Mercury is contained in some of the fish we eat, whether caught in local lakes and streams or bought in a grocery store. Mercury is also contained in some of the products we use, which may be found in your home, at the dentist, and at schools. This article provides links to information about sources of mercury exposure, potential health effects, fish that may contain mercury, consumer products that contain mercury, and ways to reduce your exposure to mercury.

General information about mercury and mercury exposure

Forms of mercury. Mercury is a naturally occurring element that is found in air, water and soil. It exists in several forms:

• elemental or metallic mercury,
• inorganic mercury compounds, and

• organic mercury compounds.

Sources of mercury.

Mercury is an element in the earth's crust. Humans cannot create or destroy mercury. Pure mercury is a liquid metal, sometimes referred to as quicksilver that volatizes readily. It has traditionally been used to make products like thermometers, switches, and some light bulbs.

Exposure to mercury. Mercury in the air eventually settles into water or onto land where it can be washed into water. Once deposited, certain microorganisms can change it into methylmercury, a highly toxic form that builds up in fish, shellfish and animals that eat fish. Fish and shellfish are the main sources of methylmercury exposure to humans. Methylmercury builds up more in some types of fish and shellfish than others. The levels of methylmercury in fish and shellfish depend on what they eat, how long they live and how high they are in the food chain.

Check out the following chart on the next page:

EPA works with the U.S. Food and Drug Administration (FDA) and with states and tribes to issue advice to women who may become pregnant, pregnant women, nursing mothers, and parents of young children about how often they should eat certain types of commercially-caught fish and shellfish. Fish advisories are also issued for men, women, and children of all ages when appropriate. In addition, EPA releases an annual summary of information on locally-issued fish advisories and safe-eating guidelines to the public. Fish is a beneficial part of the diet, so EPA & FDA encourage people to continue to eat fish that are low in methylmercury.

Another less common exposure to mercury that can be a concern is breathing mercury vapor. These exposures can occur when elemental mercury or products that contain elemental mercury break and release mercury to the air, particularly in warm or poorly-ventilated indoor spaces.

Health effects of mercury. Mercury exposure at high levels can harm the brain, heart, kidneys, lungs, and immune system of people of all ages. Research shows that most people's fish consumption does not cause a health concern. However, it has been demonstrated that high levels of methylmercury in the bloodstream of unborn babies and young children may harm the developing nervous system, making the child less able to think and learn. More information

Ecological effects of mercury.

Birds and mammals that eat fish are more exposed to mercury than other animals in water ecosystems. Similarly, predators that eat fish-eating animals may be highly exposed. At high levels of exposure, methylmercury's harmful effects on these animals include death, reduced reproduction, slower growth and development, and abnormal behavior.

Reducing mercury releases. EPA issues regulations that require industry to reduce mercury releases to air and water and to properly treat and dispose of mercury wastes. EPA also works with industry to promote voluntary reductions in mercury use and releases, and with partners in state, local and tribal governments to improve their mercury reduction programs. EPA works with international organizations to prevent the release of mercury in other countries. The public can contribute to mercury reduction efforts by purchasing mercury-free products and correctly disposing of products that contain mercury by reducing demand for products whose production leads to the release of mercury into the environment.

People in the U.S. are mainly exposed to methylmercury, an organic compound, when they eat fish and shellfish that contain methylmercury. Whether an exposure to the various forms of mercury will harm a person's health depends on a number of factors (below). Almost all people have at least trace amounts of methylmercury in their tissues, reflecting methylmercury's widespread presence in the environment and people's exposure through the consumption of fish and shellfish. People may be exposed to mercury in any of its forms under different circumstances. The factors that determine how severe the health effects are from mercury exposure include these:

• the chemical form of mercury;
• the dose;
• the age of the person exposed (the fetus is the most susceptible);
• the duration of exposure;
• the route of exposure -- inhalation, ingestion, dermal contact, etc.; and
• the health of the person exposed.

Mercury exists in three chemical forms. They each have specific effects on human health.

1. Methylmercury

1. Elemental mercury
2. Other mercury compounds (inorganic and organic)

Methylmercury effects

For fetuses, infants, and children, the primary health effect of methylmercury is impaired neurological development. Methylmercury exposure in the womb, which can result from a mother's consumption of fish and shellfish that contain methylmercury, can adversely affect a baby's growing brain and nervous system. Impacts on cognitive thinking, memory, attention, language, and fine motor and visual spatial skills have been seen in children exposed to methylmercury in the womb. Recent human biological monitoring by the Centers for Disease Control and Prevention in 1999 and 2000 (PDF) shows that most people have blood mercury levels below a level associated with possible health effects. 

Outbreaks of methylmercury poisonings have made it clear that adults, children, and developing fetuses are at risk from ingestion exposure to methylmercury. During these poisoning outbreaks some mothers with no symptoms of nervous system damage gave birth to infants with severe disabilities, it became clear that the developing nervous system of the fetus may be more vulnerable to methylmercury than is the adult nervous system.

Symptoms of methylmercury poisoning

In addition to the subtle impairments noted above, symptoms of methylmercury poisoning may include;

• impairment of the peripheral vision;
• disturbances in sensations ("pins and needles" feelings, usually in the hands, feet, and around the mouth);
• lack of coordination of movements;
• impairment of speech, hearing, walking; and
• muscle weakness.

People concerned about their exposure to methylmercury should consult their physician.

Elemental mercury effects

Elemental (metallic) mercury primarily causes health effects when it is breathed as a vapor where it can be absorbed through the lungs. These exposures can occur when elemental mercury is spilled or products that contain elemental mercury break and expose mercury to the air, particularly in warm or poorly-ventilated indoor spaces.

Symptoms of elemental mercury effects

Symptoms include these:

• tremors;
• emotional changes (e.g., mood swings, irritability, nervousness, excessive shyness);
• insomnia;
• neuromuscular changes (such as weakness, muscle atrophy, twitching);
• headaches;
• disturbances in sensations;
• changes in nerve responses;
• performance deficits on tests of cognitive function.

At higher exposures there may be kidney effects, respiratory failure and death. People concerned about their exposure to elemental mercury should consult their physician.

Effects of other mercury compounds (inorganic and organic)

High exposures to inorganic mercury may result in damage to the gastrointestinal tract, the nervous system, and the kidneys. Both inorganic and organic mercury compounds are absorbed through the gastrointestinal tract and affect other systems via this route. However, organic mercury compounds are more readily absorbed via ingestion than inorganic mercury compounds.

Symptoms of high exposures to inorganic mercury

Symptoms of high exposures to inorganic mercury include:

• skin rashes and dermatitis;
• mood swings;
• memory loss;
• mental disturbances; and
• muscle weakness.

People concerned about their exposure to inorganic mercury should consult their physician.