Toxic Effects of Lead and Mercury

James Roberts, MD, MPH

Disclosures

December 31, 2009

Mercury

Mercury (Hg) is another heavy metal that is highly toxic to humans -- especially to the central and peripheral nervous systems. One of the primary sources of human exposure to mercury is the consumption of seafood, which is of c particular concern to pregnant women because methyl mercury crosses the placenta and makes its way into the fetal brain.

Examples of Mercury Poisoning

A well-known epidemic of mercury poisoning affected thousands of people in Minamata, Japan, beginning in the 1950s. It was caused by ingestion of fish and shellfish contaminated by mercury from industrial wastewater. The aftermath of this disaster continues to be studied.[26]

In 1972, hundreds of people in Iraq died and thousands were poisoned from eating bread that was mistakenly baked from flour and seed treated with a methyl mercury fungicide.[27,28]

Paint is another potential source of mercury poisoning, because organic mercury compounds are sometimes added to latex paint as a fungicide. In 1989, a 4-year-old boy in Michigan developed mercury poisoning after being exposed to paint fumes when the interior of his home was painted.[29] The EPA banned the use of mercury in indoor paint in 1991, and exterior paints that contain mercury must be labeled.[30]

Forms of Mercury

Mercury exists in several forms. First is the volatile elemental form, which is liquid at room temperature. A second form is inorganic compounds, such as mercury salts.[31] Third is organic compounds, such as methyl mercury, ethyl mercury, and phenyl mercuric acetate. The organic forms of mercury are the most hazardous to humans because they are the most readily absorbed by the body and have the greatest uptake into the central nervous system. Seafood consumption is the primary source of exposure to methyl mercury.[27,30]

Mercury Cycle in the Environment

Mercury is naturally circulated and released from the earth's crust, particularly during volcanic emissions. Atmospheric concentrations from this release represent an important pathway in its global transportation. After mercury diffuses into the atmosphere, it returns to earth as rainfall and is deposited into lakes and other bodies of water. Mercury becomes methylated when ingested by small fish and shellfish. The level of mercury increases as larger species eat the smaller ones, reaching its highest levels in predatory fish.[32]

Man-Made Sources of Mercury

Although mercury is a natural part of the environment, human activities greatly increase the amount to which people are exposed. For example, mining and the burning of fossil fuels, such as coal, release mercury into the atmosphere. In addition, acidification of lakes from acid rain increases the level of methyl mercury that accumulates in fish. Mercury is also a component or byproduct of numerous industries, including chlor-alkali plants, paper mills, plastic manufacturing plants, and other chemical plants.[28,30]

Elemental mercury is used in thermometers, sphygmomanometers, thermostats, and compact fluorescent light bulbs.[33] Efforts have been taken to remove mercury-containing devices from doctor's offices and hospitals, most of which now use digital thermometers and blood pressure monitors. Mercury is used in some religious practices, which usually involve carrying the mercury in a sealed pouch or sprinkling it throughout the home.[34] Mercury is also present in dental amalgams, but studies have found no adverse effects from this use.[35]

Mercury was once a component of numerous topical medications that are no longer available in the United States. One continuing use of organic mercury is ethyl mercury (thimerosal), which is sometimes used as a preservative in vaccine manufacturing. Thimerosal has been removed from nearly all vaccines (the flu vaccine is the one exception) due to a public perception of risk, although the small amounts used in vaccines have never been shown to be harmful. Epidemiologic data have failed to support a relationship between thimerosal and health problems, including autism.[36,37,38]

Clinical Manifestations of Mercury Toxicity

Mercury can cause devastating neurologic effects, especially in the fetus and newborn. Symptoms of mercury poisoning vary depending on whether the exposure has been acute or chronic, and on the form of mercury.

Elemental mercury. Inhaled elemental mercury vapor produces a corrosive bronchitis and interstitial pneumonitis. In addition, some central nervous system effects, such as excitability, tunnel vision, and tremor, may occur.

Chronic vapor exposure primarily affects the central nervous system. Symptoms may include depression, weight loss, muscle weakness, and severe muscle wasting with decreased deep tendon reflexes.[39] Hemorrhagic colitis has also been reported following elemental mercury exposure.[40]

Other possible symptoms included tremors of fine motor muscles, with eventual progression to generalized tremors or spasms in other muscle groups, personality and behavioral changes, memory loss, and delirium. Thyroid dysfunction and thyroid enlargement also have been reported.

Many of the signs described above were first recognized in workers in the felt hat industry, which used mercurous nitrate to cure the felt.[30]

Subcutaneous exposure following accidental trauma will cause localized pain, redness, and swelling. If the mercury is identified and removed, chelation is usually not needed.[41,42]

Inorganic mercury. Inorganic mercury compounds have been used in skin-lightening creams, antiseptics, antibacterials, diuretics, "calomel" teething powders, and cathartics. Although these are no longer available in the United States, they continue to be used in other countries.

Ingestion of these compounds leads to a grayish discoloration of the mucous membranes, nausea, vomiting, and diarrhea. Teething powders have led to acrodynia, in which the extremities become red, swollen, and painful.[43]

Ingestion of inorganic mercury also can cause erythema of the palms and soles, irritability, edema, hyperkeratosis, vasodilatation, fever, and splenomegaly.[24,32,37] The patient may also experience tachycardia; hypertension; and in adults, ataxia, visual field defects, dysarthria, and paresthesias.

Organic mercury. Methyl mercury is the most important source of environmental exposure. It is a neurotoxin and effects include paresthesia, difficulty concentrating, ataxia, dysphagia, muscle weakness, fatigue, vision and hearing loss, tremors, athetoid movements, paralysis, seizure, coma, and death. Excessive exposure during pregnancy is known to have potentially devastating consequences, including cerebral palsy in the newborn.[27,28,44]

Treatment

Treatment of mercury poisoning is focused on reducing exposure. Persons at high risk are urged to reduce their intake of seafood that contains high levels of mercury (see below).

If liquid mercury should spill, such as from a broken thermometer, the mercury should be scooped up and discarded in an appropriate hazardous waste collection site. It is important that mercury not be poured down a drain, where it can contaminate the water supply, or vacuumed, because heat from the vacuum will volatilize the mercury.

Chelation therapy with DMSA and other agents has been used in rare cases to treat mercury poisoning.[38.39,45,46] Although a reference range exists for mercury in blood and urine, no level has been established at which chelation should be initiated.

Guidelines for Fish Consumption

The most severe effects of methyl mercury are seen at birth or shortly thereafter. Both the Food and Drug Administration and the EPA have published consumer advisories about fish consumption among the 4 groups considered most susceptible: women of childbearing age, pregnant women, nursing mothers, and young children.

Fish to avoid include shark, swordfish, king mackerel, or tilefish, all of which are high in mercury. Fish that are low in mercury, such as shrimp, canned light tuna, salmon, pollock, and catfish, are encouraged because fish and shellfish are rich in protein and omega-3 fatty acids. People in high-risk groups should eat no more than 6 ounces a week of albacore ("white") tuna, which contains more mercury than light tuna. Finally, the public is encouraged to check for local advisories regarding consumption of fish caught in nearby waters.[47]

Summary

Mercury is a neurotoxic agent that is particularly harmful to the developing nervous system. Prevention of excessive intake to the baby -- by reducing consumption of high-mercury fish before, during, and after pregnancy -- is the cornerstone of care. Reducing the use of medical equipment that contains mercury also can aid in prevention.

For a general reference for these and other pediatric environmental health issues, the reader may also find additional information in the Handbook of Pediatric Environmental Health, published by the American Academy of Pediatrics.

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