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Slight Nerve Effect with Elemental Mercury Exposure from Fillings

By Laura Hall
January 2010

left to right, Drs. Birnbaum and Franzblau enjoying a pre-seminar discussion
Birnbaum and Franzblau enjoyed a pre-seminar discussion. Both have long been involved with the UM Dioxin Exposure Study investigating dioxin contamination in Michigan caused by the Dow Chemical Company. (Photo courtesy of Steve McCaw)

Dr. Alfred Franzblau
Franzblau explained the placement of the electrode for the nerve conduction test. (Photo courtesy of Steve McCaw)

NIEHS Acting Director Steve Kleeberger
NIEHS Acting Director Steve Kleeberger, with his interest in pollutant effects on lung physiology, listened carefully to Franzblau's talk. (Photo courtesy of Steve McCaw)

NTP member, Mary Wolfe
NTP member, Mary Wolfe, with her toxicology background, was also intent in absorbing what Franzblau had to say. (Photo courtesy of Steve McCaw)

members of the audience
Members of the audience looked on while Franzblau fielded questions. (Photo courtesy of Steve McCaw)

Matt Longnecker at seminar
Matt Longnecker was one of several members of the Epidemiology Branch that attended the seminar.
(Photo courtesy of Steve McCaw)

On Dec. 4, Alfred Franzblau, M.D( Exit NIEHS., discussed the effect of chronic low-level elemental mercury (Hg) exposure on dental professionals. In his study, researchers measured urine mercury concentrations and nerve function in 2,974 volunteers and found a very slight, but significant deleterious effect on sensory nerves. The University of Michigan (UM) professor's talk was titled "The Impact of Low-Level Mercury Exposure on Peripheral Nerve Function."

"I've got a bunch of mercury fillings, but I'm not going to rush around and get them taken out," said Franzblau. "I'm not worried about it. The effect is so small that I'm reassured about mercury amalgam effects for adults."

However, Franzblau said that this small nerve effect in adults raises some concern about elemental mercury exposure in more susceptible groups, such as children. In particular, the fetus with its developing nervous system could be at risk from maternal exposure. Also, exposure from organic mercury is not eliminated from concern with this study.

Dental amalgams are most common Hg exposure

Restorative tooth fillings, dental amalgams, are the most common source of elemental mercury exposure for the general population. Franzblau pointed out that the amalgams are 50 percent Hg -- one large filling alone may contain as much as one gram of Hg. Previous studies have shown that the amount of Hg in the urine increases linearly with the number of fillings a person has.

In addition to exposure from their own fillings, dentists and dental hygienists are occupationally exposed to Hg when they make the amalgams used for their patients.

Mercury is a nerve poison

Franzblau explained that mercury occurs naturally and exists in various forms that are all poisonous to the nervous system. The indicator, or biomarker, for human exposure from elemental mercury is the urine mercury concentration. In contrast, the hair is used as the biomarker for organic mercury exposure such as that caused by eating contaminated fish.

Nerve damage measurements

Franzblau used measurements of nerve conduction to determine nerve damage.

Scientists tested the median nerve by sending an electric signal down the nerve between electrodes placed on the index finger and farther down the arm. The ulnar nerve was similarly measured by using the fifth finger. Reduced amplitude, the size of the electronic signal peak, and longer latency, the time for the signal to travel in milliseconds, indicate nerve damage.

The median nerve is the nerve affected in carpal tunnel syndrome, a common cause of hand discomfort and functional impairment.

Mercury concentration in dental professionals

The dental professionals had a mean urine Hg concentration of 3.63 microgram per liter, which is approximately twice the urine Hg concentration that women in the general population have. The 1,748 women tested for urine Hg in another study, the National Health and Nutrition Examination Survey( Exit NIEHS (NHANES), had a mean concentration of 1.55 microgram per liter.

The study used information from the NHANES investigations on urine mercury concentration and other parameters as a control to represent the general population.

It combined data collected for 10 years at the American Dental Association (ADA) annual meetings on the dental professionals. The data collected by Franzblau and his colleagues at the UM, ADA, and SmartHealth, Inc. allowed this study to be the largest of its kind, enabling them to see the slight adverse effect.

Slight nerve damage

Increased urine Hg resulted in very slight but significant nerve damage. The study results showed that for every one microgram per liter increase in urine Hg of the dental professionals, the median sensory nerve latency increased by 0.006 milliseconds. Increasing Hg exposure resulted in slower nerves, but the amplitude of the nerve signal was unaffected. 

Franzblau and clinical occupational medicine

Franzblau studies occupational musculoskeletal, neurological, and respiratory diesease. NIEHS/NTP Director Linda Birnbaum, Ph.D., introduced Franzblau as her long-time colleague who "has broad experience in occupational medicine and epidemiology and has done a lot of work with carpal tunnel syndrome."

(Laura Hall is a biologist in the NIEHS Laboratory of Pharmacology currently on detail as a writer for the Environmental Factor.)

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