Concerns about low-level toxicity haunt discussions of another ubiquitous source of mercury exposure: silver dental fillings. Elemental mercury, which makes up half of silver fillings, releases mercury vapor, just as liquid mercury does. The vapor from dental amalgams is the primary source of the one to eight micrograms of mercury per liter of blood, that is, according to some sources, in the average American adult. That amount uncomfortably overlaps the Environmental Protection Agency’s current safe level of 5.8 micrograms per liter. But the EPA’s safety level is based on methylmercury exposure, about which more is known. No human studies have assessed prolonged exposure to low levels of mercury vapor. One study hints at subtle neural and behavioral anomalies in dentists, who collectively use 300 metric tons of mercury in amalgams each year and who often have two to five times the typical concentration of mercury in their urine.
“I think the methylmercury in fish is probably our least toxic exposure,” says Haley, who broadcasts the hazards of dental fillings.
Silver-mercury fillings have never been tested for safety. “The amalgam question will never be solved until we do a clinical trial like those we do with other medical devices,” says Aposhian.
“It’s really unclear what’s going on with dental amalgams,” says Stern, who notes that the issue is complicated by the potential for panic and lawsuits. “It’s a snake pit.”
One of the lessons of Minamata is that mercury, like lead, is harder on fetuses than on the women carrying them, or adults in general. In the Japanese event, women with no overt symptoms of poisoning gave birth to severely disabled children. “It was evident there was a major difference in susceptibility between the developing brain and the mature brain,” says Philippe Grandjean, an epidemiologist at the Harvard University School of Public Health. “When we saw serious poisonings in Minamata, that made us wonder whether mercury could be like lead.”
Studies of lead have shown that IQ decreases approximately two or three points for every doubling of prenatal and early postnatal exposure. To see if mercury has comparable effects, Grandjean, along with Pál Weihe at the University of Southern Denmark, is conducting the largest study to date of children’s cognition and behavior in a population routinely exposed to low levels of mercury. His work in the Faeroe Islands of Denmark includes 1,000 mother-child pairs and spans almost 20 years. In a typical year, Faeroe islanders consume 1,000 pilot whales, or one whale for every 50 islanders. “They belong to one of the most fish-eating populations in the world,” says Grandjean.
Whale meat is one of the most highly contaminated seafoods because whales are at the top of the food chain. Even so, the mercury content of whale meat is considerably lower than that of the hypertoxic Minamata fish. An earlier study of shark eaters in New Zealand suggested that relatively high levels of mercury in a mother’s hair during pregnancy correlated with a loss of three IQ points in her child. High levels, in that study, were identified as six parts per million and above in the hair shaft.
Grandjean gave a battery of sophisticated cognitive and developmental tests to the Faeroese children when they were 7 and 14. His results indicate that IQ drops 1.5 points for every doubling in prenatal exposure to mercury. The 2000 National Research Council report concluded that the risk documented by Grandjean “is likely to be sufficient to result in an increase in the number of children who have to struggle to keep up in school.”
“We learned there is a response at low levels,” says Grandjean. “It’s not a huge loss, but it’s certainly not negligible.”
Yet in another large, long-term epidemiological study conducted on the Seychelles Islands in the Indian Ocean, Clarkson has so far found no effect on neurological development from prenatal exposure to low levels of mercury in seafood. “We can’t exclude effects from 20 parts per million or even 12 parts per million,” he notes. But he concludes there is no graded risk that extends to the lowest exposure levels.
The 2000 research council report evaluated the Faeroe, Seychelles, and New Zealand studies and recommended that the EPA set safety standards based on Grandjean’s more sobering findings. The agency did. Then, for good measure, it added a 10-fold uncertainty factor—a safety margin to protect against scientific unknowns and individual differences in response to a toxin. The uncertainty factor lowers the threshold to a figure of 5.8 micrograms per liter of blood and 1.2 parts per million in hair.
The problem with safety factors is that they create a toxicological limbo between demonstrably harmful doses and levels that have been declared safe. Thus, when Centers for Disease Control surveys find that one in 12 American women of childbearing age—8 percent—have blood mercury levels above the safety threshold, the implications aren’t clear, either for them or for the children they bear. Epidemiologist Tom Sinks says, “It doesn’t tell us there’s a hazard.”
“The whole idea of a safety factor is to protect people,” Clarkson says. “You can’t turn it around to use as an indication of who’s at risk. If you’re just above it, you aren’t necessarily in trouble.”
That kind of hedging, along with disagreement among population studies, leaves regulators with plenty of wiggle room. The FDA, for example, uses a more relaxed safety standard for mercury based on studies from the 1970s and 1980s. Where the EPA safety level for daily exposure is 0.1 microgram per kilogram (about 2.2 pounds) of body weight, the FDA’s standard is about 0.4 microgram per kilogram per day. The difference is four times as much mercury.
Concern about early exposure to mercury doesn’t end at birth. Until recently, many infants received regular injections of mercury on a state-mandated, medically sanctioned schedule. The mercury came from a compound called thimerosal that has been used as a preservative in vaccines and other medicines since the 1930s. In 1999 the FDA recommended that thimerosal no longer be used in pediatric vaccines, and manufacturers have removed it from all but the influenza vaccine. But some scientists and many more aggrieved parents are convinced that thimerosal in childhood vaccines has already caused, or at least catalyzed, the U.S. epidemic of autism.
An estimated 400,000 Americans today have autism, a once rare neurological disorder characterized by social withdrawal, difficulty communicating, and involuntary, repetitive movements. Although the exact numbers are in dispute, the rate of diagnosis seems to have climbed sharply in the last decade. In California the incidence of autism was six times higher in 2002 than in 1987.
During that period, federal health officials added four new kinds of vaccines to the childhood immunization schedule, and the amount of mercury routinely administered to infants in the first six months of life more than doubled. Throughout the 1990s, a 3-month-old baby might receive as much as 63 micrograms of mercury in a single visit to a doctor—roughly 100 times the daily EPA safety level. By the age of 6 months, properly immunized children were exposed to at least 188 micrograms of mercury in a series of at least nine injections. Although the 1999 FDA action minimized such exposure, some infant flu vaccines still contain 12.5 micrograms of mercury per dose—more than 10 times the daily EPA safety level for a 20-pound baby.