College Central®

Ask around. The Network works.®

Mercury in Fish -– Which Fish, and Why?

Mark Jeantheau -- This article presents information on mercury levels in fish, including mercury in tuna. It lists guides to help you determine which are fish high in mercury, and points to other resources for mercury in fish.

Unless you're like The Addams Family's Uncle Fester, who delighted in occasionally having a mercury thermometer as a snack, any mercury exposure you may suffer probably comes from eating certain types of fish. Unfortunately, that favorite "sandwich fish" of ours — "solid white" tuna — is one of the most contaminated little fishies.

Many of you may remember the mercury-in-tuna advisory a few years back from the US EPA and FDA that said we should limit consumption of tuna. But by how much? And what other fish have a mercury problem? How might we be affected by eating mercury-tainted fish? How did things get this way, and what can we do about it?

The first question we'll tackle is how planet earth turned into planet mercury.

How We Get Mercury in Fish, the Environment, and Us

We humans may get small amounts of direct exposure to elemental mercury — for instance, from a broken mercury thermometer, by breathing polluted air from a nearby coal-fired power plant, or from the mercury-based preservative found in many vaccines. The majority of our mercury exposure, however, is from eating certain species of fish. But mercury is not a natural toxin in fish, so how did it get there?

The chain of events goes something like this:

Mercury gets into the air as a byproduct of industrial activities such as power generation from coal; garbage incineration; recycling of older automobiles; and some types of mining, manufacturing, and chlorine production. Smaller volumes of mercury are also released through volcanic eruptions and rock weathering.

From there, the airborne mercury is deposited on land and water, where microorganisms convert it into a more biologically active form, methylmercury.

The methylmercury then works its way up the food chain. Because organisms tend to store mercury, not excrete it, concentrations of mercury get higher and higher as larger, longer-living creatures gobble up smaller ones. By the time you get to a top predator fish like a swordfish, albacore tuna, or shark, the concentrations are quite high. Fish can also absorb methylmercury directly from water as it passes over their gills, though this is a lesser avenue of accumulation.

When humans eat mercury-contaminated fish, they get the mercury that's in the fish's flesh.How We Get Mercury in Fish, the Environment, and Us

Effects of mercury — Who's most at risk?

According to Physicians for Social Responsibility (PSR), research has shown that:

-- short-term exposure to high concentrations of mercury vapor has harmful effects on the nervous system, digestive system, respiratory system, and kidneys;
-- long-term exposure to mercury can permanently damage the brain and kidneys at any age;
-- long-term animal studies have also found that exposure to organic mercury (such as methylmercury) at high levels can cause nervous system damage; damage to the kidneys, stomach, and large intestine; changes in blood pressure and heart rate; adverse effects on male reproductive organs, sperm, and developing fetuses; and an increase in the number of spontaneous abortions, and stillbirths.

Because mercury is particularly troublesome for developing nervous systems, fetuses, infants, and young children are most at risk. According to PSR, epidemiologic studies have found that children exposed to even low levels of mercury before birth experience subtle symptoms of neurological damage. Specific effects include poor performance on neurobehavioral tests (particularly on tests of attention), fine motor function, language, and memory.

The National Academy of Sciences states that mercury in pregnant women disrupts the development of brain cells in their babies. A report by Centers for Disease Control concluded that one in six women of childbearing age have mercury in their blood above the level that would pose a risk to a developing fetus. Clearly, pregnant women and children should exercise extra caution when it comes to fish consumption.

Finding the right fish

Choosing the right types and amount of fish to eat is essential. Here are several useful tools that will help you reduce mercury exposure for you and your family by helping you determine which fish are safe to eat and in what quantities.

PSR Guide: The Physicians for Social Responsibility (PSR) has a fish guide document (opens as PDF) that includes a wallet-sized card that rates fish for mercury contamination. However, be sure to pay attention to the little "over-fished" symbol next to some species so you don't solve your mercury problem only to contribute to the decline of a disappearing fish species.

