Fish and shellfish are by far the biggest source of mercury in the human diet, but they aren’t the only one. Rice, certain vegetables, and even wine contain measurable amounts. The type of mercury that matters most in food is methylmercury, a form your body absorbs easily and eliminates slowly, allowing it to build up over time.
Why Fish Top the List
Mercury enters waterways from both natural sources (volcanic activity, weathering rock) and industrial pollution (coal-burning power plants, mining). Once in the water, bacteria convert it into methylmercury, which gets absorbed by tiny organisms at the base of the food chain. Small fish eat those organisms, bigger fish eat the small fish, and each step concentrates the mercury further. This process, called biomagnification, means the largest, longest-lived predators at the top of the chain carry the highest loads. A swordfish or shark can accumulate mercury concentrations 4 to 69 times greater than the prey beneath it in the food web, partly because these animals simply live longer and have more years of accumulation.
Highest-Mercury Fish to Limit or Avoid
The FDA places seven types of fish in its “Choices to Avoid” category for having the highest mercury levels:
- King mackerel
- Marlin
- Orange roughy
- Shark
- Swordfish
- Tilefish (from the Gulf of Mexico)
- Bigeye tuna
These are all large, predatory, or exceptionally long-lived species. Orange roughy, for example, can live over a century, giving it decades to accumulate mercury in its tissue.
The Canned Tuna Question
Not all tuna is equal when it comes to mercury. Canned white (albacore) tuna averages about 0.407 parts per million of mercury, while canned light tuna (typically skipjack) averages 0.118 ppm. That’s roughly a three-and-a-half-fold difference. Albacore is a larger species than skipjack, which explains the gap. If you eat tuna regularly, choosing light or chunk light over white significantly cuts your exposure.
Lower-Mercury Seafood
Plenty of fish and shellfish fall well below the high-mercury threshold. Salmon, sardines, anchovies, herring, tilapia, shrimp, pollock, catfish, and cod are all generally low in mercury. The FDA recommends eating two to three servings of these lower-mercury options per week to get the nutritional benefits of seafood, particularly omega-3 fatty acids, without excessive mercury exposure.
Many of these fish also happen to be rich in selenium, a mineral that binds to mercury and can partially offset its toxic effects. When selenium is present in a molar ratio greater than 1:1 relative to mercury, it helps protect the antioxidant enzymes that mercury would otherwise disable. Most ocean fish naturally contain more selenium than mercury, which is one reason moderate fish consumption remains recommended despite the mercury concern.
Non-Seafood Foods With Mercury
Fish dominates the conversation, but research using national health survey data (NHANES 2011-2012) found that even among people who hadn’t eaten any seafood recently, certain foods were linked to higher blood mercury levels. Mixed rice dishes showed a statistically significant association with both blood and urinary mercury. Red and leafy vegetables, as well as vegetable oils, were also associated with elevated blood mercury. Wine was linked to higher methylmercury levels specifically, and liquor showed a connection to total blood mercury.
Rice deserves special attention. It’s grown in flooded paddies, and the waterlogged soil creates conditions where bacteria readily convert mercury into its methylated form, which the rice plant then absorbs through its roots. For most people eating a varied diet, the amounts from rice are small compared to what you’d get from a serving of high-mercury fish. But in populations that eat rice as a dietary staple and consume little seafood, rice can become the dominant source of mercury exposure.
How Mercury Affects Your Health
Methylmercury targets the central nervous system. At high or sustained exposure levels, it can cause problems with coordination and movement, impaired thinking and memory, vision and hearing loss, and seizures. The developing brain is especially vulnerable. In pregnant women, methylmercury crosses the placenta, and fetal exposure has been linked to cerebral palsy, developmental delays, and abnormally small head size. This is why guidelines around mercury in food focus heavily on pregnant and breastfeeding women and young children.
For most adults eating a typical diet, mercury exposure stays well below the levels associated with these serious effects. The EPA’s reference dose for methylmercury is 0.1 micrograms per kilogram of body weight per day, a threshold designed to include a wide safety margin. For a 150-pound person, that works out to roughly 6.8 micrograms per day. A single serving of swordfish can contain 100 micrograms or more, which illustrates why frequency matters as much as portion size.
Does Cooking Reduce Mercury?
It depends on the method. Baking and frying reduce the amount of mercury your body can actually absorb (its bioaccessibility), with higher-heat methods having a stronger effect. In one study, baking dropped mercury content from about 214 to 193 nanograms per gram. Steaming and marinating, on the other hand, actually increased mercury concentration by nearly 20%, likely because moisture loss concentrates whatever mercury remains in the flesh.
What you eat alongside fish also matters. Boiled broccoli and fresh garlic both reduced mercury bioaccessibility in lab testing, while lemon juice slightly increased it. The working theory is that plant compounds (phytochemicals) can bind to mercury in the gut and reduce how much gets absorbed. So pairing fish with a side of vegetables may offer a small but real protective benefit.
Practical Takeaways for Your Diet
The simplest way to manage mercury exposure is to pay attention to which fish you eat and how often. Avoid the seven highest-mercury species entirely if you’re pregnant or feeding young children, and limit them if you’re not. Choose lower-mercury fish like salmon, sardines, or shrimp for your regular rotation. When buying canned tuna, opt for light over white. If rice is a major part of your diet, varying your grains (switching in quinoa, barley, or oats some days) can reduce cumulative exposure. And baking or frying fish with a generous side of vegetables is a better bet than steaming it with a squeeze of lemon, at least from a mercury standpoint.