Not All Fish Puts You at Risk of Mercury Poisoning

No need to quit sushi.

silver fish on yellow background with mercury poisoning splatters salmon pregnancy tuna fetus

Photo Illustration by Kelly Caminero/The Daily Beast

If you’ve ever eaten sushi, you may be familiar with rumors and rumblings about how eating too much can give you mercury poisoning. Pregnant women are typically advised to steer clear of any spicy tuna roll cravings because.

It has to be dangerous to eat too much of a certain kind of fish, right? Well, sort of. Mercury poisoning, and fear of it, is laced with misconceptions, in part because the metal comes in different forms and thus, has different modes of poisoning someone.

Mercury is a natural metal found in the earth, Judith Zelikoff, a professor in NYU’s School of Medicine’s department of environmental medicine, told The Daily Beast. It’s potentially dangerous to ingest because it inhibits inhibits selenoenzymes, which protect the immune system, causing toxicity. Methylmercury is the most common type of organic mercury that is found in the earth and what is commonly in fish that most commonly leads to mercury poisoning. There’s also inorganic mercury, or elemental mercury, that is found in old-school thermometers.

“The problem comes from environmental exposures,” Zelikoff said, pointing to mercury-containing coal combustion as the problem. “So when [coal] is burned and combusted, it gets into the air. Once it’s in the air it falls to the ground, not only to the soil but also to the surface of water and rivers and lakes.

Not every kind of fish has mercury.

“There are microorganisms in the water that convert the mercury to methylmercury, and that’s the kind accumulated in fish, so now you have an exposure pathway to humans.”

What Zelikoff is referring to is the fact that as the mercury moves up the food chain, it gets more concentrated. So small fish ingest the methylmercury, are eaten by bigger fish and eventually ingested by humans at its most concentrated—and dangerous—form, Zelikoff explained.

But not every kind of fish has mercury. Mercury content depends on where the fish is coming from. A 2017 study by the Biodiversity Research Institute and the Institute of Public and Environmental Affairs looked at mercury levels in women ages 18-44 (called childbearing age, by the study) in 25 countries. The study found elevated mercury levels across many regions, specifically where gold mining and industrial pollution was highest, like in Indonesia, the Pacific Islands, Kenya and Myanmar. Women in Alaska had elevated levels of mercury, too, most likely tied to a diet high in fish and sea mammals, like seals. Pollution is to blame in Albania, Chile, Nepal, Nigeria, Kazakhstan, and Ukraine, where elevated mercury levels in women have been linked to suspected fish contamination in the water.

But what kind of fish should you steer clear of? Advice varies on this. The Food and Drug Administration suggests that shark, swordfish, king mackerel, and tilefish contain high levels of mercury, with albacore tuna having higher levels of mercury than canned tuna, and shrimp, canned light tuna, salmon, pollock, and catfish having lower levels of mercury. The Environmental Protection Agency has a location-based advisory to confirm mercury levels where you live (or eat).

There’s a difference between elevated mercury levels and mercury poisoning.

The risk of mercury poisoning also depends on who you are. High mercury levels are most concerning for women who are of a reproductive age, Zelikoff said, as the mercury can pass through the placenta and into the fetus.

“The mercury accumulates and concentrates in the fetus, which can cause all kinds of neuro impairments and birth defects,” Zelikoff said.

A mother may not show symptoms, but a fetus could, Zelikoff said. Researchers at Harvard Medical School found lower cognition in babies whose mothers had high levels of mercury during pregnancy.

There’s no data to support that mercury poisoning affects men differently than women, but that women are the ones that become pregnant means there’s an issue to be more cautious about, Zelikoff said.

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There’s a difference between elevated mercury levels and mercury poisoning, though, said Ruben Olmedo, a Mt. Sinai professor of emergency medicine who specializes in toxicology.

“This is where the fallacy comes in as to whether you’re poisoned or not,” he said. “Elevated means that at some point you probably ended up ingesting something that had mercury, but it doesn’t mean your poisoned. Poisoned means those levels, that mercury or whatever you ingested actually got to the organs, which is where mercury likes to go, to your central nervous system and your kidneys.”

If you do have symptoms of mercury poisoning like neurological dysfunction, confusion, or memory loss, or paresthesia (tingling in hands or feet), a toxicologist would run a test on your urine for 24 hours to confirm either mercury toxicity or poisoning before moving ahead with treatment. Patients with poisoning are treated with chelation therapy, which binds to mercury to remove it from the body.

“Checking metals in blood or in urine is not something routine that’s done, so if you have symptoms, especially if you’re saying you eat fish or sushi twice a day, then you should ask for it to be checked,” Zelikoff advised.

Your body will rid itself of mercury over the course of about a year and a half, Zelikoff said. Chelation therapy can also be done if you have elevated levels without poisoning. However, the therapy can also bind to essential elements in the body, which you may not  want excreted.

The bottom line: Be aware of what fish you’re eating. Know where it came from, especially if you’re traveling to other countries, and talk to your doctor if you’re concerned, specifically if you’re thinking about pregnancy or are already pregnant.