From Medscape Medical News

FDA Hearing on Mercury-Based Fillings Will Question Safety Threshold

Robert Lowes

Authors and Disclosures

December 11, 2010 — At a 2-day meeting next week of a federal dental products panel, there will be no vote on whether to ban or restrict the use of dental amalgam — a mercury-based filling material — as advocated by its opponents.

Instead, the US Food and Drug Administration (FDA) panel will wade through voluminous reports and oral testimony on December 14 and 15 in an attempt to clarify 3 key issues:

1. How much elemental mercury are people with dental amalgam exposed to?
2. What is the appropriate safety threshold for this exposure?
3. How should clinical studies of possible health risks posed by dental amalgam be valued and interpreted?

The conclusions reached by the panel could prompt the FDA to further regulate the highly controversial material, which critics say causes diseases ranging from autism to Alzheimer's disease. Dental amalgam already is banned in Sweden, Norway, and Denmark. In October, the city of Costa Mesa, California, passed a resolution asking local dentists to voluntarily switch to other filling materials.

Dental amalgam is composed of roughly 40% to 50% liquid elemental mercury and a powdered alloy of mostly silver, copper, and tin. The American Dental Association estimates that amalgam is used for 30% of dental restorations — a far cry from the day when it accounted for the majority of these procedures.

Still, an estimated 120 million Americans have amalgam in their mouths, according to Jim Love, an attorney for a mercury-free dentistry group called the International Academy of Oral Medicine and Toxicology.

At high levels, mercury vapor released by elemental mercury can poison the brain and kidneys. The FDA states, however, that people with amalgam fillings do not experience such dangerous exposure levels, and that the material is safe for adults and children aged 6 years and older.

That said, the agency in 2009 reclassified dental amalgam as a medical device from a lower-risk class I to a moderate-risk class II, which entailed label information about how fetuses and young children may be more sensitive to its neurotoxic effects. What lies behind this precaution is the ability of bloodstream mercury in a pregnant woman with amalgam fillings to accumulate in the fetus. Likewise, breast-fed children of mothers with dental amalgam also are exposed to mercury.

The 2009 reclassification does not go far enough for dental amalgam opponents, who filed 4 petitions with the FDA to revisit the issue. They are challenging the agency's estimates of mercury vapor exposure resulting from dental amalgam, the safety threshold it uses, and the soundness of the clinical studies that formed the basis of the reclassification. Their petitions ask the agency to ban the use of dental amalgam, or else classify it as a high-risk class III medical device and restrict its use in pregnant women, children younger than 6 years, and individuals already known to be sensitive to mercury.

For its part, the American Dental Association maintains that there has been no new research since 2009 warranting further FDA regulation of dental amalgam, which it calls safe and less expensive than alternate filling materials. An amalgam ban, it argues, would drive up the cost of dental restorations, making them unaffordable for many Americans.

Current Safety Threshold Protective Enough?

One of the petitioners is the International Academy of Oral Medicine and Toxicology, which commissioned a study of dental amalgam and mercury exposure prepared by the environmental division of an engineering and construction firm called SNC-Lavalin. Lead author G. Mark Richardson, PhD, has conducted similar studies for Health Canada, a former employer and the Canadian equivalent of the US Department of Health and Human Services. Dr. Richardson will testify at next week's dental panel hearing.

Dr. Richardson's SNC-Lavalin study estimated that 67 million Americans with dental amalgam inhale mercury vapor at levels exceeding the reference exposure level (REL), or safety threshold, of 0.3 μg/m³ set by the US Environmental Protection Agency (EPA). The FDA has used this REL to determine the health risk for dental amalgam. However, Dr. Richardson writes that the EPA's REL, also called a reference concentration (RfC), is too high because it is based in part on studies of chloralkali workers who were exposed simultaneously to chlorine gas and mercury. Chlorine gas, he writes, reduces the toxic effects of mercury.

The California EPA puts the REL for mercury vapor at 0.03 μg/m³ — 10 times lower than the EPA's threshold. Although that state agency relied on the same chloralkali industry studies in its determination, it also introduced an additional uncertainty factor of 10 to account for the higher susceptibility of developing nervous systems in fetuses and young children. Using this safety threshold, Dr. Richardson estimated that 122 million Americans with dental amalgam are exposed to unsafe levels of mercury vapor.

Three independent toxicology experts who will testify at next week's hearing at the FDA's request have already weighed in on the REL issue and reached different conclusions. Earlier this year, as part of a "homework" assignment, the agency asked them whether the REL set by the US EPA was "protective" of all groups — including fetuses and young children — who are sensitive to the adverse health effects of mercury.

One expert, Robert Yokel, PhD, replied in his written answers that the EPA's REL is indeed protective, but that some individuals with dental amalgam are exposed to higher levels of mercury vapor. A second expert, William Farland, PhD, stated that "it is impossible to say with certainty whether the EPA's RfC is protective for all groups," and he recommended reevaluating the issue. The third expert, Gary Ginsberg, PhD, concluded that "the current RfC appears to not be adequately protective," and that "several lines of evidence and calculation approaches lead to an RfC 3 to 10 times below" the one derived by the EPA.

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