Monday, March 30, 2009

Mercury Pollution in the Environment

If eight percent of women ages 18-49 -- child-bearing age -- have mercury levels exceeding federal health guidelines, how many children have potentially inherited high levels of mercury from their mothers?

“Tens to hundreds of thousands," says Helen Hsu-Kim, professor of civil and environmental engineering at Duke.

Hsu-Kim discussed all aspects of mercury pollution during a March 26 talk in Giles, a freshman dorm. Hers was the last of this year’s Chautauqua series, a program designed to facilitate discussion and give Duke freshmen greater access to accomplished faculty.

Seafood is perhaps the widest-known source of mercury exposure, Hsu-Kim said. However, additional sources include occupational inhalation, broken thermometers or compact fluorescent light bulbs, dental amalgam fillings, some flu vaccines, and transmission from mother to child through the placental barrier. Animals at the top of the food chain, such as humans, are most vulnerable to mercury accumulation because they eat large numbers of animals that have already accumulated mercury from their own diets.

Mercury poses the greatest threat to human health when it accumulates in the body. Elemental mercury is not accumulated, and is excreted by the body within days or weeks of consumption. Methyl mercury, on the other hand, can remain in the body for years if incorporated into the central nervous system. [More about the effects of mercury]

Overexposure to mercury can have dangerous repercussions. Typically, victims of mercury poisoning experience confusion, anxiety, depression, and impaired movement, speech, and vision.

The Mad Hatter, from Alice in Wonderland, is believed to have been inspired by mercury’s toxic effects, according to Hsu-Kim. Mercury used to be involved in the hat-making process, when it was applied for curing felt. Hatters could not avoid exposure as they worked, and many later suffered from significant neurological impairment. From this phenomenon, the phrase “mad as a hatter” emerged, and perhaps the inspiration for Lewis Carroll’s memorably “mad” character.

In some places, mercury exists naturally in the environment, and elemental mercury is occasionally released. However, the main releases of mercury are anthropogenic: coal power plants and artisanal gold mining. (In some parts of California, there are still high concentrations of mercury from the Gold Rush era!)

Mercury is a byproduct of coal combustion. It is released into the air via smokestacks, and also is present in fly ash. The controversy surrounding fly ash is that it is not treated as hazardous waste. Often, power plants will dump fly ash in large outdoor retention ponds-- there are around 1300 in the U.S. alone. However, what happens when this ash contaminates the surrounding environment?

Last Christmas, a retention pond in Tennessee gave way, releasing one billion gallons of coal ash sludge into a bordering river. People’s homes and backyards were inundated with a potentially toxic layer of muck. This material should be considered hazardous, according to Hsu-Kim, not stored in large pools that obviously are vulnerable to leaking or breakage.

And what about the mercury that coal plants release into the air? Unlike elemental mercury, which the body successfully excretes, the mercury from power plants is converted into methyl mercury-- the most dangerous kind. While the U.S. has reduced its coal-related mercury emissions by half over the past decade, developing countries are still highly reliant on coal for power production, and are responsible for an increasing percentage of mercury emissions.

Efforts have been undertaken to capture mercury before it leaves the smokestack. But this also creates an issue, according to Hsu-Kim: dispersed mercury in the air, or concentrated mercury in solid form? The latter still must be disposed of in some way.

“How do we balance energy security, waste management, and materials reuse?” Hsu-Kim said. It’s a question that largely remains to be answered.

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