Mercury is a naturally occurring element found in the earth’s crust. It exists in several forms, each with different properties and exposure risks. The most recognizable form is elemental mercury, a silver-white liquid metal historically used in thermometers and some electrical switches. Other forms include inorganic mercury salts found in soil and water, and organic mercury compounds like methylmercury, which is the form most people encounter through their diet.
Sources of Mercury Exposure
The primary way people are exposed to mercury is by consuming fish and shellfish containing methylmercury. This organic form is created by microorganisms in water and soil. It accumulates in aquatic life and becomes magnified up the food chain in a process called bioaccumulation. As smaller fish are eaten by larger ones, the concentration of methylmercury increases.
Consequently, large predatory fish contain higher levels of mercury. These include:
- Shark
- Swordfish
- King mackerel
- Certain tuna species
Conversely, smaller, non-predatory fish and shellfish like shrimp, salmon, and catfish have lower levels, offering safer alternatives for seafood consumption.
Inhalation of mercury vapor is another route of exposure, particularly for elemental mercury. When products containing it, such as older thermometers or fluorescent light bulbs, break, the liquid metal can evaporate into an invisible, odorless toxic gas. Industrial activities like coal-fired power plants and waste incineration also release mercury into the atmosphere. These particles can travel long distances before settling back onto land and water.
Direct skin contact presents a less common pathway for mercury to enter the body. This involves inorganic mercury compounds found in some unregulated consumer products, such as certain skin-lightening creams and antiseptic products containing mercury salts. When these products are applied, the mercury can be absorbed through the skin and enter the bloodstream.
The Path of Mercury in the Body
Once mercury enters the body, its journey and impact depend on its chemical form. Methylmercury is almost entirely absorbed through the gastrointestinal tract. From there, it enters the bloodstream and is distributed throughout the body, easily crossing sensitive biological barriers.
Elemental mercury is readily absorbed by the lungs and rapidly passes into the bloodstream. Once in the blood, both elemental mercury and methylmercury bind to red blood cells and plasma proteins for transport. The body distributes mercury to various organs, but it has a strong affinity for specific tissues where it accumulates over time.
The kidneys are a primary site of accumulation for inorganic mercury, which can form after the body metabolizes other forms of the element. The primary destination for methylmercury is the central nervous system.
A defining characteristic of methylmercury is its ability to penetrate the blood-brain barrier, a protective membrane that shields the brain. This allows it to accumulate directly in brain tissue. It can also cross the placental barrier in pregnant individuals, exposing a developing fetus to the toxin.
Health Consequences of Mercury Toxicity
The harm from mercury begins at the cellular level. Mercury has a high affinity for sulfur-containing molecules, and it binds to sulfhydryl groups within proteins and enzymes. This binding action changes the structure and function of these molecules, disrupting cellular processes like energy production and DNA repair. This interference generates oxidative stress, a state of chemical imbalance that leads to cell damage and death.
Because methylmercury accumulates in the brain, the most pronounced health effects are neurological. In adults, symptoms of mercury toxicity often include tremors, particularly in the hands, and a loss of coordination that can affect balance and movement. Cognitive functions can also be impaired, leading to memory loss, difficulty concentrating, and irritability. Another common sign is peripheral neuropathy, which manifests as a tingling sensation in the hands, feet, and around the mouth.
Beyond the nervous system, the kidneys are highly susceptible to damage from inorganic mercury. The accumulation can lead to increased protein in the urine and, in cases of severe or prolonged exposure, can result in kidney failure.
The effects of mercury are especially damaging to developing nervous systems. When a fetus is exposed to methylmercury in the womb, the consequences can be permanent. The toxin can disrupt the migration and organization of neurons in the developing brain. This can result in a range of outcomes, including developmental delays, reduced IQ, and, in severe cases, symptoms resembling cerebral palsy.
Diagnosis and Medical Intervention
Diagnosing mercury toxicity involves measuring the amount of the metal in the body. Physicians use different tests depending on the suspected form and timing of exposure. Blood tests are effective for detecting recent or ongoing exposure to methylmercury, reflecting the amount currently circulating in the bloodstream. For long-term exposure, hair analysis is often used, as mercury incorporates into growing hair shafts, providing a timeline of exposure over months.
Urine tests are the preferred method for assessing exposure to elemental and inorganic mercury. The kidneys filter these forms from the blood, and their concentration in urine can indicate the total body burden, especially from occupational or industrial exposure.
The first step in any medical intervention is to identify and eliminate the source of the mercury exposure. This could mean changing dietary habits to avoid high-mercury fish, removing a person from a contaminated work environment, or safely clearing a home of spilled elemental mercury.
For cases of severe, acute poisoning, a treatment known as chelation therapy may be administered. This procedure involves giving a patient specific chemical agents that bind tightly to heavy metals in the bloodstream. This newly formed compound is water-soluble, allowing the kidneys to filter it from the blood and excrete it in the urine. Chelation is a serious medical procedure with potential side effects and is reserved for patients with clear evidence of toxicity, always under strict medical supervision.