Mercury is a naturally occurring metal that exists in various forms, posing a health risk primarily as an environmental toxin. The most common form humans encounter is methylmercury, an organic compound that accumulates in the food chain. Testing the concentration of mercury in the blood provides a snapshot of recent exposure, reflecting the amount absorbed into the bloodstream, mainly through diet. Health professionals use this blood test to monitor population exposure and assess individual risk.
Understanding Mercury Measurement in Blood
Measuring mercury in the blood helps differentiate between recent or high-level exposure and the low background levels found in most people. The standard unit of measurement used by laboratories is micrograms per liter (\(\mu\)g/L). Total mercury is typically measured in whole blood, with the majority of this total concentration representing methylmercury in individuals exposed through fish consumption.
The average concentration of total blood mercury in the general United States adult population is quite low, typically estimated to be less than 1 \(\mu\)g/L. Blood mercury levels below 5 \(\mu\)g/L are generally considered to reflect non-hazardous, background exposure in people without unusual dietary or occupational habits.
Defining Elevated and Toxic Thresholds
A blood mercury level is defined as elevated when it exceeds the concentration established by agencies as a threshold for potential concern. The U.S. Environmental Protection Agency (EPA) established a reference dose for methylmercury exposure, which translates to a blood mercury concentration of 5.8 \(\mu\)g/L. Levels above this guidance value suggest a need to identify and reduce the source of exposure, especially for sensitive populations like pregnant women and young children.
This level of 5.8 \(\mu\)g/L is not a diagnostic threshold for poisoning but indicates a concentration where long-term intake may exceed safe limits based on population studies. For individuals with no symptoms, blood mercury levels up to 10 \(\mu\)g/L are sometimes considered the upper end of the normal range, depending on dietary intake. Concentrations significantly higher than 10 \(\mu\)g/L indicate a high exposure that requires attention from a health professional.
A blood mercury level is considered toxic when it exceeds concentrations clearly linked to observable health effects. Concentrations above 50 \(\mu\)g/L are often cited as a threshold where neurological damage is possible, particularly from long-term exposure to the organic form of mercury. In cases of severe, acute poisoning, blood mercury levels can reach thousands of \(\mu\)g/L, requiring immediate medical intervention.
Health Consequences of High Mercury Levels
Sustained high levels of methylmercury target the nervous system, which is especially vulnerable in developing fetuses and young children. Exposure during pregnancy can result in developmental issues, affecting cognitive thinking, memory, and fine motor skills. The developing brain is more susceptible to damage than the adult brain.
In adults, chronic high exposure leads to a range of neurological symptoms. Initial symptoms often include paresthesia (numbness or “pins and needles”) in the hands, feet, or around the mouth. As the concentration increases, individuals may experience ataxia (lack of coordination or trouble walking), tremors, and changes in vision or hearing.
The kidneys are also a primary target organ for mercury toxicity, particularly from the inorganic and elemental forms. High levels can cause damage ranging from increased protein in the urine to kidney failure. Other long-term effects of mercury exposure include memory loss, mood swings, and general muscle weakness.
Primary Sources of Mercury Exposure
For the general population, the primary source of high mercury exposure is the consumption of fish and shellfish contaminated with methylmercury. Methylmercury bioaccumulates in aquatic food chains, meaning that larger, longer-lived predatory fish tend to have the highest concentrations. Common dietary sources that can lead to elevated blood mercury levels include:
- Shark
- Swordfish
- King mackerel
- Large tuna
A second significant source of exposure is occupational or environmental contact with elemental or inorganic mercury. This exposure occurs in industrial settings, such as mining or the chlor-alkali industry. Individuals handling dental amalgam may also have higher exposure risks, as can those exposed to elemental mercury vapor from spills of mercury-containing devices.