September 2006 / Links updated 2012 WHAT I DIDN'T KNOW ABOUT MERCURY
Until I decided to write the present piece, I thought I knew as much as I needed about the risk I faced from eating mercury-laden fish. I knew, for instance, that mercury -- which passes easily from mother to child in utero and through breast milk -- is a neurotoxin that can damage the developing brain. I also thought I knew that I, personally, had nothing to worry about, as I eat very little fish that's high in mercury and don't plan to have more kids.
I have been chastened.
Surprise #1: My mercury exposure has been above the level considered safe by the Environmental Protection Agency for three of the past four weeks, according to NRDC's mercury calculator. (This is just an estimate based on averages, not a measurement of my actual exposure, which would require that I be tested.)
It wasn't that I overindulged. There was only one week when I ate fish more than once (ironically, my "safe" week), and my portions were all moderate in size.
The problem is with the fish I chose to eat -- in particular, shark and swordfish. As large, predatory species, they contain much higher levels of mercury than small fish, such as anchovies and sardines, because of the way mercury moves up the food chain. I knew this but didn't realize that a single 6-ounce portion in a week would be enough to put me over the top -- by a factor of four.
(In passing, I should note that shark was a bad choice from a sustainability standpoint, as well, as shark populations are dangerously low.)
Even the single portion of smoked bluefish I had one week was borderline. And bluefish has the added problem of contamination with PCBs.
Surprise #2: The danger from mercury is not just to developing brains. A growing body of evidence suggests an association between mercury exposure and heart disease. So even middle-aged people, like my husband and myself, should take care what they eat.
These two surprises were enough to make me shun the fish stand at the greenmarket last Saturday. But that was foolish: eating fish low in mercury is good for the heart, and low-mercury choices abound.
So, which fish are good choices? There's a longish list, including some popular fish, such as sole, tilapia, clams and oysters. When it comes to canned tuna, chunk light is much better than white albacore, as it typically has about a third of the mercury (though Consumer Reports recently found that some cans of chunk light can contain as much mercury as most albacore). Just be careful how much canned tuna you consume, as quantity matters -- even more for kids than adults because they weigh so little.
And what's bad? Well, swordfish and shark top the list, along with king mackerel, marlin, orange roughy and tilefish. Steer clear of them altogether if you are trying to become pregnant, are already pregnant or are nursing, and obviously, don't feed them to young children. Even if you aren't planning on children, if you are a woman of reproductive age, it would be wise to limit your intake in case you change your plans or become pregnant without planning -- as it takes months to shed your body of mercury.
There are many other fish besides the ones I've listed here, including many with moderate levels of mercury that are safe to eat in limited quantities. So, do yourself a favor and consult NRDC's guide to mercury in fish. Knowledge is power -- and a key to good health.
Where does mercury come from?
Mercury is a natural substance, one of the basic elements (Hg), and is bound up in the earth's crust where it does no harm. It becomes a problem when it is released into the atmosphere and settles in bodies of water. It can then be converted by microorganisms to a more dangerous form (methyl mercury) and ingested by fish, which are consumed by fish-eating animals, including humans.
The release of mercury occurs through natural processes, such as volcanic eruptions and forest fires. However, human activities since the beginning of the Industrial Age have tripled the amount of mercury in the atmosphere, according to the U.S. Geological Survey.
There are a number of human sources of mercury emissions, from old chlorine plants to gold mining, but the single largest one is coal-fired power plants. This is just one reason why coal is such a bad source of electricity. Global warming is another.
Sheryl Eisenberg, a long-time advisor to NRDC, posts a new This Green Life every month. Sheryl makes her home in Tribeca (NYC), wherealong with her children, Sophie and Gabby, and husband, Petershe tries to put her environmental principles into practice. No fooling. Read more about Sheryl.
Mercury (at left). If a mercury thermometer breaks, the mercury will bead like this and release toxic vapors. Learn how to protect yourself.
Risk to children in the United States. One in 17 women of childbearing age has more mercury in her blood than the safety level set by the government of 5.8 micrograms per liter of blood, according to a 2005 report from the Centers for Disease Control and Prevention. Recent science suggests that even 3.4 micrograms might pose a risk to the fetus because mercury concentrates in the umbilical cord. Nearly one in 10 women of reproductive age in the United States has that level or higher.
Don't give up on fish. The American Heart Association recommends that you eat fish at least two times a week because fish—especially fatty varieties—are high in omega-3 fatty acids, which can help prevent heart attacks. Fatty fish that are low in mercury include mackerel (but not king mackerel), herring, sardines and salmon.
Suspect your mercury is high? You can get a self-testing kit from Greenpeace for $25. Your lab results will be kept confidential but used in a study, anonymously, to determine if mercury exposure levels in the United States exceed standards set to protect public health. If you have serious reason for concern, consult your doctor. In general, you should be able to bring your mercury down to safe levels in 6 months if you alter your diet to avoid mercury.
Consult local advisories. If you fish for sport and eat your catch, consult local fish advisories for mercury contamination warnings. This is also a good idea if you buy fish caught in local waters.
Coal plants are the biggest human source of mercury emissions in the world.
--------------------------------- Sheryl Eisenberg is a web developer and writer. With her firm, Mixit Productions (http://www.mixitproductions.com), she brought NRDC online in 1996, designed NRDC's first websites, and continues to develop special web features for NRDC. She created and, for several years, wrote the Union of Concerned Scientists' green living column, Greentips, and has designed and contributed content to many nonprofit sites.