Common Questions About Contaminants in Fish

What are contaminants?

Despite the health benefits of seafood, eating fish that are contaminated with toxins and chemicals can pose considerable health risks. Contaminants may include:

• metals such as mercury and lead,
• industrial chemicals such as PCBs, and
• pesticides such as DDT and dieldrin.

Many of our oceans, lakes and rivers are surprisingly tainted. As a result, some fish are so contaminated that Environmental Defense recommends limited or no consumption (see Health Alerts chart).

What are the risks of eating seafood contaminated with mercury and PCBs?

Contaminants such as mercury, PCBs and dioxins build up in your body over time and may result in health problems, ranging from small, hard-to-detect changes to birth defects and cancer. Key facts about the risks include:

• It can take 5 years or more for women in their childbearing years to rid their bodies of PCBs, and 12-18 months to significantly reduce their mercury levels.
• Mothers who eat contaminated fish before becoming pregnant may have children who are slower to develop and learn. Developing fetuses are exposed to stored toxins through the placenta.
• Women beyond their childbearing years and men face fewer health risks from contaminants than children do. Following the advice below will minimize your exposure and reduce the health risks associated with these contaminants.

What about mercury in canned tuna?

The two most popular types of canned tuna – white and light – vary greatly in their average mercury content. Overall, it’s best to exercise caution in how much tuna you (or especially your children) consume.

• Canned white tuna consists of albacore, a large species of tuna that accumulates moderate amounts of mercury. Therefore, Environmental Defense recommends that both adults and children limit their consumption of canned white tuna.
• Canned light tuna usually consists of skipjack, a smaller species with approximately one-third the mercury levels of albacore. Therefore, Environmental Defense recommends only that young children (ages 0-6) limit their consumption of canned light tuna.

A word of caution: Some canned light tuna reportedly contains yellowfin tuna, which has similar mercury levels to albacore. These products are sometimes (but not always) labeled ‘gourmet’ or ‘tonno’, and their consumption should be limited by adults and children.

Do the health benefits of omega-3s outweigh the risks associated with contaminants in seafood?

Fish is generally healthy to eat, but there are some types you should eat infrequently, if at all. Consider the following:

• For young children and women of childbearing age, consumption of mercury-contaminated fish can severely impact a child's development.
• Older women and men may find it an acceptable tradeoff to exceed recommended seafood meal limits to increase their omega-3 intake.
• People at high risk of cardiovascular disease must weigh the cancer risk of eating fish high in PCBs with the benefits of eating fish high in omega-3s, in which case the benefits of omega-3s may outweigh the cancer risk (1 in 100,000 - the level recommended by the EPA). However, these chemicals are known to cause serious health problems besides cancer, so the tradeoffs are not simple.
• The good news is that there are several low-contaminant, high-omega-3 seafood options available (see Eco-Best list), so there’s no need to risk eating contaminated fish.

Where do contaminants come from?

Contaminants end up in water in a variety of ways:

• Industrial and municipal discharges, agricultural practices, and storm water runoff can all deposit harmful substances directly into the water.
• Rain can also wash chemicals from the land or air into streams and rivers. These contaminants are then carried downstream into lakes, reservoirs and estuaries.

Fish take in these substances in several ways, and their contaminant levels depend on factors like species, size, age and location.

Mercury, for example, is naturally converted by bacteria into methylmercury. Fish absorb methylmercury mostly from their food, but also from the water as it passes over their gills. Generally, larger and older fish have had more time to bioaccumulate mercury from their food and the water than smaller and younger fish. In addition, large predatory fish (like sharks and swordfish) near the top of marine food chains are more likely to have high levels of mercury than fish lower in marine food chains due to the process of biomagnification.

Fish can also absorb organic chemicals (such as PCBs, dioxins and DDT) from the water, suspended sediments, and their food. In contaminated areas, bottom-dwelling fish are especially likely to have high levels of such toxins because these substances run off the land and settle to the bottom. These organic chemicals then concentrate in the skin, organs and other fatty tissues of fish. Wild striped bass, bluefish, American eel, and seatrout tend to be high in PCBs, since they are bottom-tending fish often found in contaminated rivers and estuaries.

I've heard that the high amount of selenium in seafood counteracts the harmful effects of mercury. Is this true?

There is limited evidence to date that selenium in seafood provides significant protection against the negative effects of methylmercury (the toxic form of mercury found in fish).

Selenium, an essential nutrient, is present in the cells of all mammals. When bound to certain proteins, selenium acts as an antioxidant by detoxifying free radicals. (Free radicals are highly-reactive atoms or molecules that can damage cells.) Organ meats and seafood are the best sources of selenium, and the USDA ranks 16 different seafood sources in the top 25 selenium-containing foods [PDF].

A form of selenium – selenide – has also been shown to neutralize the toxicity of some forms of mercury. As part of its 2006 report Seafood Choices: Balancing Benefits and Risks, the Institute of Medicine reviewed the scientific evidence that selenium reduces the risks associated with methylmercury in seafood.

The expert panel concluded that although selenium may diminish some of the toxic effects of some forms of mercury and other heavy metals, the mechanisms for these interactions are poorly understood. In addition, there was little or no evidence showing that selenium affected the toxicity of other seafood contaminants such as PCBs or dioxins. Therefore, it is premature to conclude that selenium acts as a safeguard against methylmercury. Choosing fish that are low in contaminants is still the best course of action (see our list of best and worst choices).

Posted: 01-Jan-1900; Updated: 01-Jan-1900