MERCURY FILLINGS

Dental amalgams, known as silver fillings, are composed of roughly 50 percent mercury. Studies of people with mercury-containing dental fillings show a correlation between the number and size of the fillings and the amount of mercury excreted in their urine. The relationship suggests that the mercury is derived from mercury vapor released from the fillings. Some evidence shows that the level of mercury in the brain tissue of fetuses, newborns, and young children is also directly proportional to the number of surfaces of amalgam fillings the mother has.

Circumstantial evidence also implicates mercury in autism. Some of the symptoms of autism and mercury poisoning are similar, and Haley has garnered evidence from hair samples that autistic children do not clear mercury from their bodies as efficiently as most kids do. They may have a genetic susceptibility that allows more mercury to accumulate in their tissues, he says. That could make them more vulnerable to mercury-laced vaccines and the continuous low-level exposure from their mothers’ dental fillings. “It is amazing to me that no one has taken the tissue of autistic children to see if there is excess mercury there,” Aposhian told a committee at the Institute of Medicine in Washington, D.C., last year. “That’s one thing that really has to be done.”

There are other sources of uncertainty. The form of mercury in thimerosal—an organic compound called ethyl mercury—is the least studied of all mercury’s incarnations. When scientists argue about its toxicity, they typically rely on data from methylmercury, which may not be an equivalent form of exposure. Experts even disagree about whether ethyl mercury can cross the blood-brain barrier. (It probably does.) “There are no good ways to measure ethyl mercury in tissue,” toxicologist Polly Sager of the National Institute of Allergy and Infectious Diseases told the Institute of Medicine committee.


The Institute of Medicine concluded last May that no claim could be made for a causal link between mercury-laced vaccines and autism, but several independent researchers had complained that their access to federal vaccine databases, which could provide evidence of a link, had been repeatedly blocked. A few scientists, including Haley and neuropharmacologist Richard Deth of Northeastern University in Boston, continue to study possible mechanisms for the connection. Deth reported last year, for example, that in human nerve cells thimerosal blocks a chemical reaction called methylation that is critical to gene activity and that is also disabled by exposure to lead.

The report that first triggered worries about a connection between vaccines and autism was published in the British medical journal The Lancet in 1998. It described eight children whose behavioral problems surfaced within two weeks of receiving the measles-mumps-rubella vaccine. The Lancet and most of the article’s coauthors ultimately disowned the study because its lead author had not divulged that he was also being paid to conduct research for parents seeking to sue vaccine manufacturers. Nonetheless, the number of parents in the United Kingdom willing to immunize their babies with the vaccine dropped from 90 percent in 1998 to less than 80 percent in 2004.

TESTING TROUBLE

Chronic low-level exposure to mercury is difficult to quantify because analyses of blood, urine, and hair will reflect only recent acute exposure, not past exposure. If acute mercury poisoning is diagnosed, administering compounds that bind to mercury and draw it out of the tissue—a process called chelation therapy—can remove elemental or inorganic mercury. However, chelation cannot remove methylmercury.

Mercury has a strong affinity for the brain, especially the fetal brain. Methylmercury has been shown to alter the construction of structural components of the brain called microtubules and influence the development of neurons.