemical brain drain throughout the world in a book published earlier this year.

"Given that there are substantial populations worldwide who are exposed to hazardous levels of mercury, we can't wait for the convention to correct that," Grandjean said.

He advocates taking secondary measures—like advising pregnant women what fish are healthy to eat (such as young, small fish, and those that don't prey upon other fish). One way to protect pregnant women would be to test their hair for mercury, to see if they might need to change their diet to reduce exposure to the heavy metal. This test could be done as cheaply as other widespread lab tests, Grandjean said.

"The Minamata convention includes only weak and general recommendations for health surveillance, but this new [study] makes it even more urgent to institute procedures where pregnant women can obtain dietary advice to avoid mercury exposure, preferably with the option of testing for mercury in hair," Grandjean said.

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