The Impacts of Exposure to Mercury

Mercury has significant impacts on the human nervous system. Research in other countries has shown that some people who ate either fish contaminated with large amounts of methylmercury or seed grains treated with methylmercury developed permanent damage to the brain and kidneys. Different forms of mercury have different effects on the nervous system because they do not all move through the body in the same way.

When metallic mercury vapors are inhaled, they readily enter the bloodstream and are carried throughout the body and can move into the brain. Metallic mercury vapors or organic mercury may affect many different areas of the brain and their associated functions, resulting in a variety of symptoms. These include personality changes (irritability, shyness, nervousness), tremors, changes in vision (narrowing of the visual field), deafness, muscle incoordination, loss of sensation, and difficulties with memory.

Short-term exposure (hours) to high levels of metallic mercury vapor in the air can damage the lining of the mouth and irritate the lungs and airways, causing tightness of the chest, a burning sensation in the lungs, and coughing. Other effects from exposure to mercury vapor include nausea, vomiting, diarrhea, increases in blood pressure or heart rate, skin rashes, and eye irritation. Damage to the lining of the mouth and lungs can also occur from exposure to lower levels of mercury vapor over longer periods (for example, in some occupations where workers were exposed to mercury for many years).

The kidneys are also sensitive to the effects of mercury, because mercury accumulates in the kidneys and causes higher exposures to these tissues, and thus more damage. All forms of mercury can cause kidney damage if large enough amounts enter the body. If the damage caused by the mercury is not too great, the kidneys are likely to recover once the body clears itself of the contamination.

In addition to effects on the kidneys, inorganic mercury can damage the stomach and intestines, producing symptoms of nausea, diarrhea, or severe ulcers if swallowed in large amounts.

There is little information on the effects in humans from long-term, low-level exposure to inorganic mercury.

Potential Health Effects from Exposures to Methylmercury in Children (from conception to 18 years of age)

Exposure to methylmercury is more dangerous for young children than for adults, because methylmercury easily passes into the developing brain of young children and may interfere with the development process. As a result, methylmercury is the form of mercury most commonly associated with a risk for developmental effects. Exposure can come from foods contaminated with mercury on the surface (for example, from seed grain treated with methylmercury to kill fungus) or from foods that contain toxic levels of methylmercury (such as some fish, wild game, and marine mammals). Mothers who are exposed to methylmercury and breast-feed their infant may also expose their child.

The severity of the effects of methylmercury exposure depends on the level of mercury exposure and the length of exposure. In instances in which the exposure is great, the effects may involve delays in the age of first walking and talking, brain damage with mental retardation, incoordination, inability to move, eventual blindness, involuntary muscle contractions and seizures, muscle weakness, and inability to speak. These very severe effects were reported in large-scale poisoning instances in which pregnant and nursing women were exposed to extremely high levels of methylmercury in contaminated grain used to make bread (in Iraq) or in seafood sold to the general population (in Japan). Researchers are currently studying the potential for less serious developmental effects resulting from eating lower levels of methylmercury in foods, including effects on a child's behavior and ability to learn, think, and solve problems.

The main source of exposure to methylmercury for pregnant women and young children is from eating fish. Many fish purchased in the market in the United States do not have mercury levels that pose a risk to anyone, including pregnant women. However, tuna, shark, and swordfish are presently the subject of consumption advisories for mercury. Little comprehensive testing of commercially sold fish is done at this time.

Since mercury accumulates in the muscles of fish, larger fish that feed on smaller fish and live for long periods usually have larger concentrations of methylmercury than fish that feed on plants. For example, shark and swordfish normally contain the highest levels of mercury out of all ocean fish. The FDA regulates the safety of most fish sold commercially in the United States. Only fish or wildlife containing relatively high levels of methylmercury are of concern.

Scientists have an ongoing debate about the value of fish in the diet versus any risk from increased exposure of pregnant women to methylmercury that may be in the fish.

For more information on the health impacts of exposure to mercury, go to:

The Agency for Toxic Substances and Disease Registry (ATSDR) website at http://www.atsdr.cdc.gov/toxprofiles/tp46.html

The United State Environmental Protection Agency website at: http://www.epa.gov/mercury/index.html