Mercury is no longer an ingredient in childhood vaccines, but seafood contains high enough levels of the toxic heavy metal to pose a risk to women of childbearing age for immune-system disorders, according to new research.
 

The findings, published by University of Michigan researchers in the journal Environmental Health Perspectives, indicate even low levels of mercury generally considered safe are associated with autoimmunity conditions that cause the body's immune system to attack healthy cells by mistake.
 

Such disorders — including arthritis, multiple sclerosis, inflammatory bowel disease, and lupus — strike nearly 50 million Americans and predominately women, and are among the 10 leading causes of death in the U.S.
 

"We don't have a very good sense of why people develop autoimmune disorders," says lead researcher Emily Somers, an associate professor at the UM Medical and Public Health Schools.

"A large number of cases are not explained by genetics, so we believe studying environmental factors will help us understand why autoimmunity happens and how we may be able to intervene to improve health outcomes. In our study, exposure to mercury stood out as the main risk factor for autoimmunity."
 

For the study, researchers analyzed the medical charts of women ages 16-49 from the National Health and Nutrition Examination Survey from 1999-2004. Greater exposure to mercury was associated with a higher rate of autoantibodies — proteins made by a person's immune system when it fails to distinguish between its own tissues and potentially harmful cells.
 

"The presence of autoantibodies doesn't necessarily mean they will lead to an autoimmune disease," Somers said. "However, we know that autoantibodies are significant predictors of future autoimmune disease, and may predate the symptoms and diagnosis of an autoimmune disease by years.
 

"For women of childbearing age, who are at particular risk of developing this type of disease, it may be especially important to keep track of seafood consumption."
 

Many fish consumption recommendations are aimed at pregnant women, those who may become pregnant, nursing moms, and young children.
 

The Food and Drug Administration and the Environmental Protection Agency say pregnant women can safely eat up to 12 ounces (340 grams) of seafood a week. Fish such as swordfish, king mackerel, and tilefish contain the highest levels of mercury while shrimp, canned light tuna, and salmon have lower levels.