July 27, 2005

Mercury

You've probably heard warnings about eating too much fish because of mercury. In Minnesota, for children under 15 and women who would like to have kids the recommendations are as follows--Sunfish: once a week. Pike: once a month. Shark: do not eat. The contamination is more severe farther north. Further, mercury is concentrated by the food chain. It builds up in fat, and so predators like pike or tuna tend to have higher levels than caribou. These two factors (location, diet) put Arctic people at great risk. Measurements of mercury levels in the blood of Inuits living on Greenland show levels above those known to produce neurological damage.

I never used to know where the mercury was coming from. Turns out there are three main anthropogenic sources:

1. Fossil fuel combustion, especially coal
2. The extraction of metals from ore, especially non-ferrous ('not like iron') metals.
3. Waste treatment, for example batteries, thermometers and tooth fillings.
x. There are also natural sources, like the land surface, ocean surface and volcanoes, and mercury emitted previously can be re-emitted by surfaces. The best current estimate is that natural emissions are 88 % of anthropogenic.

(Its true, if you're like me, you are constantly being exposed to mercury vapor from amalgam tooth fillings. The concentrations go up when you drink hot liquids. Its probably not necessary to have your fillings replaced with plastic though as the vapor is in the form of elemental mercury which is not as toxic as methyl mercury or inorganic mercuric oxides or halides.)

Of the countries bordering the Arctic region (Canada, Denmark, Finland, Iceland, Norway, Russia, Sweden, US), the US emits the most mercury overall, and the most per capita. In 1999 the US emitted 109 metric tons of mercury, much more than our closest rival Russia at 39 metric tons and far beyond Canada at 8. The US is even outdoing mother nature-- the natural emission of mercury from volcanoes is estimated at 94.6 metric tons per year. The US annual emissions amount to 370 mg of mercury apiece. In the per capita rankings, Russia, Canada and Denmark tie for second place at about 200 mg per person per year. Beyond the 'Arctic' countries, Asia is responsible for about half of the annual global emission of mercury of ca. 2200 tons.

Two of my students are doing research on the atmospheric chemistry and transport of mercury, from the point of emission in populated regions to deposition in the Arctic environment. One of them will be visiting the US EPA later this year before going to Alaska to make field measurements of atmospheric mercury concentrations.

4 Comments:

At August 19, 2005 6:39 PM , Anonymous Andrea said...

In May we went to the BWCA and ate several fish. My friend didn't realize at the time that she was pregnant. Should she be worried?

 
At August 22, 2005 3:26 PM , Blogger Matt_J said...

I can't say for sure but there is probably no reason to worry. It was an isolated occasion, and the health recommendations have a safety factor built in, a factor of ten to 100 greater than the consumption level that would show noticeable effects. What kind of fish were they? Bass, walleye and perch are not as bad as northerns. The documented cases that I know of are in Greenland Inuits, where they eat fish & seal several times a day, every day.

 
At August 22, 2005 11:43 PM , Anonymous Andrea said...

We had all three - mostly northern, though. Bass won the taste test.

 
At August 23, 2005 3:30 PM , Blogger Matt_J said...

Another reason not to worry, I have been told, is that they make the recommendations based simply on the total amount of mercury in the fish. The analysis does not take into account what chemical form the mercury is in. For example, livers typically contain a lot of mercury but it is an oxidized, inorganic form that has a low toxicity, so its not part of the problem. The problem is organic mercury (e.g. methyl mercury) that can screw up the nervous system. You remember how we used to fight to see who got to eat the fried northern livers? No big deal.

 

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