(Published September 2008)
Scientific Evidence on Fish Contaminants
Mercury and persistent organic pollutants (POPs) such as PCBs are common contaminants of freshwater as well as ocean fish. These pollutants have been released to the environment in large quantities by industrial activities, and fish from more than half of inland lakes and rivers in the United States contain detectable levels of these or other chemicals. Many are contaminated at or near levels of concern. These contaminants can travel beyond national borders, persist for long periods in the global environment, and accumulate to toxic levels in aquatic ecosystems and fish. Many are potent neurotoxicants, and the developing brains of fetuses, infants, and young children are most sensitive to the effects of exposure. When these pollutants are ingested, their concentrations build up in the body over time. As a result, past as well as current dietary habits influence the body burden of these contaminants, particularly in the case of POPs.
Mercury. Mercury is a persistent heavy metal that occurs in elemental form as well as various organic and inorganic forms.11 Most of the human-caused mercury pollution in our environment is emitted from industrial smokestacks.12 The EPA has concluded that coal-fired power plants are the nation’s largest source of unregulated mercury emissions attributable to human activity. Other major sources include mining, smelting, and waste incineration.13
Mercury that is released into the atmosphere from various industrial activities can be deposited onto soil or into waterways.14 Biological processes then convert it to organic forms, such as methylmercury, which bio-accumulates through the food chain. Consequently, mercury concentrations are highest in large, long-lived predatory fish. In 2006, mercury contamination led 48 states to issue 3,080 fish consumption advisories for rivers, lakes, and coastal areas.15 Table 1 shows the mean mercury levels measured in various species of fish and shellfish, as determined by the EPA and FDA from a variety of data sources.16 In the absence of a single “official” standard, ARHP and PSR selected the breakpoints between lower, moderate, and highest mercury levels that are used to categorize the species in the chart.
|Table 1. Mercury Levels in Commercial Fish and Shellfish
|MEAN MERCURY LEVEL
(parts per million [ppm])
|Lower Mean Mercury Levels (None detected [ND] to 0.29 ppm)
|Bass (saltwater; includes sea bass/striped bass/rockfish)
|Crab (blue, king, and snow)d
|Flatfish (includes flounder and sole)d
|Mackerel chub (Pacific)d
|Mackerel chub (South Australia)d
|Mackerel, Spanish (South Atlantic)d
|Tuna (canned chunk light)d
|Weakfish (sea trout)
|Moderate Mean Mercury Levels (0.3 to 0.59 ppm)
|Mackerel, Spanish (Gulf of Mexico)d
|Tuna (canned, white albacore)d
|Highest Mean Mercury Levels (>0.6 ppm): AVOID EATING
|Mackerel-King (Atlantic & Gulf of Mexico)d
|Tilefish (Gulf of Mexico)b
|a) FDA testing has been extremely limited (<10 samples tested) and may not reflect actual contamination levels. b) Some species have been overfished in recent years, and thus may not be good choices for those concerned about fishery sustainability. Visit www.mbayaq.org for more information. c) These fatty fish may be low in mercury but high in PCBs or other persistent organic pollutants. d) These fish contain >240 mg of omega-3 fatty acids. See Table 2 for specific amounts per serving.
Adapted from EPA/FDA, 2006 and Nutrition Action Health Letter, October 2007.
Methylmercury readily crosses the placenta and enters the fetal brain, where it impairs normal development. Epidemiological studies suggest that prenatal exposure to even low levels of mercury may result in subtle deficiencies in motor skills, attention, language skills, learning capacity, and memory, as well as other symptoms of neurological damage in children.17–20 These effects of prenatal exposure have been shown to persist into adolescence, suggesting that at least some neurotoxic effects of intrauterine exposure to methylmercury are irreversible.20 The National Research Council (NRC) has estimated that neurobehavioral effects in the fetus could occur at methylmercury levels of as low as 58 parts per billion in cord blood.21 The NRC cautions that children of women who consume large amounts of fish and seafood during pregnancy are at particular risk. Mercury also passes through breast milk, which is an additional reason breastfeeding mothers should minimize fish consumption.
Data presented in the 1999–2002 Centers for Disease Control and Prevention’s third National Exposure Report indicated that measures of methylmercury exposure were below 58 µg/L in all tested children between ages 1 and 5 as well as all women of childbearing age.22,23 This is good news, as this level is below the “level of concern” for harmful health effects. Six percent of childbearing-age women had levels >5.8 µg/L, however, suggesting that close monitoring is needed to continue to define the safe level of mercury in the blood.22,23
PCBs. PCBs are a large group of fat-soluble chemicals that were produced from the 1920s to the 1970s for use as lubricants and insulators in electrical equipment. Production of PCBs has been banned in the United States, but large quantities remain in equipment manufactured before the ban was implemented and in the environment.24 PCBs are highly toxic. They accumulate in fatty fish such as salmon and bluefish, as well as beef and dairy products. PCBs can cause a number of different health effects, depending on the extent of exposure and individual sensitivity.25,26 They are representative of the lipophilic POPs that can produce negative health effects ranging from subtle biochemical and cellular changes to more serious long-term effects, such as cancer and delays in childhood development. Developing fetuses as well as infants and young children may be particularly vulnerable to the adverse effects of these chemicals because their bodies are immature and rapidly growing.27 Early life exposure to PCBs can cause harmful neurological effects, leading to learning deficits, poor memory, and behavioral problems.
In 2006, 39 states issued 1,023 consumption advisories for PCBs in freshwater and coastal fish.15 PCBs and related chemicals tend to accumulate and persist, especially in deep, coldwater bodies such as the Great Lakes and the northern oceans. Although wild salmon from Alaska and elsewhere is contaminated with PCBs, data from the EPA indicate that farmed salmon contains significantly higher concentrations of these contaminants,28 probably as a result of contaminants in fish feed.