Safety Matters When Selecting Fish and Sushi

Mercury in Fish

Once mercury enters a waterway, naturally occurring bacteria absorb it and convert it to a form called methyl mercury. This transition is particularly significant for humans, who absorb methyl mercury easily and are especially vulnerable to its effects.

Mercury then works its way up the food chain as large fish consume contaminated smaller fish. Instead of dissolving or breaking down, mercury accumulates at ever-increasing levels. Predatory fish such as large tuna, swordfish, shark and mackerel can have mercury concentrations in their bodies that are 10,000 times higher than those of their surrounding habitat.

Mercury and Human Health

Humans risk ingesting dangerous levels of mercury when they eat contaminated fish. Since the poison is odorless, invisible and accumulates in the meat of the fish, it is not easy to detect and can't be avoided by trimming off the skin or other parts. Once in the human body, mercury acts as a neurotoxin, interfering with the brain and nervous system.

Exposure to mercury can be particularly hazardous for pregnant women and small children. During the first several years of life, a child's brain is still developing and rapidly absorbing nutrients. Prenatal and infant mercury exposure can cause mental retardation, cerebral palsy, deafness and blindness. Even in low doses, mercury may affect a child's development, delaying walking and talking, shortening attention span and causing learning disabilities.

In adults, mercury poisoning can adversely affect fertility and blood pressure regulation and can cause memory loss, tremors, vision loss and numbness of the fingers and toes. A growing body of evidence suggests that exposure to mercury may also lead to heart disease.

Mercury and a High-Fish Diet

A July 2005 report from the federal Centers for Disease Control and Prevention (CDC) concluded that one in 17 women of childbearing age have mercury in their blood above 5.8 micrograms per liter of blood -- a level that could pose a risk to a developing fetus. This is an improvement from the prior report in 2003 which showed that one out of 12 women had mercury in their blood at this level. Newer science indicates, however, that mercury actually concentrates in the umbilical cord blood that goes to the fetus, so mercury levels as low as 3.4 micrograms per liter of a mother’s blood are now a concern. Nearly one in 10 women of reproductive age in the United States has mercury in her blood at or above this level, according to the new CDC study.

Dr. Jane Hightower, a doctor of internal medicine at the California Pacific Medical Center in San Francisco also linked fish consumption to elevated mercury levels when she tested her own patients. Her 2003 study found that 89 percent of the participating patients -- chosen because of their fish-heavy diets -- had elevated mercury levels. Many had levels as much as four times that which the Environmental Protection Agency considers safe.

Calculate your mercury intake

Using NRDC's Mercury Calculator, you can estimate whether you're consuming too much mercury. For an accurate reading, you should request a blood mercury test from your physician. Women with a high blood mercury level who are planning to start a family may decide to postpone pregnancy for a few months until levels drop. The good news, according to Dr. Hightower and other health professionals, is that high mercury levels are reversible: cutting consumption of mercury-contaminated fish causes blood mercury to drop, though it can take six months or more.

Reduce your family's consumption of high-mercury fish

Since children get most of their mercury from canned tuna, it is important for parents to limit their children's consumption to less than one ounce of canned light tuna for every 12 pounds of body weight per week, in order to stay below the level of mercury the EPA considers safe. That means that a child who weighs 36 pounds should not eat more than 3 ounces (half a standard-sized can of chunk light tuna) per week. Children should avoid albacore or white tuna because the levels of mercury are higher.

A woman who is pregnant or is likely to get pregnant should eat no more than two cans of light tuna per week, or 2/3 of a can per week of white albacore tuna if she wants to stay below the EPA's level of concern for mercury. Keep in mind that the amount of mercury in a single can varies depending on the type of tuna and where the fish was caught. Albacore or solid white tuna is most likely to have higher concentrations, and chunk light tuna, lower concentrations.

To learn about mercury levels in other fish, and be sure you are buying the safest fish, read our Consumer Guide to Mercury in Fish. Also, use the searchable directory in the sidebar to check the fish advisories for your state. They are testing the water and the fish populations in your area.

Put safety first when choosing sushi

Raw tuna and other sushi fish are also something to watch out for. Often the apex predators of the food chain, these fish tend to be high in mercury. Whenever possible, avoid sushi choices that are highest in mercury, using NRDC's Mercury in Sushi Guide when you shop (downloadable as a PDF here).

Stop mercury pollution at its source

Call your electric company and switch to clean energy from renewable sources.  Learn more about the sources of mercury pollution, become an NRDC activist and work for tougher laws and rules to clamp down on mercury pollution, and get involved in efforts to oppose the construction of new coal-fired power plants.