No one can see mercury in a fish dinner with the naked eye, but Jane Browning, executive director of the Learning Disabilities Association of America, has seen its effects.

"We see a rise in learning and other developmental disabilities. We believe those are attributable to environmental factors, environmental toxicants,'' Browning said yesterday at a news conference.

She said about 12 percent to 15 percent of all children and adults have learning and other developmental disabilities, some of which are caused by mercury exposure. Exactly how many are affected is uncertain.

"We're talking about lifelong impact, people who struggle throughout their lives because of neurological deficits incurred in the womb,'' she said.

Browning -- as well as leaders from the National Education Association and the Arc of the United States, an advocacy group for those with intellectual and developmental disabilities -- yesterday released a new guide for parents called "Mercury and Learning Disabilities.'' The guide is available on the Internet at www.mercuryhurts.org.

The guide states that coal-burning power plants are the nation's "largest single source of uncontrolled mercury pollution.''

Some of the mercury coming out of smokestacks lands in surface water and is converted into a more toxic form, methylmercury, which is eaten by fish, which are eaten by people.

Methylmercury is a neurotoxin, meaning it damages or destroys nerve tissue.

While the federal government says that nearly all fish and shellfish contain traces of mercury, some types have more than others. Larger fish that have lived longer contain more mercury.

The Environmental Protection Agency and the Food and Drug Administration have special warnings for women who might become pregnant, women who are pregnant, nursing mothers and young children. They can be found on the Internet at www.epa.gov/waterscience/fishadvice/advice.html.

The women and children are warned not to eat fish with high levels of mercury, including shark, swordfish, king mackerel and tilefish.

They also are advised to eat no more than 12 ounces per week of fish and shellfish that are lower in mercury.

Of those 12 ounces, no more than 6 ounces should be albacore or white tuna because it contains more mercury than canned light tuna.

Young children are advised to eat smaller portions.

The federal government also recommends paying attention to advisories on fish caught locally.

The warnings apply to women of child-bearing age because it takes months for mercury to leave the body, said Larry Silver, clinical professor of psychiatry at Georgetown University Medical Center and former president of the Learning Disabilities Association.

For a pregnant woman, Silver said, the biggest impact of mercury is during the first two months of pregnancy, when the child's brain is developing. He said a woman may not realize she is pregnant when she eats seafood containing mercury.

The EPA/FDA brochure puts the risks this way: "For fetuses, infants and children, the primary health effect of methylmercury is impaired neurological development. Methylmercury exposure in the womb, which can result from a mother's consumption of fish and shellfish that contain methylmercury, can adversely affect a baby's growing brain and nervous system.

"Impacts on cognitive thinking, memory, attention, language and fine motor and visual spatial skills have been seen in children exposed to methylmercury in the womb.''

In the news conference, Martha Keating, senior scientist of the Clean Air Task Force, a Boston-based nonprofit group, said, "We simply can't solve the problem of mercury contamination in fish without significantly reducing emissions.''

Browning said she wants to see stringent rules and is opposed to the idea of trading between clean plants and dirty plants.

"It's anathema to us to think that certain populations are being selected out as if it's OK to subject them to higher levels of these emissions than other populations. If you live where there's a dirty plant, too bad for you and your children. It's just not acceptable,'' Browning said.