What are the risks associated with mercury exposure?

Thermometers provide warnings, pregnancy guidelines suggest ways to avoid it, and regulations are in place to limit its presence in the workplace.

Mercury, in its various forms, should not be handled carelessly.

Mercury, in its elemental form, is represented by the symbol Hg on the periodic table, atomic number 80. It is a silvery liquid at room temperature and occurs naturally. Some estimates suggest that natural sources, such as volcanic eruptions and erosion of rocks, release about 2,100 tons of mercury annually [source: UDEQ].

Although it's challenging to precisely quantify mercury emissions, most experts agree that human activities contribute more mercury to the atmosphere than natural processes, potentially releasing up to 2,900 tons annually [source: UDEQ]. This mercury can originate from mining, manufacturing for products (like thermometers), or energy-related sources such as coal-fired power plants.

Mercury is a neurotoxin, meaning it harms the nervous system. It can affect the brain and lead to both physical and emotional disorders. Whether natural or human-made, mercury presents a risk to humans. The severity of the problem depends on the mercury’s form, its concentration, and the individuals exposed to it.

Although mercury exposure and its compounds can be extremely harmful, it is not a widespread issue. In this article, we will explore where mercury is typically found, how exposure usually occurs, and the potential consequences. Additionally, we will discuss the precautions individuals can take to minimize their risk of mercury exposure.

The risks linked to mercury exposure differ significantly across the general population. One group faces far more severe effects than the others.

Mercury Exposure: Safety Risks

Not all mercury exposures have the same impact. The way mercury enters the body can significantly affect the severity of its effects.

Mercury vapor, whether from industrial emissions, spills, or a broken thermometer, can be incredibly harmful to anyone. This form is especially dangerous because it is inhaled through the lungs, allowing much of it to reach the brain. Side effects from inhaling mercury can occur quickly. In contrast, swallowing small amounts of mercury typically has little to no effect on adults.

This is how most people ingest mercury: through their diet. The primary source of exposure is fish contaminated with methylmercury — a mercury compound created in the bodies of small marine organisms that have consumed elemental mercury. This mercury is deposited into water bodies when rain or snow carries it from the atmosphere or through contaminated runoff from land.

As these tiny organisms are consumed by larger fish, and those fish are in turn eaten by even larger fish, methylmercury works its way up the food chain. Eventually, it reaches humans who eat seafood. Most humans carry some mercury in their bodies, but only a few experience enough exposure to face noticeable side effects.

Few adults are at risk, but young children and fetuses are far more vulnerable to mercury poisoning. It's easy to avoid fish with high mercury levels — typically large predatory fish with long life spans, like tilefish, swordfish, and king mackerel — in the diets of toddlers. The greater concern often arises during pregnancy, when the developing brain of a fetus is especially sensitive to even the smallest exposure to this neurotoxin.

This is a real concern: As recently as 2000, the U.S. Environmental Protection Agency or EPA estimated that 8 percent of women of childbearing age may have blood mercury levels exceeding the safe threshold, set at 5.8 micrograms per liter (ug/L) [source: EPA]. And what's considered safe for adults isn't necessarily safe for a fetus or an infant, as mercury can also transfer through breast milk.

Mercury poisoning can cause the following effects:

Fortunately, there are ways to lower the risk of harm. It just requires some understanding of where the greatest dangers lie.

How Can I Prevent Mercury Exposure?

Mercury poisoning isn't something that happens regularly. As far as environmental hazards go, it's relatively uncommon. But it can occur: a spill in a school science lab, a broken thermometer, a deteriorating tooth filling, or a leak at a factory.

However, the most common way people are exposed is through seafood, particularly the largest and oldest fish. These fish have consumed smaller fish that have ingested methylmercury-contaminated fish. While the location where the fish is caught is crucial, generally, the fish to watch out for include [source: FDA]:

You don't have to eliminate these from your diet entirely, but it’s wise to limit consumption (no more than one serving per week).

Fish with mid-range methylmercury levels, which should be limited to two or three servings per week, include:

Fish and shellfish that can be consumed more freely include:

Of course, pregnancy changes the rules. Most pregnant women choose to avoid the highest-mercury fish entirely, and significantly reduce their intake of mid-range fish. However, it's important for pregnant women not to cut all fish from their diet, as fish is a rich source of omega-3s, which are essential for a developing baby.

For a full list of mercury content in seafood, refer to FDA: Mercury Levels in Commercial Fish and Shellfish.

In addition to avoiding fish with high mercury levels, you can lower your risk of mercury exposure by refraining from vacuuming up mercury from a broken thermometer or fluorescent bulb, consulting your dentist about any fillings, steering clear of religious practices (typically voodoo or Santeria) that involve mercury sold as azogue or botanicas, and avoiding attractive mercury-filled glass pendants often sold in Mexico. Seriously, carrying poison on your body in a fragile container?