Mercury in Tuna and Sardines
Q: I eat tuna fish and sardines for lunch at least 4 times per week. Am I at
risk for mercury poisoning, and if so, what will happen? How much fish is safe
Dr. Donnica: Mercury contamination in fish has existed for centuries, although
it has been exacerbated in the past centuries by pollution and other industrial
activities. Mercury poisoning is an important, but controversial, issue. It also
varies from state to state and even depends on the body of water. The groups
who should be most concerned are women who are pregnant, breastfeeding, or trying
to become pregnant. According to a US government advisory panel, pregnant women
who eat "too much" tuna may expose their unborn babies' brains to possibly harmful
levels of mercury. How much is too much? The panel advised a limit of two 6-ounce
cans of tuna each week if tuna was the only fish eaten or one can if other seafood
was eaten that might also contain mercury. Small children (under 7) are also
highly susceptible to mercury toxicity.
Mercury toxicity mainly affects the nervous system. In adults, mild symptoms
can include loss of or abnormal sensation in the hands and feet, tiredness,
or blurred vision. Severe poisoning involves vision, hearing and speech impairment,
and may later result in coma and death. Long-term exposure to methyl mercury
may cause kidney damage. In children, mercury toxicity may result in decreased
brain size, delayed physical development, mental impairment, abnormal muscle
tone, and coordination problems.
According to estimates from the Center for Disease Control and Prevention,
approximately 8% of American women of childbearing age have enough blood mercury
to put their babies at risk for learning difficulties. Eating contaminated
seafood is the main source of mercury exposure. The extent of contamination
not only depends upon the source of the fish, but its size and age: Bigger and
older fish eat or have eaten more smaller fish and thus have accumulated more
Despite this, fish is considered a highly nutritious food. Women are advised
to eat up to 12 ounces a week of cooked fish, including canned tuna, shellfish
and other ocean fish.
Created: 3/8/2003  - Donnica Moore, M.D.