Tuna carries more mercury than nearly any other commonly eaten fish, and that single fact drives most of the concern. But mercury isn’t the only issue. Depending on how much you eat, what species you choose, and how it’s processed, tuna can also pose risks from histamine poisoning, excess sodium, microplastic contamination, and significant environmental harm.
Mercury Levels Vary Widely by Species
All tuna contains some mercury, but the range across species is dramatic. Canned light tuna (typically skipjack) averages 0.126 parts per million of mercury. Canned albacore, often labeled “white tuna,” comes in at 0.350 ppm, nearly three times higher. Fresh yellowfin sits at a similar 0.354 ppm. At the top end, fresh bigeye tuna averages 0.689 ppm, making it one of the most mercury-dense fish you can buy.
The reason is simple biology. Tuna are large, long-lived predators. Mercury accumulates as it moves up the food chain: tiny organisms absorb it from the water, small fish eat those organisms, and tuna eat those fish. The bigger the tuna and the longer it lives, the more mercury it stores in its muscle tissue. Bigeye tuna, which can live over a decade and weigh hundreds of pounds, concentrate far more than a young skipjack.
What Mercury Actually Does to Your Body
The form of mercury in fish is methylmercury, which your gut absorbs efficiently and which crosses into your brain. Once there, it overstimulates nerve cells, flooding them with calcium and triggering oxidative stress that damages and kills neurons. It also depletes one of the body’s key antioxidant defenses, a molecule called glutathione, by binding to it directly. The bound complex gets excreted from the body, leaving cells more vulnerable to further damage.
In adults, chronic low-level exposure can cause fatigue, difficulty concentrating, tingling in the hands and feet, and impaired coordination. The stakes are higher for developing brains. Methylmercury crosses the placenta and can interfere with fetal brain development, which is why guidelines for pregnant and breastfeeding women are stricter. The EPA and FDA recommend these groups eat 8 to 12 ounces per week of low-mercury seafood, choosing from species like shrimp, salmon, and canned light tuna rather than albacore or bigeye.
Selenium Doesn’t Reliably Cancel Out Mercury
You may have heard that selenium in tuna protects against mercury toxicity. There’s a kernel of truth here: selenium binds to mercury and can partially offset its effects. But the science is far less settled than supplement marketers suggest. The amount of selenium needed to protect against a given mercury concentration is unclear, and it likely varies between individuals. Researchers have also found wide variation in the selenium-to-mercury ratio within the same species, making it unreliable as a safety measure. Canned light tuna has a favorable ratio (around 23:1 selenium to mercury), but yellowfin tuna steaks can drop to roughly 3:1. Using these ratios for safety decisions is considered premature by toxicologists, particularly because there appears to be no safe threshold below which methylmercury causes zero harm.
Histamine Poisoning From Improper Handling
Tuna is one of the fish most commonly involved in scombroid poisoning, a type of food poisoning caused not by bacteria themselves but by the histamine they produce. When tuna isn’t kept cold enough after being caught, bacteria break down proteins in the flesh and generate toxic levels of histamine. This can happen in as little as 6 to 12 hours without proper refrigeration, and cooking doesn’t destroy histamine once it’s formed.
Symptoms mimic an allergic reaction: facial flushing, a burning or peppery taste in the mouth, sweating, dizziness, nausea, and headache. More severe cases can progress to hives, diarrhea, abdominal cramps, blurred vision, and swelling of the tongue. The episode typically resolves within hours, but it can be alarming and occasionally requires treatment. If your tuna tastes unusually sharp or peppery, stop eating it.
Canned Tuna Adds Sodium and Microplastics
Fresh tuna contains about 13 milligrams of sodium per ounce. Canned tuna packed in water jumps to 70 mg per ounce, and oil-packed versions hit 118 mg. A standard can holds roughly 5 ounces, so a single serving of oil-packed tuna delivers nearly 600 mg of sodium before you add anything else. That’s over a quarter of the daily recommended limit. If you’re watching your blood pressure, this adds up fast.
Microplastics are a newer concern. Researchers examining canned tuna from multiple countries have identified synthetic particles in the edible tissue, including nylon, thermoplastics, and other industrial polymers. The health effects of ingesting microplastics are still being studied, but their consistent presence in canned fish is one more reason the product isn’t as clean as the label suggests.
Environmental Costs of Tuna Fishing
Tuna fishing creates substantial collateral damage. In one well-documented Pacific longline fishery, the ratio of target catch to bycatch was 1:1. For every tuna pulled from the water, one non-target animal was caught alongside it. Over a decade, that fishery hooked an estimated 48,400 sharks per year, along with 20,450 billfish, 2,350 sea turtles, and dozens of marine mammals and seabirds annually.
Sea turtles face particularly grim odds. While they’re caught in lower numbers than sharks, they’re discarded dead at substantially higher rates, especially by distant-water fleets. Silky sharks, a species already in decline, were the third-most caught species in the local fleet studied, with over 13,000 caught annually. And 62% of all “species of special interest,” a category covering vulnerable wildlife, were discarded dead or dying.
Stock health varies by region and species. Atlantic bigeye tuna is officially classified as overfished, with a rebuilding plan that has no projected completion date. Western Atlantic bluefin tuna’s status is listed as unknown. Other populations, like skipjack and yellowfin in the Western Atlantic and Central Western Pacific, are currently not overfished, but industrial fishing pressure on these stocks remains intense.
The “Dolphin Safe” Label Has Limits
The “dolphin safe” label on canned tuna is legally regulated in the United States, but its scope is narrower than most consumers assume. The standard prohibits intentionally encircling dolphins with purse seine nets and requires that no dolphins were killed or seriously injured during the sets where the tuna was caught. It does not address bycatch of sharks, turtles, or other marine life. It does not address mercury content, microplastics, or the broader ecological footprint of the fishery. It tells you one specific thing: dolphins were not deliberately targeted on that particular fishing trip.
How Much Is Too Much
For most adults, eating tuna once or twice a week is considered manageable if you stick to lower-mercury species like skipjack (canned light). Albacore and yellowfin carry roughly three times more mercury, so one serving per week is a more cautious ceiling. Bigeye tuna, commonly served as sushi, is best treated as an occasional indulgence rather than a regular meal.
Children should eat smaller portions, around two child-sized servings per week of low-mercury options. Pregnant or breastfeeding women can safely eat 8 to 12 ounces of low-mercury fish weekly but should avoid bigeye entirely and limit albacore to about 6 ounces. The risk isn’t from a single meal. It’s from consistent, repeated exposure that lets mercury accumulate faster than your body can clear it, a process that takes roughly 70 days per half-life in the bloodstream.