What are the dangers of mercury exposure?

Not all mercury exposures are equal. How the mercury enters a person's system can matter a lot.

Mercury vapor, whether from industrial exhaust or spills or a broken thermometer, can be extremely damaging to anyone. It's the most dangerous form because it's absorbed through the lungs, allowing much of it to reach the brain. It doesn't take long to experience side effects from inhaling mercury. Ingesting small amounts of mercury, on the other hand, can have little or no effect at all -- on adults.

That's how most people consume mercury: They eat it. Exposure is almost always via fish that have been contaminated with methylmercury -- a form of mercury produced in the bodies of tiny sea life that have ingested the elemental form, deposited in bodies of water when rain or snow carry it down from the atmosphere or across land in contaminated runoff.

As those tiny organisms are eaten by larger fish, and those fish are eaten by larger fish and so on, the methylmercury moves up the food chain. Eventually, it reaches seafood-eating humans. Most humans have at least some mercury in their bodies. But few experience so much exposure that they suffer from side effects.

Few adults, that is. Very young children and fetuses are much more susceptible to mercury poisoning than adults. It's easy enough to keep fish with high mercury content -- typically large predator fish with long life spans, like tile fish, swordfish and king mackerel -- out of the diets of 2-year-olds. The problem, more often, is during pregnancy, when a fetus' developing brain is highly vulnerable to even a miniscule amount of neurotoxin.

And it is something to be concerned about: As recently as 2000, the U.S. Environmental Protection Agency or EPA estimated that 8 percent of women of child-bearing age could have blood-mercury levels above a safe amount, determined to be 5.8 ug/L, or micrograms per liter [source: EPA]. And what's safe for an adult is not necessarily safe for a fetus -- or an infant, since mercury can also be passed on in breast milk.

Effects of mercury poisoning can include:

Luckily, there are ways to reduce the risk of harm. It just takes a little knowledge about where the greatest danger lies.

Mercury poisoning doesn't happen every day. As far as environmental dangers go, it's a fairly rare occurrence. Still, it does happen: a spill in a school science lab, a broken thermometer, a faulty old tooth filling, a leak at a factory.

By far, though, the most common route of exposure is through seafood, particularly the largest and oldest fish, which have spent the most time eating smaller fish that have been eating even smaller fish contaminated with methylmercury. The location where the fish is caught matters quite a bit, but in general, the fish to be most wary of include [source: FDA]:

You don't need to cut them out entirely, but careful consumption (no more than one serving per week) is smart.

Mid-range methylmercury fish, which should be limited to two or three servings per week, include:

And the fish and shellfish to eat freely include:

Of course, pregnancy is a game changer. Most pregnant women choose to avoid the highest-mercury fish altogether, and greatly limit their ingestion of the mid-range fish. But it's important that pregnant women do not eliminate all fish from their diet, because fish contains a lot of omega-3s, which are crucial to a developing baby.

For a complete list of seafood mercury contents, see FDA: Mercury Levels in Commercial Fish and Shellfish.

Other than avoiding the fishiest fish, you can also reduce the risk of mercury poisoning by never vacuuming up mercury from a broken thermometer or fluorescent bulb, asking your dentist about your fillings, avoiding religious rituals (typically voodoo or Santeria) involving mercury sold as azogue or botanicas, and staying away from the pretty mercury-filled glass pendants often sold in Mexico. Really, people -- carrying poison around on your body in a breakable container?

For more information on mercury poisoning and related topics, look over the links on the next page.