Warnings about methylmercury-contaminated fish are not just for young children and expectant mothers, as research shows that eating methylmercury-contaminated fish also causes problems in adults.

Diet, particularly large fish, is the major source of methylmercury. The contaminant damages or destroys nerve tissue and affects the visual cortex and the cerebellum, the part of the brain that controls complex movements and balance.

Exposure to mercury may also result in attention and language deficits, impaired memory, inability to process and recall information, and impaired visual and motor function, and adults who regularly eat contaminated fish may have decreased concentration, dexterity and verbal memory.

In the study, researchers examined the effects of long-term exposure to small amounts of methylmercury in adults.

The study involved 129 people living in fishing communities in Brazil. Researchers took hair samples and analyzed them for mercury and also used tests to assess mental functions such as learning and attention.

About one out of four participants were found to have mercury levels that exceeded the 'safe' level set by the World Health Organization (WHO) for women and children. WHO is looking into decreasing the current exposure limit.

Moreover, people with higher levels of mercury in their hair scored lower on the tests for motor skills, memory and concentration.

Mercury contamination is a growing concern. In the United States, 43 states have advisories in effect for mercury-contaminated fish that warn the general population or sensitive groups of people to reduce or avoid consumption. This is close to a 60 percent increase from 1993 when only 27 states had mercury advisories.

In 2001, the U.S. Food and Drug Administration issued a warning to pregnant women and women of childbearing age to restrict their intake of shark, swordfish, king mackerel and tilefish due to mercury contamination. Women who eat severely contaminated fish may give birth to children with birth defects including cerebral palsy and mental retardation.

Other advisories extend the warning to children, although the current study findings suggest that it should be extended to adults as well.

Researchers concluded that exposure to methylmercury at levels often encountered by adults in North America may result in adverse effects on neurobehavioral performance.

Environmental Health: A Global Access Science Source June 2003, 2:8 (4)