Mercury facts

The Idaho Statesman

30th October 2004

• How does mercury threaten public health?

Methylmercury in the water and sediment is taken up by plankton. Minnows and juvenile fish eat large quantities of plankton over time. Larger predatory fish consume many smaller fish, accumulating methylmercury in their tissues. The older and larger the fish, the greater the potential for high mercury levels in their bodies. Fish are caught and eaten by humans, causing methylmercury to accumulate in human tissues. Since mercury is tightly bound to proteins in all fish tissue, including muscle, there is no method of cooking or cleaning them that will reduce the amount of mercury in a meal. From the 1950s to the 1970s, several mass poisonings took place in Japan and in Canada involving methylmercury from consumption of fish from contaminated waters. In the United States, the number of states that have issued health advisories limiting consumption of fish has risen steadily from 27 states in 1993 to 41 states in 1999.

• How serious a problem is it?

Mercury is considered the most dangerous heavy metal because it moves freely through the environment. Currently, public health officials are most concerned about chronic exposures to low levels of mercury from fish. Preliminary estimates of mercury levels in hair and blood samples from the 1999 National Health and Nutrition Examination Survey suggest that approximately 10 percent of all women have mercury levels within one tenth of potentially hazardous levels. The National Research Council issued a report estimating that as many as 60,000 newborns a year in the United States are now at risk for adverse neurological effects from dietary mercury, mostly from fish.

• Give me the numbers

In 1996, EPA set its guideline for methylmercury in the diet at 0.1 micrograms of mercury per kilogram of body weight per day. A 1994 EPA survey found an average of 0.26 parts per million of mercury in freshwater fish nationwide. Ocean fish have average mercury levels of 0.21 ppm. An average woman weighing 132 pounds can eat 6 micrograms of mercury per day without exceeding the EPA recommended dose. If each gram of fish contains 0.2 micrograms of mercury, our average woman could only eat 30 grams of fish per day without exceeding the EPA dose. Since there are 28 grams in an ounce, a woman planning to have children would only be safe eating seven ounces a week, or one serving. The walleye sampled in Salmon Falls Reservoir had .55 ppm of mercury. So the average woman who ate one meal a week from Salmon Falls would exceed EPA's recommended dose, which is conservative by world standards. The World Health Organization's recommended dose is 4.7 times higher than EPA's.

• The mercury from gold mines is elemental. How does it turn into the more lethal methylmercury?

Chemical reactions in the atmosphere and bacteria in water transform the mercury at varying rates. It drops from the sky onto land and in waterways, then rain and runoff carry the mercury into lakes and rivers.

• How do Nevada's gold mines compare with other industrial sources of mercury?

One mine in 1998 was emitting 9,400 pounds of mercury a year, while another emitted more than 2,200 pounds. Coal-fired power plants are the national focus of mercury pollution, but each plant emits only an average of 250 pounds of mercury annually.

• How have the mines reduced their mercury emissions?

Cinnebar, the natural source of mercury, is mixed in with the gold ore. When the ore is roasted, mercury is released and some is collected, some is left in the waste rock while some goes up the stack. The mines have added scrubbers to their smokestacks that remove the mercury from their pollution plume. The miners also have changed their processing so they can collect more mercury or leave more in the waste rock.

History lesson

Ever heard of the Mad Hatter? This Alice in Wonderland character got the name from the phrase "Mad as a hatter," popular during the 1830s. It came from the fact that hatters really did go mad. Mercurous nitrate was used in curing felt for hats. Prolonged exposure to the mercury vapors caused hatmakers to develop severe and uncontrollable muscular tremors and twitching limbs, called "hatter's shakes."They also suffered from distorted vision, confused speech and even hallucinations