Note: PSR's fish guide PDF mentions the URL "" as a place to go for more information. That URL is out-of-date (and now in fact points to a pro-mercury propaganda site). The correct "Mercury In Fish" information paper from PSR can be found at

Tuna Tabulator/Tuna Calculator: The Natural Resources Defense Council has put together a table for tuna consumption based on the US EPA's mercury guidelines, which are more cautious that the FDA's guidelines (which, unfortunately, tend to have dominant influence on recommendations made by mainstream food experts). The NRDC table lets you figure out the safe amount of tuna for you or your child based on body weight and tuna type (solid white or chunk light). Similarly, the Environmental Working Group has a calculator-style tool at that helps you do the same thing.

Caught fish: Remember that some geographical areas are exposed to more mercury than others, so fish that you, your family, or your friends catch from local bodies of water could be more contaminated than the national average. Many states, including Grinning Planet's home state of Kentucky, have issued local-fish advisories tied to mercury.

US EPA data show that more than three-quarters of fish sampled from the nation's lakes have mercury levels that are high enough to pose a threat women of childbearing age and children younger than three. (And the rest of us can take our chances, I guess.) You can check your state at for advisories about the safety of fish in your local lakes, rivers, and coastal areas. If no advice is available, FDA recommends that you eat no more than 6 ounces per week of any fish you catch from local waters and that you not consume any other fish during that week.

Fingering the bad guys on mercury contamination

On the surface, it's easy to identify the black hats in the mercury-pollution game: they're the ones who are dragging their feet on reducing mercury air pollution. That typically includes any industry whose processes emit mercury, but the unrelenting lobbying by the coal industry against tougher mercury standards has been particularly odious.

To be fair, the public hasn't done much to demand reductions in mercury pollution, and most of us have enjoyed the benefits of cheap-but-dirty coal-generated electricity for decades (as well as the fruits of the other industrial processes the cause mercury pollution). Still, many manufacturing sources have been greatly cleaned up, and the focus is now squarely on coal-fired power plants, which are the largest source of mercury air emissions worldwide.

Mercury releases -- natural vs. manmade

Coal-industry boosters and other defenders of mercury-polluting industries sometimes assert that human-caused releases of mercury are insignificant compared to releases from natural sources like volcanoes. Not so, according to the United Nations Environment Programme, which estimates that human activities have tripled the amount of atmospheric mercury. The US Geological Survey agrees — they estimate human activities have doubled or tripled the amount of mercury in the atmosphere over the past 150 years relative to the pre-industrial baseline.

It's a shame that we have let the mercury problem fester for so long. It would have been worth a few extra bucks every month on our power bills to keep the fish supply clean. But it's better that we take action late instead of never. The technology exists to reduce emissions from dirty coal-fired power plants by 90% within a few years — much better than the "70% reduction of mercury by 2018" plan previously pushed by the Bush Administration. We should accept nothing less the 90% solution.

You can keep up with the latest in "mercury in fish" stuff and other air-pollution issues NRDC's Air page at

Mark is a writer, financial analyst, Web developer, environmentalist, and, as necessary, chef and janitor. Grinning Planet is an expression of Mark's enthusiasm for all things humorous and green, as well as a psychotic desire to work himself half-to-death. Hobbies include health foods, music, getting frustrated over politics, and occasionally lecturing the TV set on how uncreative it is. For jokes, cartoons, and more great environmental information, visit If you're not already on it, you can sign up for our free mailing list so you don't miss anything.

© 2009 Mark Jeantheau

Return to top

The views and opinions expressed in these articles do not necessarily reflect those of College Central Network, Inc. or its affiliates. Reference to any company, organization, product, or service does not constitute endorsement by College Central Network, Inc., its affiliates or associated companies. The information provided is not intended to replace the advice or guidance of your legal, financial, or medical professional.