Mercury is a deadly poison inside every one of us. It's in things like fish, dental fillings, and vaccines. Scientists agree that a little bit is okay, but warn not to over-do it.
So how much is too much?
Fish is an important part of the diet of millions of Americans. And it should be! It's one of the healthiest foods we can eat. It's high in protein, low in fat and rich in those wonderfully healthy omega-3 fats, particularly DHA which has been proven to increase cognitive function. Fish is great for your heart and your brain and should be on the plates of kids and adults every week.
However, fish does contain mercury so some caution should be taken when choosing your fish and deciding how often to eat it.
Big Fish, Little Fish
Much of the mercury in fish comes from coal-fired power plants. The emissions fall to the ground and are then washed into the waterways. Then it's absorbed by tiny organisms which are eaten by small fish, so the mercury stores in their bodies.
When larger fish eat smaller fish, mercury accumulates, and builds-up to high levels in the big fish. So the fish at the top of the food chain contain the most mercury.
Dr. S. Rutherfoord Rose is a professor and the chair of the Division of Clinical Toxicology at Virginia Commonwealth University and is the director of the Virginia Poison Center.
"Small amounts of mercury, our bodies deal with very well," he said. "But when it accumulates it can be very toxic to multiple parts of the body including the brain."
Dr. Rose explained that bodies that are still developing are at greatest risk. So young children, women who are nursing, pregnant or may become pregnant should not eat any big fish like shark, swordfish, king mackerel or tilefish. They should not have more than two six-ounce servings a week of the smaller fish, such as shrimp, salmon, pollack and catfish.
When choosing tuna, canned light tuna - which is the darker, cheaper variety - is low in mercury, and can be eaten twice weekly. But white tuna, albacore or sushi tuna, has more mercury and should be eaten no more than once a week for the high-risk groups mentioned above.
But what about the mercury in fish you catch in your local lake or river? If there's reason for concern, health departments will post signs on the banks of certain waterways, warning fishermen to be careful when eating fish caught there.
Most state health departments will also post warnings about specific waterways on their Web page. If you are uncertain, play it safe and eat only one meal a week of fish you catch and no other fish during that week.
"If you're out in the middle of the ocean you're in pretty good shape," Dr. Rose said. "In coastal waters you have to be more careful. But if it gets into lakes and streams and if it's near a mining facility or a chemical plant, you probably have to use even more caution."
The Skinny on Dental Fillings
In addition to eating fish, many of us are also exposed to mercury here at the dentist's office when we get a filling.
Amalgam fillings, also known as silver fillings, contain 50 percent mercury. But don't panic if you have them. The American Dental Association and the Food and Drug Administration say it's safe. So watch out if your dentist suggests replacing your silver fillings because of the mercury.
Dr. Edgar Boggs is a family dentist concerned about patients paying more than they should for dental services, especially during these tough economic times. He explained that while most dentists don't swindle their patients, consumers should beware of the few bad apples out there.
"I think the mercury fear or scare tactic is unreasonable and criminal," he said. "I mean, you go in and tell somebody they have to replace all their amalgams with composite fillings, I think it's criminal, I think it should be against the law."
Dr. Boggs said silver fillings only need to be replaced if they are broken or leaking. Of course, some patients elect to have silver fillings replaced by composite fillings, which are tooth-colored, purely for cosmetic reasons. But that is a decision that should be made for aesthetic reasons, not health reasons.
Vaccines, Mercury and Autism
Some vaccines contain mercury, but not nearly as many as in the past. Years ago, mercury was common as a preservative in vaccines. But it is largely gone now. Many parents believe mercury in vaccines causes autism. However Dr. Rose points out that most research refutes that.
"There's not been an association found in scientific studies," he said. "That doesn't deter people who think they've been poisoned by mercury and they're going to think that and it's probably prudent not to have mercury in those products if we can avoid it. But at the same time it's probably unfair to pin mercury as a concrete cause of illness."
So mercury is a poison that is part of our daily lives, such as in fish, dental fillings and vaccines. But it's important to separate fact from fear.
*Original broadcast March 17, 2010.