But will this put to rest the concerns many people have?
It's a sensitive issue. Methyl mercury is the type found in fish, and has been found harmful to the brain in large amounts. The EPA advises women to avoid eating mercury-rich fish during pregnancy for that reason.
However, mercury in dental fillings is a different type of chemical compound -- an amalgam or blend of copper, silver, and mercury. Dentists have used this blended metal for more than 150 years. But over the years, concerns about mercury fillings have been raised, writes Meryl Karol, PhD, an epidemiologist with the University of Pittsburgh School of Public Health.
Karol chaired the expert panel whose research review was released today. Scientists from the National Institutes of Health's National Institute of Dental and Craniofacial Research, the Center for Devices and Radiological Health of the FDA, the CDC, and the Public Health Service's chief dental officer, were among those experts.
Only 300 studies published since 1996 had sufficient merit to be included in their report -- studies that analyzed mercury in urine samples as a marker for mercury exposure. Methyl mercury from fish is not found in urine samples, explains Karol.
Large population studies, animal toxicity studies, and studies of effects from various levels of mercury exposure were included. Researchers also looked at whether it was biologically possible for mercury vapor from fillings to cause brain disorders and other health problems, she notes.
The panel's conclusion: "Current research is insufficient to attribute various complaints to mercury in dental amalgam," writes Karol. Even those people with allergic reaction to dental amalgam "did not have high levels in their blood," she adds.
Also among the panel's conclusions:
- The evidence showed that mercury vapor is released from dental work and absorbed in the body. However, about 95% of people in the studies had mercury levels at or lower than the level deemed harmful by the WHO. The long-term use of nicotine chewing gum (over two years) combined with intense chewing and more than 20 dental amalgam surfaces presents the greatest chance that urine mercury measurements exceed the general population and approach a level seen in people who have occupational exposure to mercury. However, the reports state that adverse health effects for long-term nicotine gum chewers was not evaluated.
- Bruxism (grinding teeth) and dental amalgam placement and removal appear to have less impact on exposure than the use of nicotine chewing gum.
- Allergic sensitivity to dental amalgam seems to affect a small percentage of people.
- Insufficient research has been done to support or refute whether dental amalgam causes antibiotic resistance in the human gut or that it may cause any autoimmune disease including multiple sclerosis.
- Three studies also failed to support a role of dental amalgam as a factor in the development of Parkinson's disease or Alzheimer's disease.
- Human studies have failed to support or refute a link between dental amalgam with brain damage in a developing fetus.
- Both methyl mercury from fish and mercury from dental amalgam have been found in breast milk. Rat studies show that high exposure of mercury vapor among pregnant rats and monkeys induces behavioral abnormalities -- but no studies have looked at whether low-level exposures affect brain development.
Although some people undergo chelation therapy to treat their symptoms, animal studies have shown that chelation therapy works to bind and remove mercury from the kidneys, but not from the brain, Karol notes. However, chelation carries a host of problems -- possible adverse health problems including headaches, dizziness, nausea, and the loss of essential metals.
Chemicals used in chelation therapy have been harmful to the developing fetus, she adds.
Why Mercury Fillings Are Safe
Mercury amalgam fillings are "100% safe," says J. Rodway Mackert, DMD, PhD, professor of dental materials at the School of Dentistry of the Medical College of Georgia in Augusta. Mackert is a spokesman for the American Dental Association.
"Dental amalgam restorations are considered safe," says a WHO consensus statement. In rare instances, there is an allergic reaction to the filling, the statement notes. However, "the small amount of mercury released from amalgam restorations, especially during placement and removal, has not been shown to cause any other adverse health effects."
"According to the best available scientific evidence, there is no relationship between silver dental fillings and Alzheimer's," says the Alzheimer's Association web site.
"There is no scientific evidence to connect the development of multiple sclerosis (MS) with mercury-based dental fillings," says the National Multiple Sclerosis Society web site. "Poisoning with heavy metals, such as mercury, lead, or manganese, can damage the nervous system and produce symptoms, such as tremor or weakness, similar to those seen in MS. However, the underlying mechanism of nerve damage is different from MS, as is the pattern of illness caused by heavy metal poisoning."
In creating amalgam fillings, mercury is chemically combined with silver, Mackert explains. It's the chemical reaction between powdered silver and tin -- and liquid mercury -- that creates a material stable and safe enough for the human mouth.
"When liquid mercury is combined with silver, the chemical reaction reduces the amount of mercury that is released by nearly 1 million-fold," Mackert tells WebMD. "That's why it can be used in fillings."
Questions about mercury fillings began surfacing in 1979, when technology became available to measure the mercury vapor that the fillings emitted, he explains. "When we could measure the vapor, we found it wasn't zero, but it was a very small number. That was the concern. People began looking at how much vapor is released and the effects."
Since then, numerous well-conducted studies have shown that there is no need for concern, says Mackert. "In order to produce [toxic mercury] levels in fillings, you would have to have 450 to 500 fillings in your mouth. Humans don't have that many teeth. Most people have 32 teeth, and with their wisdom teeth out, it's 28. So it doesn't add up," he tells WebMD.
Among the studies:
- In 2003, T.W. Clarkston, one of the world's leading mercury toxicity experts, published a study in The New England Journal of Medicine looking at mercury exposure from food, paints, vaccines, and dental fillings. The conclusion: "Patients who have questions about the potential relation between mercury [vapor from amalgam fillings] and degenerative diseases can be assured that the available evidence shows no connection."
- An important twin study conducted in Sweden -- comparing nearly 600 adopted and "reared together" twin sets, all about age 66 - concluded: "This study does not indicate any negative effects from dental amalgam on physical or mental health or memory functions in the general population over 50 years of age." That 1996 study appeared in the journal Community Dental and Oral Epidemiology.
- Another study published in 2003 looked at neurological and cognitive function among 550 healthy working adults. That study showed no association between mercury fillings and "any detectable deficits in cognitive or fine motor functioning." The study appeared in the journal Environmental Health Perspectives.
There's more data: "The amount absorbed in the average person's diet is 5 to 6 micrograms per day," says Mackert. "Mercury is in vegetables, fish, and in other meats in very small amounts. Mercury fillings release from 1 to 3 micrograms per day."
But, given concerns about pregnancy and mercury, are women at greater risk? "Studies of female dentists who are occupationally exposed to mercury amalgam don't show any higher incidence of adverse pregnancy outcomes," Mackert tells WebMD. "Yet they are exposed to more mercury in fillings than other women are."