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. 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.
• The health of the person exposed.

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

1. Methylmercury.
2. Elemental mercury.
3. 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 CDC shows that most people have blood mercury levels below a level associated with possible health effects. More recent data from the CDC support this general finding.

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, and it became clear that the developing nervous system of the fetus may be more vulnerable to methylmercury than the adult nervous system.

Symptoms of methylmercury poisoning

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

• Impairment of 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.
• 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 (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.
• Muscle weakness.

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

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