Mercury: A Persistent Toxin

by Bill Chameides | October 29th, 2008
posted by Erica Rowell (Editor)

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Mercury poisoning has a long history. The phrase “mad as a hatter” (circa 1830s) derives from the slurred speech, tremors and other neurological symptoms displayed by the day’s hatmakers – maladies caused by long-term exposure to mercury used in hat-making. Today, the highly toxic metal continues to plague us, and new studies indicate even very low doses of mercury can be risky. But there’s at least one way we can safeguard against it – remember, you are what you eat.

Mercury is a toxin. Short-term acute exposure can cause pneumonia-like symptoms. Low-dose chronic exposure causes neurological damage along the lines of what the nineteenth-century hatmakers exhibited. A study from 2005 found that hundreds of thousands of U.S. babies were born with reduced IQs because of in vitro mercury exposure.

Studies as far back as the 1920s have recorded the serious effects of mercury poisoning. German chemist Alfred Stock, who suffered from chronic mercury poisoning, conducted decades of research on the toxin. Yet today we’re still battling how to keep safe around this toxic substance.

Where Does Mercury Come From?

Mercury is both naturally occurring and manmade. One of the most common ways we’re exposed to it is through eating fish. Because mercury is present in every ocean, lake and stream, all fish contain some level of the toxin. And since mercury accumulates in fish tissue, large fish that eat smaller fish (like tuna and swordfish) tend to have more of it.*

Figuring out how the mercury gets into our waterways is trickier.

Many contaminated oceanic fish ingest natural mercury. However, the 2,000 tons of mercury pollution spewed annually from coal-fired power plants and other industrial facilities around the world, penetrate into lakes and streams, thus contaminating fish, especially those closest to the source.

Every year federal and state agencies issue fish advisories and sometimes close areas to fishing because of mercury contamination. (See here and here.)

Today we have a good idea of the local effects of mercury contamination. But how far mercury pollution can travel -– and how much damage its long-range transport can do — is more of a mystery. China, for instance, the world’s largest emitter of mercury, might play a significant role in fish contamination in America or might not.

Up until now, it has been difficult to know just where mercury contamination in a given location came from. A new study published in Environment Science & Technology may change all that. Now it may be possible to use the abundances of mercury isotopes (mercury atoms with different numbers of neutrons) to trace mercury contamination back to its source.

Reducing Mercury Exposure

So far, the best way we have to reduce our exposure to mercury is by eating less fish. Attempts to steer clear of mercury via reduced seafood consumption began in earnest in the 1960s. One effort took the form of issuing fish advisories and distributing information about the dangers of contaminants. Other ways involved changing mining and manufacturing practices.

More recently, EPA’s Clean Air Interstate Rule (CAIR) and Clean Air Mercury Rule (CAMR) represented another effort that targeted industrial sources of mercury. Together, these two rules aimed to significantly cut pollution from coal-fired power plants through a cap-and-trade system, reducing mercury emissions by 70 percent by 2018.

Many public health experts and environmentalists opposed CAMR because, they argued, allowing a toxin like mercury to be traded would create “hotspots” or areas of concentrated pollution. A proposed alternative regulating mercury as a “hazardous air pollutant” would require emission reductions at all sources.

Earlier this year, the U.S. Court of Appeals vacated both CAIR and CAMR, citing violations of the Clean Air Act. Just this month, the court agreed to reconsider its CAIR ruling, but since no D.C. Circuit Court judge had favored a rehearing of CAMR earlier, it’s unlikely that CAMR will be considered again.

While regulating mercury emissions remains up in the air, a couple new studies suggest problems with mercury remain serious and demand attention.

Mercury Linked to Heart Disease

Adding to the long-known dangers of mercury is a new link to heart disease. Several population studies in Finland over the past few years show that men with high mercury concentrations (based on hair sampling) were more likely to die from heart disease compared to men with lower mercury levels (see here and here).

Another study, recently published in the American Journal of Physiology, found that even “low dose” exposure of mercury may up the risk of heart disease. The researchers found that rats exposed to initial doses of 4.8 micrograms/kg of mercury followed by daily doses of .07 micrograms/kg developed impaired artery function, a key factor of heart disease, after 30 days.

Mercury Levels in U.S. Women: Some Good and Bad News

In August, an EPA study [pdf] of American women of childbearing age turned up more sobering findings: EPA estimated that 7 million women (or one out of every 10) have blood mercury levels in excess of the 3.5 parts per billion (ppb) threshold that may pose a risk to fetuses.

There was a bit of positive news too, though: the number of women with dangerous blood mercury levels (~ 5.8 ppb) had decreased by a factor of two during the periods 1999-2000 and 2003-2004. The authors speculated that the drop came about by women eating less of the most heavily contaminated fish (e.g., swordfish, shark, tilefish, albacore tuna).

My Recommended Steps for Reducing Your Exposure to Mercury

While we wait for environmental protections like the Clean Air Act to do what they were intended to do –- keep us safe from pollutants –- you can help protect yourself by monitoring your intake of contaminated fish. Studies show it is clearly having an effect, so:

  • Pay close attention to fish advisories,
  • Don’t fish near a coal-fired power plant,
  • Eat sushi and other large oceanic fish in moderation, and
  • Get your hands on a seafood selector [pdf] and/or sushi selector.


*This post has been revised to reflect the following correction:

Corrections: November 5, 2008

Originally, we explained that “since mercury accumulates in fish tissue, large fish that eat smaller fish (like tuna, salmon, and swordfish) tend to have more of it.” That was largely true, but in fact not all

large fish that eat smaller fish have higher levels of mercury. Salmon, for instance, does not have elevated levels of mercury. The levels of contamination depend on what kinds of fish the larger fish eats.

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