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Harvard Commentaries
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Reviewed by the Faculty of Harvard Medical School

Medical Myths Medical Myths

The Mercury Myth

January 06, 2015

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center

When I was a kid, mercury was fun. I can recall going to the dentist and being given a thimble-sized blob of mercury in a cup, which I could pick up, push around, spill onto the floor, split into several smaller blobs and watch merge with each other. It was unusual stuff, with a consistency half-way between liquid and solid, and unlike anything else in my experience. It easily kept me occupied until it was my turn to see the dentist. Perhaps my dentist and my mother were unaware of the hazards of mercury at the time. Fortunately, it seems to have caused me no harm.

The toxic effects of mercury have become well-known over the last several decades. It would (and should) never be used as a toy for kids now. Mercury poisoning can cause irreversible nerve damage to a developing baby's vulnerable brain and nerve damage leading to numbness, tingling, imbalance, hearing loss and poor vision in adults.

But we are still exposed to mercury from a number of sources as the table below shows.

Where Is Mercury Found?

  • Fish – Fish have naturally-occurring amounts of mercury in their bodies. Larger fish have more mercury than smaller ones. Different fish of the same species may vary in their mercury content. For example, white or albacore tuna has more mercury than light tuna.
  • Dental fillings – Since the 1800s, dentists have been filling cavities with mercury. Fillings may contain a combination of silver, mercury, copper and tin; 50% of these "amalgams" are mercury. After brushing, mercury vapors are released. The popularity of these fillings has declined in recent years in favor of "resin" fillings mostly because they blend in with the tooth color.
  • Thermometers – Older thermometers may contain mercury, although exposure is only likely if you break one. The use of mercury in thermometers has been banned in most places and digital thermometers are much more accurate anyway.
  • Household items – Clock pendulums, fluorescent light bulbs, and some batteries and medical devices (including blood pressure instruments) may contain mercury. Small amounts of mercury may be found in thimerosal, a topical disinfectant and preservative present in many vaccines and some over-the-counter products including contact lens solutions, mercurochrome, and a number of nasal sprays and herbal medicines. It's even in some cosmetics, toiletries and children's sneakers that light up.

But just how much exposure to mercury is too much and how far should you go to avoid mercury exposure? These are controversial issues that I'll sort out here.

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Separating Fact from Fiction

Consider the following statements:

  • Dental fillings that contain mercury are a hazard to your health.
  • Fish containing any amount of mercury should not be eaten.
  • Over-the-counter (OTC) products that contain mercury are a proven health hazard and should be taken off the market.

Each of these may sound reasonable considering that mercury is a known toxin. But how much of a threat to your health do they really pose? Consider what the available research says.

    • A U.S. Food and Drug Administration (FDA) expert panel that met in early September 2006 reviewed 34 studies on the safety of mercury dental fillings. It found that although blood and urine levels of mercury rise with increasing numbers of mercury-containing fillings, the levels are well below those considered harmful. Still, experts continue to study the issue.
    • Experts have also concluded that mercury in fish could pose a threat, especially for kids or a developing fetus, if fish is eaten in excess. Shark, swordfish, tilefish and king mackerel have the highest amounts and should be avoided by pregnant women or young children, according to the FDA. "Medium" levels of mercury may be found in lobster, tuna, halibut, haddock, bluefish, saltwater trout, crabs, grouper and orange roughy. People should eat no more than 2 servings per week. Light tuna is a better choice than white tuna if mercury exposure is a concern. The FDA recommends no more than 1 serving of white tuna per week for pregnant women and children.

Multiple studies with negative results do not prove that fillings or other sources of mercury are harmless, but at some point it's difficult to ignore the evidence. Perhaps the effect is so small that it's difficult to detect the evidence of harm. Maybe definitive studies on these issues will never be done because of the difficulty and the expense of designing them. Even if you design a study with thousands of people and detect nerve damage or other health problems in people who eat fish compared with people who don't, it might be impossible to link the problems directly to the mercury. Other differences in diet between the two groups could be responsible.

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The Bottom Line

For now it's unclear whether mercury exposure should be on your short list of major health concerns. It's hard to avoid mercury completely, but it's unlikely that you're being exposed to harmful levels unless you eat three or more servings of white tuna a week or you're exposed to mercury regularly at work. One thing is for sure: my kids won't be playing with mercury at the dentist's office.

Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

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