Seafood isn’t universally bad for you, but it does carry real risks that other protein sources don’t. Mercury contamination, bacterial infections from raw shellfish, industrial pollutants, allergens, and even tiny plastic particles all show up in fish and shellfish to varying degrees. Whether seafood is a net positive or negative depends heavily on which species you eat, how it’s prepared, and your individual health profile.
Mercury Builds Up in Certain Fish
The most well-known risk from seafood is methylmercury, a form of mercury that accumulates in fish tissue over time. Larger, longer-lived predator fish concentrate the most because they eat smaller contaminated fish throughout their lives. Mercury is a neurotoxin, and it’s especially dangerous during fetal development and early childhood because it crosses the blood-brain barrier by mimicking an amino acid the body normally transports freely. The brain’s own transport system essentially gets tricked into pulling mercury inside.
Not all seafood is equally contaminated. FDA testing shows enormous variation across species. Scallops average just 0.003 parts per million (ppm) of mercury, shrimp and clams come in at 0.009 ppm, and salmon hovers around 0.014 to 0.022 ppm. These are extremely low levels. On the other end, swordfish, shark, king mackerel, and certain tuna species can be 50 to 100 times higher. The practical takeaway: if you stick to smaller fish and shellfish, mercury exposure is minimal. The problem comes from regularly eating large predatory fish.
Federal guidelines recommend that pregnant women, breastfeeding mothers, and young children eat 2 to 3 servings per week from a “Best Choices” list of lower-mercury species like salmon, sardines, shrimp, tilapia, and catfish. That framing is telling: even for the most vulnerable populations, the advice isn’t to avoid seafood entirely but to choose carefully.
Industrial Pollutants in Fish
Mercury gets the headlines, but fish also carry persistent organic pollutants like PCBs and dioxins. These are industrial chemicals that settled into waterways decades ago and still cycle through the food chain. They accumulate in fat tissue and have been linked to immune disruption and increased cancer risk at high exposures.
Interestingly, wild salmon actually carries higher concentrations of these pollutants than farmed salmon, contrary to what many people assume. A Norwegian study found that wild salmon had roughly three times the dioxins and dioxin-like PCBs of farmed fish. Wild salmon averaged 1.48 pg TEQ/g of combined dioxins and PCBs compared to 0.51 pg TEQ/g in farmed salmon. Both levels fell well below EU safety limits, but the pattern surprises people who think of wild-caught as automatically cleaner.
Raw Shellfish Can Be Genuinely Dangerous
Raw oysters carry one of the most serious food safety risks of any commonly eaten food. Vibrio vulnificus, a bacterium found naturally in warm coastal waters, is the single most fatal foodborne pathogen in the United States. It accounts for 95% of all seafood-related deaths and has a fatality rate approaching 50%, even with aggressive medical treatment. Death can occur within 24 to 72 hours of eating a single contaminated oyster. An estimated 84,000 Americans contract foodborne Vibrio infections each year, and most cases go unreported.
The risk is highest for people with liver disease, weakened immune systems, or conditions that affect iron metabolism. For healthy adults, Vibrio infection is far less likely to be fatal, but it can still cause severe vomiting, diarrhea, and fever. Cooking shellfish thoroughly eliminates the bacteria.
Histamine Poisoning From Improperly Stored Fish
Scombroid poisoning is a form of food poisoning that mimics an allergic reaction. It happens when fish like tuna, mackerel, and bonito aren’t refrigerated properly after being caught. At temperatures above 4°C (40°F), bacteria on the fish convert a naturally occurring amino acid called histidine into histamine. The histamine levels can spike so high that eating the fish triggers flushing, headaches, cramps, and sometimes difficulty breathing within minutes.
What makes scombroid tricky is that the fish may look and smell perfectly normal. Cooking doesn’t break down histamine once it’s formed. The only reliable prevention is an unbroken cold chain from the moment the fish leaves the water to the moment it reaches your plate.
Shellfish Allergies Are Common
Shellfish allergy is one of the most prevalent food allergies in adults. About 6% of the U.S. population shows sensitization to shellfish proteins, and roughly 2% report a confirmed allergy. The primary trigger is a muscle protein called tropomyosin, found in both crustaceans (shrimp, crab, lobster) and mollusks (clams, mussels, oysters). Because tropomyosin is structurally similar across these species, people allergic to shrimp often react to other shellfish as well.
Unlike some childhood food allergies, shellfish allergy typically develops in adulthood and rarely resolves on its own. Reactions range from hives and stomach upset to anaphylaxis.
Microplastics in Shellfish
Filter-feeding shellfish like mussels, clams, and oysters pull tiny plastic particles from the water as they feed. Studies of commercial mussels have found roughly 3 to 5 plastic fibers per 10 grams of tissue, with some samples from China containing up to 11 particles per gram. A typical serving of mussels (about 225 grams of meat) could contain around 1 gram of plastic at the higher end of contamination.
The long-term health effects of ingesting microplastics are still being studied, but the particles can carry surface-bound pollutants and may provoke inflammation in the gut. Smaller fish eaten whole, like sardines and anchovies, also contain microplastics in their digestive tracts, though the concentrations vary widely by region.
Purines and Gout Risk
Seafood is one of the richest dietary sources of purines, compounds your body breaks down into uric acid. When uric acid levels in the blood get too high, crystals can form in the joints and trigger gout attacks. The purine content across seafood species varies dramatically, from as low as 7.7 mg per 100 grams in some shellfish to as high as 1,400 mg per 100 grams in others. Anchovies are among the worst offenders at 321 mg per 100 grams. Canned clams, by contrast, contain just 62 mg per 100 grams.
If you have gout or elevated uric acid levels, the species you choose matters enormously. Sardines, anchovies, herring, and organ meats from fish are the highest-risk options. Shrimp, crab, and most white fish fall in a moderate range and are generally tolerable in smaller portions.
Antibiotics in Farmed Seafood
Aquaculture operations, particularly shrimp farms in Southeast Asia, have historically used antibiotics to prevent disease in densely stocked ponds. Between 2002 and 2019, antibiotics were cited as a reason for import refusal in about 29% of shrimp-related cases flagged by the FDA. One study found that 93% of tested imported shrimp samples contained at least one detectable antibiotic residue, including compounds banned for use in food animals in the United States.
However, more recent surveys paint a less alarming picture. A preliminary survey of frozen shrimp from U.S. retail stores found no detectable antibiotic residues in any samples tested. The discrepancy likely reflects both improved aquaculture practices and the effectiveness of import screening. The concern isn’t acute toxicity from a single meal but the broader public health issue of antibiotic resistance driven by widespread agricultural use.