Mercury is a naturally occurring element and a potential toxin. When it enters the human body, it does not remain there indefinitely. The process, speed, and method of elimination depend entirely on the specific chemical form of the mercury involved. Mercury’s chemical structure dictates its path of absorption, its distribution within the body’s tissues, and the biological mechanism used to clear it. The body’s ability to process and excrete this element is highly varied, ranging from rapid clearance to retention that can last for years.
The Different Forms of Mercury
Mercury exists in three primary forms that pose a risk to human health, and the route of exposure is distinct for each one.
Elemental mercury, also known as metallic mercury, is the silver liquid found in older thermometers and certain dental amalgam fillings. Exposure occurs mainly through the inhalation of its invisible vapor, which is readily absorbed through the lungs at a rate of approximately 80%. If elemental mercury is accidentally swallowed, its absorption through the digestive tract is very low, often less than 0.01%.
Inorganic mercury refers to mercury compounds, such as mercury salts, that are often water-soluble. This form can enter the body from industrial exposures or be created internally as the body metabolizes other forms of mercury. When ingested, the absorption rate of inorganic mercury salts in the gastrointestinal tract is relatively poor, typically ranging between 7% and 15%. Once absorbed, this form tends to accumulate primarily in the kidneys.
The third major form, methylmercury, is an organic compound and is the most common exposure route for the general population. This form is created when microorganisms in water and soil convert elemental and inorganic mercury, causing it to bioaccumulate up the aquatic food chain. Humans are exposed almost exclusively through the consumption of fish and seafood. Methylmercury is absorbed extremely efficiently through the digestive tract, with absorption rates approaching 90% to 100%.
Biological Clearance Mechanisms
The body has developed specific physiological processes to handle the elimination of mercury, with the pathway directly linked to the chemical form. For methylmercury, the process begins in the liver, the primary site of its metabolism. Here, methylmercury is conjugated with the antioxidant glutathione, and this complex is then secreted into the bile. This biliary excretion is the main initial route of elimination for organic mercury.
The process is complicated because a significant portion of the methylmercury excreted into the intestine via bile is reabsorbed back into the bloodstream through enterohepatic circulation. To be truly eliminated, methylmercury must first be slowly broken down into the less toxic, inorganic form by intestinal bacteria. Once demethylated, the resulting inorganic mercury is then excreted in the feces.
Inorganic mercury, whether absorbed directly or converted from other forms, utilizes different clearance routes. This form is eliminated primarily through two major pathways: the urine and the feces. The kidneys are a major site of inorganic mercury accumulation and play a central role in filtering the toxin for urinary excretion. Small amounts of inorganic mercury can also be cleared from the body through sweat, saliva, hair, and nails.
Methylmercury can easily cross the blood-brain barrier and the placental barrier because it forms a complex that the body recognizes as an essential amino acid. In contrast, inorganic mercury salts are not lipid-soluble and do not readily cross these barriers. All forms of mercury, however, eventually accumulate in the brain as inorganic mercury after transformation, where its retention time is significantly longer than in other tissues.
The Time Factor: Biological Half-Life and Retention
The duration mercury remains in the body is quantified by its biological half-life, which is the time required for the amount of a substance in the body to decrease by half. The half-life is highly dependent on the specific form of mercury and the tissue being measured. Methylmercury has a relatively long half-life in the bloodstream, estimated to be around 80 days. This means that after a single exposure, it takes approximately 80 days for the concentration of methylmercury in the blood to drop by 50%.
The half-life of inorganic mercury in the body’s general circulation is typically shorter, often estimated to be in the range of 30 to 90 days following exposure to elemental mercury vapor. For inorganic mercury salts, the half-life is often cited as about 60 days. However, these systemic half-lives do not reflect the retention time in all organs, especially the brain.
Once mercury, regardless of its original form, is converted to the inorganic state and sequestered in the brain, its clearance dramatically slows. The biological half-life of inorganic mercury in the human brain is not measured in days but in years, with estimates ranging from several years to potentially decades. This extensive retention time is due to the strong binding of inorganic mercury to certain proteins.
The overall time it takes for mercury to leave the body is influenced by several individual factors. Continuous or chronic low-level exposure prevents the body from achieving complete clearance, leading to a higher total body burden. The efficiency of the liver in processing methylmercury and the function of the kidneys in filtering inorganic mercury are important variables that affect the individual rate of elimination. Significant variability in half-life estimates exists across the population, highlighting person-to-person differences in clearance ability.