Factories, mines, and waste incinerators spew mercury into the air and water, and some of it finds its way into the seafood we eat. Most research on the dangers that mercury poses to the brain and central nervous system has focused on pregnant women and children—but very little has been done on older adults whose nervous systems are often more sensitive to toxins than those of younger people. Researchers from Johns Hopkins University and the New York State Department of Health decided to take a look.

What the researchers wanted to know: Are elevated blood levels of mercury in older adults associated with an impairment of mental functioning or nervous system functioning?

What they did: The researchers gave 474 Baltimore residents between the ages of 50 and 70 a dozen tests of mental and manual agility, which measured, among other things, nonverbal reasoning, letter fluency, visual memory, and finger dexterity. They correlated scores on these tests with mercury levels, obtained by taking blood samples, and the amount of fish that participants said they had eaten in the past year.

What they found: People who ate more fish did have higher levels of mercury in their bodies, but they did not score differently on most of the tests of brain or nervous system function. Participants with higher levels of mercury did worse on tests of visual memory but better on tests of manual dexterity. However, the researchers speculated that these apparently contradictory results might be due to chance, especially given that other tests showed insignificant differences between people with high and low levels of mercury.

What it means to you: If you're an older adult and like seafood, eat up! Fish have a high concentration of omega-3 fatty acids, which can help protect against heart disease. And, this study should reassure you that most older adults probably can consume seafood without worrying about neurological effects.

Caveats: This study took blood mercury levels at only one point in time so they could not tell whether scores on these tests would change for individual people as their blood mercury levels changed.

Find out more: The Environmental Protection Agency has information about what types of fish have high levels of mercury and how much it is OK to eat.

Previously, researchers believed that mercury from dental fillings could contribute to cognitive decline or even to Alzheimer's disease. Read a University of Kentucky study done several years ago that found no relation.

Read the article: Weil, M. et al. "Blood Mercury Levels and Neurobehavioral Function." Journal of the American Medical Association. April 20, 2005, Vol. 293, No. 15, pp. 1875–1882.

Abstract online: http://jama.ama-assn.org