TUESDAY, April 19, 2005 (HealthDay News) -- A first of its kind study of nearly 500 Americans over the age of 50 finds no neurobehavioral damage linked to their blood levels of mercury.
The finding may have implications both for industrial emissions standards and for guidelines on consumption of fish -- a major dietary source of environmental mercury.
Although more research is required, "the take-away message from our study is that the current government levels -- where they are regulating mercury right now -- might be OK," said lead researcher Megan Weil, who conducted the study while a doctoral candidate at the Johns Hopkins University School of Public Health.
The report appears in the April 20 issue of the Journal of the American Medical Association.
Mercury released into the air via coal-fired generating plants and other pollution sources eventually finds its way into rivers, lakes and oceans. Once there, bacteria convert the toxin into methylmercury. Methylmercury then makes its way up the aquatic food chain, concentrating at high levels in the flesh of large, relatively long-lived fish species.
In March 2004, two federal agencies -- the U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) -- rekindled fish lovers' worries with a special joint methylmercury advisory. The bulletin urged pregnant women, nursing mothers, women who may become pregnant, and young children not to eat shark, swordfish, king mackerel or tilefish because they contain high levels of mercury.
Women and children may eat up to 12 ounces a week of fish and shellfish that are low in mercury, such as shrimp, canned light tuna, salmon, pollock and catfish, the advisory said. But they should limit their intake of albacore -- or white -- tuna to no more than 6 ounces a week because that fish is higher in methylmercury.
Weil -- now director of environmental health services at the Association of State and Territorial Health Officials -- wondered whether current mercury levels might harm the brains of older adults, too.
She pointed out that, once inside the brain, methylmercury changes its chemical composition so it cannot exit the organ. This means people experience a gradual buildup of the toxin over their lifespan. As the brain ages, it grows less able to deal with this toxic insult, Weil added.
"This is the first study done on this in the U.S., and in this age group," she said. "I expected to find something."
That wasn't the case, however.
For the study, conducted in 2001-02, Weil's team measured the blood mercury levels of 474 Baltimore residents aged 50 to 70. They then examined data on participants' neurological and behavioral functioning, as evaluated by a battery of standard tests.
"The main finding was that at the levels of mercury that we found -- about average for the general population -- there were no cognitive effects," Weil said.
Experts know that at high concentrations, mercury poisoning can have devastating effects on the central nervous system. And at more moderate levels, "subtle neurobehavioral effects" arise, Weil said.
"So the fact that we didn't see anything at this lower level indicates that there is probably some cut-off point where, if you go above that, you are going to see some effects," she said.
The findings should not change any of the current recommendations for fish consumption, however.
"All the warnings are still in place for the entire population," she said. "For children especially -- their brains aren't fully developed. Even when they are still in their mother's wombs, mercury exposures can easily cross into the [fetal] brain. So it's very important for women and children to follow the FDA and EPA guidelines."
And Weil stressed her study isn't likely to be the last word on older adults, either. "This is just a first study," she pointed out. "We have all these policies and regulations that we're putting into place, and yet we really don't know much about mercury's effects in this population."
For more on mercury and fish consumption, head to the U.S. Environmental and Protection Agency.