What are the contaminants in fish? A comprehensive guide

November 25, 2025 •

6 min read

According to the EPA, virtually all fish and shellfish contain traces of mercury, a naturally occurring element that is also a byproduct of industrial processes. However, mercury is just one of several types of contaminants in fish that can affect human health. Understanding what these pollutants are, how they get into our food, and how to reduce exposure is key to making informed dietary choices.

Quick Summary

Fish can be contaminated with a variety of harmful substances, including heavy metals like methylmercury, industrial chemicals like PCBs and dioxins, pesticides, and microplastics. These pollutants can build up in the fish's tissue through a process called bioaccumulation and move up the food chain. Exposure to high levels of contaminants can pose serious health risks to humans.

Key Points

Heavy Metals: Fish can contain heavy metals like mercury, lead, and cadmium, with mercury being the most significant neurotoxin, especially in larger predatory fish.
Persistent Pollutants: Persistent Organic Pollutants (POPs), including PCBs, dioxins, and pesticides, are fat-soluble chemicals that accumulate in fish and pose long-term health risks.
Microplastic Threat: Microplastics are a pervasive contaminant that fish ingest from polluted water, potentially causing physical harm to the fish and acting as a vector for other toxins.
Informed Choices: Reducing exposure involves selecting low-contaminant fish, such as salmon or tilapia, and following local consumption advisories.
Cooking Practices: Proper preparation, like removing fat and skin for fat-soluble pollutants, can help reduce contaminant levels, though mercury cannot be cooked out.
Vulnerable Groups: Pregnant women, nursing mothers, and young children are particularly vulnerable to the effects of contaminants like mercury and should adhere strictly to recommended guidelines.
Environmental Factors: The levels of contaminants can vary based on the fish's species, age, and the pollution levels of its environment.

Common Types of Contaminants in Fish

The presence of contaminants in fish is a global concern, stemming from both natural processes and human activities. These substances can enter aquatic ecosystems through various pathways and accumulate in the tissues of marine and freshwater life. Understanding the specific types of contaminants helps in assessing the risks associated with seafood consumption.

Heavy Metals

Heavy metals are a significant category of contaminants found in fish. These elements, such as mercury, lead, and cadmium, are naturally present in the environment but are introduced in higher concentrations through anthropogenic activities like industrial discharges and burning fossil fuels. Once in the water, these metals can be converted into more toxic forms and enter the food chain.

Mercury: Methylmercury, its organic form, is highly toxic and readily absorbed by fish from the water and through their diet. It then accumulates in their muscle tissue. Larger, older predatory fish, such as sharks and swordfish, tend to have the highest concentrations due to biomagnification. Health risks associated with high mercury exposure include neurological damage and developmental problems, especially for unborn babies and young children.
Lead and Cadmium: While less bio-accumulative than mercury, lead and cadmium can still reach harmful levels in fish from contaminated environments. These can damage internal organs, like the kidneys and nervous system, in humans.

Persistent Organic Pollutants (POPs)

Persistent Organic Pollutants, or POPs, are a group of toxic, bio-accumulative chemicals that remain in the environment for long periods.

Polychlorinated Biphenyls (PCBs): Used widely in industrial applications until being banned in the 1970s, PCBs still linger in the environment. These compounds are fat-soluble and therefore accumulate in the fatty tissues of fish, reaching higher concentrations in predatory species. PCBs have been linked to cancer, immune system dysfunction, and developmental issues.
Dioxins and Furans: These unintentional byproducts of industrial processes and incineration can also accumulate in the fatty tissues of fish. They pose similar health risks to PCBs and have been associated with cancer and developmental problems.
Pesticides: Organochlorine pesticides like DDT, though banned in many places, persist in the environment and can still be found in fish. They can cause a variety of harmful effects on fish and can be passed to humans through consumption.

Microplastics

Microplastics are a growing threat to aquatic life and human health. These tiny plastic particles, less than 5mm in size, enter the marine environment from various sources, including the breakdown of larger plastics, synthetic clothing, and fishing gear. Fish ingest these particles, mistaking them for food, leading to accumulation in their guts, gills, and tissues.

Sources: Microplastics are a major component of ocean pollution, with sources ranging from industrial pellets to the tiny fibers shed from synthetic textiles.
Impact on Fish: Ingesting microplastics can cause physical harm to fish, such as blockages and intestinal damage. Studies have also found that microplastics can increase mortality rates and negatively affect reproduction in fish.
Impact on Humans: Humans are exposed to microplastics through seafood consumption and other means, though the long-term health effects are still under investigation. Concerns include inflammation and other health issues related to the plastic and the toxins they carry.

Reducing Your Exposure to Contaminants

While avoiding all fish may seem like the safest option, it's important to remember that fish is a vital source of protein, vitamins, minerals, and omega-3 fatty acids. By following certain practices, you can minimize your exposure to contaminants while still enjoying the health benefits of seafood.

