Mercury is a naturally occurring element, but modern industrial processes and environmental contamination have significantly increased human exposure, leading to serious health concerns. The biological impact of this heavy metal depends heavily on its chemical structure, which dictates how the body absorbs it, where it travels, and the damage it can cause. Understanding the different forms of mercury and their unique pathways into the body is essential for comprehending its overall toxicity.
The Different Forms of Mercury and Exposure Routes
Mercury exists in three primary chemical forms, each presenting a distinct hazard. Elemental mercury (metallic mercury) is liquid at room temperature and released as a colorless, odorless vapor. Inhaling this vapor, which can occur from a broken thermometer, industrial spill, or dental amalgam fillings, is the primary route, with approximately 80% absorption through the lungs. Once absorbed, this form easily crosses the blood-brain barrier due to its lipid solubility.
Inorganic mercury compounds are typically salts, formed when elemental mercury binds with other elements like chlorine or sulfur. Exposure occurs mainly through ingestion or direct skin contact, such as from contaminated consumer products or traditional medicines. These salts are poorly absorbed by the gastrointestinal tract, with only about 7% to 15% entering the bloodstream. This form is highly corrosive and primarily targets the gastrointestinal system and the kidneys.
The third form, organic mercury, is primarily methylmercury, and it is the most common exposure route for the general population. Methylmercury is created when microorganisms in water bodies convert inorganic mercury into this organic compound. It then bioaccumulates up the aquatic food chain, meaning larger, longer-lived predatory fish and shellfish contain the highest concentrations. When consumed, methylmercury is absorbed almost completely by the body, with rates around 95% from the gastrointestinal tract.
How Mercury Harms the Body
Once absorbed, mercury causes cellular damage by interfering with fundamental biological processes. Mercury has a high affinity for sulfhydryl groups, which are present in the amino acid cysteine and numerous proteins and enzymes throughout the body. By binding to these groups, mercury disrupts the three-dimensional structure of proteins, inactivating enzymes and crippling cellular functions. This cellular disruption leads to oxidative stress and cell death in vulnerable organs.
The central nervous system (CNS) is the primary target for both elemental mercury vapor and methylmercury because they readily cross the blood-brain barrier. Methylmercury is particularly damaging to the CNS because it forms a complex with the amino acid L-cysteine, allowing it to be actively transported into the brain. Once inside the brain, it damages neurons, especially in areas responsible for vision and motor coordination.
In contrast, inorganic mercury salts do not easily penetrate the blood-brain barrier due to their poor lipid solubility. Instead, they concentrate in the kidneys, which are the main site of accumulation and damage, leading to severe renal dysfunction. A developing fetus is highly susceptible to all forms of mercury, especially methylmercury, which crosses the placenta and interferes with the formation of the fetal brain and nervous system.
Recognizing Mercury Poisoning
The signs of mercury poisoning are diverse and depend on the chemical form and duration of exposure. Chronic, low-level exposure to methylmercury primarily manifests as neurological symptoms, including paresthesia (tingling or numbness) often beginning in the hands and feet. High-level exposure can lead to ataxia (lack of muscle coordination) and severe constriction of the visual field.
Exposure to elemental mercury vapor often produces a combination of physical and psychological effects. Neurological symptoms include tremors, which can start in the hands and spread, along with memory loss. Psychological changes are notable, including increased irritability, anxiety, and excessive shyness.
Acute exposure to inorganic mercury salts often causes immediate gastrointestinal distress, such as severe abdominal pain and vomiting. Since the kidneys accumulate this form, patients may also develop signs of kidney damage, including acute renal failure. Diagnosis involves a clinical history and laboratory testing, using blood and hair samples to assess organic mercury levels and urine samples to gauge elemental and inorganic mercury exposure.
Managing and Preventing Exposure
Preventing exposure is the most effective approach to managing mercury health risks. Public health guidelines recommend careful dietary choices to reduce methylmercury intake, particularly for pregnant women and young children. This involves limiting or avoiding consumption of large, long-lived predatory fish, which bioaccumulate high levels of the contaminant. Consumers should choose lower-mercury options, such as salmon, shrimp, and canned light tuna, to maintain the nutritional benefits of seafood.
Proper management of mercury-containing household items is another important preventive step. If an elemental mercury item, such as a thermometer or fluorescent bulb, breaks, the spill must be handled carefully to avoid inhaling the released vapor. Workplace safety protocols are also implemented in industries that use mercury to minimize occupational inhalation exposure.
For confirmed cases of significant mercury poisoning, medical management often involves chelation therapy. This treatment uses specific chemical agents, such as DMSA, designed to bind tightly to the mercury ions in the body. Once bound, the mercury-chelator complex becomes stable and water-soluble, allowing the body to safely excrete the heavy metal through the urine or feces. Chelation therapy is used to reduce the overall body burden of mercury and limit further organ damage.