More and more people are aware that eating certain types of seafood can increase your exposure to mercury, PCBs, dioxins and pesticides. Industrial activities have significantly elevated the amount of mercury and other toxics chemicals in the environment. Present in the air and water, these contaminants have made their way into some of the food we eat.
Mercury in fresh and marine waters is converted by bacteria into methylmercury, which is easily absorbed by animals as they feed and binds to fish protein. Methylmercury and persistent organic compounds, such as dioxins and PCBs, accumulate in fish tissue and become concentrated in fish high on the food chain. Residues of antibiotics and hormones used in aquaculture may be a cause for additional caution. However, there are still many seafood options that allow people to get the benefits of seafood without the risk of contaminants.
Health Canada, the U.S. Food and Drug Administration and the U.S. Environmental Protection Agency all issue warnings concerning mercury contamination in seafood. SeaChoice recommends following a precautionary approach that considers mercury contamination as well as other persistent toxins. While exact consumption advisories for all compounds have not been established, we do know that human beings are exposed to all of these toxins from many sources, and research has shown that many people already carry a total load of contaminants close to the concentration at which adverse health effects could be expected.
Following a precautionary approach, SeaChoice recommends limiting intake for all seafood that has a scientifically identified contamination concern. This may be more restrictive than national advisories, but acknowledges that many agencies also factor economic impact on industry into health advisories and that there is not sufficient science on the cumulative effects of all contaminant sources to give exact safe consumption limits:
• Young children, pregnant or nursing mothers and women of childbearing age should avoid fish with listed mercury concerns. Instead, these populations should choose fish without toxicity warnings.
• Other people should limit consumption of fish with mercury concerns to one or two meals per month.
• For more exact information, visit Environmental Defense Fund’s health advisories.
Mercury risk factors and effects
The factors that determine how severe the health effects are from mercury exposure include: the chemical form of mercury (methylmercury is more toxic than elemental mercury); the dose; the age of the person exposed (the fetus is the most susceptible); the duration of exposure; the route of exposure — inhalation, ingestion, dermal contact, etc. — and the health of the person exposed.
For fetuses, infants, and children, the primary health effect of methylmercury is impaired neurological development. Methylmercury exposure in the womb can adversely affect a baby’s growing brain and nervous system and impacts on cognitive thinking, memory, attention, language, and fine motor and visual spatial skills have been documented. Symptoms of methylmercury poisoning may include: impairment of the peripheral vision; disturbances in sensations (“pins and needles” feelings, usually in the hands, feet, and around the mouth); lack of coordination of movements; impairment of speech, hearing, walking; and muscle weakness.
Seafood to avoid
Older predatory fish have the highest levels of methylmercury (and other persistent organic compounds) because they have had more time to accumulate it. Fishes such as swordfish, shark, king mackerel and tilefish pose the greatest risk. Fisheries marked with the diamond warning symbol on our card may contain one or more species that poses a mercury-related health risk with regular consumption. For more information see the resources below.
Consumers should also be aware that the Canadian government allows fish to be marketed even if they regularly exceed Health Canada’s guideline for total mercury content, 0.5 parts per million (ppm). These species, namely, shark, swordfish, and fresh and frozen tuna (but not canned tuna) regularly show mercury levels between 0.5 and 1.5 ppm, and are issued exemptions from the guidelines. Instead, the government chooses to issue advisories for these species, recommending appropriate restrictions on consumption, and leaving it up to consumers to inform themselves of this information.
There are many excellent resources available to help you find out more about health issues related to seafood, particularly mercury contamination and how to reduce your exposure.
Mercury calculator based on the limits set by the US Environmental Protection Agency (EPA)