Choose Lower-Mercury Fish: The EPA and FDA provide guidelines for choosing fish with lower mercury levels. Pregnant women, nursing mothers, and young children are particularly advised to avoid high-mercury species like shark, swordfish, king mackerel, and tilefish. Opt for "Best Choices" such as salmon, canned light tuna, tilapia, and cod.
Vary Your Fish Choices: Eating a diverse range of fish species helps to avoid excessive exposure to any single type of contaminant. This strategy prevents the buildup of pollutants that may be more concentrated in certain species or from specific environments.
Remove Fat and Skin: For contaminants that accumulate in fat, such as PCBs and dioxins, proper cooking can help reduce exposure by up to 50%. This involves removing the skin and fatty portions before cooking and using methods that allow fat to drip away, such as broiling or grilling on a rack. Do not use the cooking drippings for sauces.
Be Aware of Local Advisories: Check with your local or state health department for fish consumption advisories, especially if you eat fish caught in local rivers, lakes, or coastal waters. These advisories provide up-to-date information on contaminant levels in specific waterbodies and species.

Wild-Caught vs. Farmed Fish Contaminants


Feature	Wild-Caught Fish	Farmed Fish
Mercury	Mercury levels tend to be more variable, largely dependent on the size, age, and species of the fish. Predatory wild fish, like shark and tuna, often have higher levels.	Levels may be lower and more consistent due to controlled diet, but the quality of feed and farm water is a factor.
PCBs/Dioxins	Contaminant levels can be influenced by the presence of historical industrial pollutants and persistent environmental contamination.	Feed can introduce PCBs and other pollutants, but well-regulated farms can control exposure.
Microplastics	Can ingest microplastics from their natural, polluted environments. Filter-feeders may show higher contamination.	Research has found microplastic contamination in farmed fish, likely from feed and farm water.
Antibiotics	Generally not an issue unless in a highly polluted environment.	A common concern due to potential use of antibiotics in aquaculture to prevent disease.

Conclusion

While consuming fish offers significant health benefits, it is crucial to be mindful of the potential presence of contaminants like heavy metals, POPs, and microplastics. These substances enter the food chain through various environmental pathways and can pose health risks, particularly for vulnerable populations. By choosing low-contaminant fish, varying your diet, and preparing fish properly, you can enjoy the nutritional advantages of seafood while minimizing the associated risks. Staying informed through local health advisories is also a key strategy for making the safest choices. With conscious decisions, you can balance the benefits and risks of eating fish, ensuring a healthier meal for yourself and your family.

Lists of Contaminants in Fish

Here are some of the most common contaminants found in fish, categorized by their type:

Heavy Metals: Methylmercury (MeHg), Lead (Pb), Cadmium (Cd), Inorganic Arsenic (iArsenic), Selenium, Tributyltin.
Persistent Organic Pollutants (POPs): Polychlorinated Biphenyls (PCBs), Dioxins, Furans, Polycyclic Aromatic Hydrocarbons (PAHs), Pesticides (e.g., DDT), Polybrominated Diphenyl Ethers (PBDEs), Perfluoroalkyl Substances (PFASs).
Plastic Waste: Microplastics, Nanoplastics.
Biological Agents: Histamine (due to improper handling), bacteria (such as Salmonella and Listeria from contamination).

Important Considerations for Safe Seafood Consumption

Know Your Source: Consider the source of your fish. Local advisories are particularly important for wild-caught fish from regional waterbodies.
Fish Species: Be aware that contaminant levels, especially for mercury, differ significantly among species. Larger predatory fish higher on the food chain tend to accumulate more toxins.
Preparation Method: Proper preparation can significantly reduce levels of fat-soluble contaminants like PCBs, but not mercury, which is stored in the muscle tissue.
Dietary Choices: For high-risk groups, such as pregnant women and young children, following specific FDA guidelines is critical.
Reduce Plastics: Supporting global efforts to reduce plastic pollution is key to addressing the microplastic problem at its source.

Conclusion: Navigating the Trade-Offs

Eating fish offers considerable nutritional benefits, but it comes with the reality of environmental contamination. The best approach is to manage risk rather than avoid fish entirely. By choosing fish wisely, preparing it correctly, and staying informed about local advisories, consumers can continue to enjoy the health benefits of seafood while minimizing their exposure to harmful substances. The balance between risk and reward is at the heart of making informed and healthy dietary decisions in a polluted world. For comprehensive advice on fish consumption, refer to guidance from public health agencies like the FDA and EPA.

Frequently Asked Questions

Larger, older predatory fish typically have the highest concentrations of contaminants, particularly methylmercury, due to biomagnification. Examples include shark, swordfish, king mackerel, and tilefish.

No, cooking methods do not remove mercury from fish. Mercury is stored throughout the muscle tissue (the edible meat), so it cannot be cut or cooked away.

Not necessarily. While farmed fish may have more consistent mercury levels due to controlled diets, the concentration of other contaminants like PCBs can depend on the quality of their feed and the farming environment.

For fat-soluble contaminants like PCBs and dioxins, you can reduce exposure by removing the skin and fat before cooking. Cooking methods that allow fat to drip off, such as broiling or grilling on a rack, are also recommended.

Microplastics are present in seafood, and some research suggests potential health risks such as inflammation and oxidative stress. However, the full extent of the risks to humans is still being studied.

POPs are toxic chemicals that persist in the environment and accumulate in fish's fatty tissue. They include industrial chemicals like PCBs and dioxins, as well as certain pesticides.

You can find fish consumption advisories from public health agencies like the U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA), as well as your local and state health departments.