Minnesota Fish: Benefits and Risks - EH: Minnesota Department of Health

Contaminants and Minnesota Fish

How do Minnesota's fish compare with other states?

Minnesota has one of the most extensive fish monitoring programs in the United States. It is not because Minnesota has some of the most contaminated fish; rather, Minnesota has more lakes and river miles than other states. All of the Great Lake states, and Ontario, face many of the same problems with mercury and PCB contamination, and all issue fish consumption advisories. However, Lake Superior is the least contaminated of the Great Lakes. Mercury contamination in Wisconsin and Ontario's inland lakes is comparable to that in Minnesota. Advisories that states issue for inter-state border waters may differ because of differences in how health risks are interpreted.

What contaminants are found in Minnesota fish?

In Minnesota, mercury is the contaminant in fish that causes the most concern. Air pollution is the major source of mercury that contaminates the fish in Minnesota’s lakes and rivers—see Sources of Mercury Pollution and the Methylmercury Contamination of Fish in Minnesota. About 70 percent of the mercury in the air is the result of emissions from coal combustion, mining, incineration of mercury-containing products and other human sources. Over time, fish can accumulate relatively high mercury concentrations. That’s why it’s important to make wise choices about the fish you eat and how often you eat it.

Fish in Lake Superior and major rivers such as the Mississippi River may contain PCBs. These synthetic oils had many uses and are found in electrical transformers, cutting oils, and carbonless paper. Although they were banned in 1976, they do not easily break down and remain in the water and lake sediments for years. PCB levels in Minnesota waters are slowly decreasing.

Residues of toxaphene in lake trout from Lake Superior suggest a potential environmental health problem with this insecticide. Toxaphene, actually a mixture of over 670 chemicals, was banned in 1990, but continues to be a problem in certain areas. The Minnesota Department of Health (MDH) continues to monitor reports, and will issue consumption advice based on toxaphene, if necessary.

Dioxins are inadvertently produced through a number of human activities as well as by natural processes. Results from an US EPA study of contaminants in fish from lakes across the US indicate that dioxins are found in every fish tested. The levels of dioxins in the fish tested from Minnesota as part of this study are low overall and low in comparison to other areas of the country. Dioxins accumulate in animal fat and are therefore also present in meat and dairy products. At this time MDH does not provide advice to limit fish consumption based on dioxins in fish. Lakes and rivers where fish have been tested for dioxins and where the levels of dioxins that were measured appear higher than typically found in Minnesota are marked in the site-specific consumption advice tables.

Perfluoroalkyl Substances (PFAS) are a family of manmade chemicals that have been used for decades to make products that resist heat, oil, stains, grease and water. Common uses include nonstick cookware, stain-resistant carpets and fabrics, as components of fire-fighting foam, and other industrial applications.

Perfluorooctane sulfonate (PFOS) is the PFAS that accumulates to levels of concern in fish. The Minnesota Pollution Control Agency is investigating the sources of perfluoroalkyl substances in fish. MDH has site-specific meal advice for fish from waters where fish have been tested for PFOS.

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Health risks of eating contaminated fish

PCBs, methylmercury, PFCs and dioxins build up in your body over time. It may take months or years of regularly eating contaminated fish to accumulate levels which are a health concern. As you follow the fish advisory, the amount of methylmercury you take into your body is safely eliminated between meals. Large amounts of mercury may harm the nervous system. Young children, developing fetuses and breast-fed babies are at most risk, because small amounts of mercury can damage a brain that is just starting to form or grow. Too much mercury may affect a child’s behavior and lead to learning problems later in life. The first symptoms of adult mercury poisoning include incoordination and burning or tingling sensation in the fingers and toes. As mercury levels increase, your ability to walk, talk, see, and hear may all be affected in subtle ways. The consumption advice given by MDH is intended to keep the mercury in your body below levels that damage the nervous system.

Exposure to PCBs is linked to infant development problems in children whose mothers were exposed to PCBs before becoming pregnant. The consumption advice for PCBs is intended to protect children from developmental problems. PCBs also cause changes in human blood, liver, and immune functions of adults. In addition, PCBs cause cancer in laboratory animals and may cause cancer in humans.

Currently, cancer will affect about one in every two people in Minnesota, primarily due to smoking, diet, and hereditary risk factors. If you follow the advisory over your lifetime, the PCBs in the fish you eat may not increase your cancer risk at all. At worst, using Environmental Protection Agency methods to calculate risk from a lifetime of eating contaminated fish, it is estimated that approximately one additional cancer case may develop in 10,000 people eating contaminated fish, according to this advisory. Eating fewer meals of contaminated fish will further decrease your cancer risk.

In scientific studies of PFCs, laboratory animals are given much higher doses than we would ever expect people to consume. At these very high concentrations, in laboratory studies, PFCs cause harmful changes, including cancer, in the liver and other organs. Developmental problems (e.g., delays in growth and maturation) have been seen in the offspring of rats and mice exposed to PFCs while pregnant.

PFOS is the PFC that accumulates in fish. When scientists look at laboratory studies they look for the lowest dose at which a health effect was seen. For PFOS, the health effects seen at the lowest dose were a decrease in high-density lipoprotein (HDL or "good" cholesterol) and changes in thyroid hormone levels in some animals. This lowest dose is the starting point for determining what level is safe for people to consume. The level is lowered to take into consideration the possibility that people may react differently to PFOS than laboratory animals. The level is further reduced to reflect concern that some people might be more sensitive to PFOS than people in general.

The MDH continually reviews ongoing research on PFCs to ensure that our guidelines reduce exposures and protect public health. As new studies and science become available, our advice may be revised to reflect additional information.

How do contaminants get into fish?

Contaminants can reach rivers and lakes from local sources such as improperly stored wastes and abandoned dumps. If a local source is identified, it may be possible to clean it up and decrease the contamination of the lake or river. However, contaminants also reach remote and pristine lakes from the atmosphere.

Once in a lake, mercury is converted to methylmercury by bacteria. Fish absorb methylmercury from their food. Mercury is tightly bound to proteins in all fish tissue, including muscle. There is no method of cooking or cleaning fish that will reduce the amount of mercury in a meal. Larger, older fish and fish which eat other fish accumulate more contaminants than smaller, younger fish which eat less contaminated prey.

Fish absorb fat-soluble chemicals like PCBs and dioxins from water, suspended sediments, and food. PCBs and Dioxins concentrate in the fat of fish, and in fatty fish such as carp and catfish. Cleaning and cooking a fish to remove fat will lower the amount of PCBs and dioxins in a fish meal.

PFCs are very stable chemicals that do not change or break down in the environment. As a result, they may build up in soil, sediments, or in other places. There are a few studies indicating that PFCs easily enter groundwater and move long distances. Some experts suggest that PFCs in air can also travel long distances, deposit on soil and leach into groundwater.

PFCs have been found in the blood of many species of wildlife around the world, including fish, bald eagles and mink in the midwestern United States. The exact way PFCs get into fish is not known at this time. As with mercury, removing the fat when cleaning or cooking does not reduce the amount of PFOS in the edible parts of the fish.

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Updated Friday, October 26, 2018 at 09:06AM