Mercury poisoning happens when your body absorbs too much mercury, most commonly through eating contaminated seafood, breathing in mercury vapor, or absorbing it through the skin from certain products. Mercury exists in three forms, each with different exposure routes: elemental (liquid metal), inorganic (mercury salts), and organic (methylmercury and ethylmercury). The form you’re exposed to and how it enters your body determine how dangerous it is.
Eating Contaminated Fish and Shellfish
The most common way people get mercury poisoning is by eating fish that contain methylmercury, the organic form of mercury and the most toxic to humans through dietary exposure. Mercury released into the environment, whether from industrial emissions, coal burning, or mining runoff, settles into oceans, lakes, and rivers. Bacteria in aquatic sediment convert it into methylmercury, which is then absorbed by tiny organisms at the bottom of the food chain.
From there, small fish eat those organisms, and larger fish eat the small fish. At each step, mercury concentrations increase. By the time you reach top predator fish like shark, swordfish, king mackerel, and tilefish, methylmercury levels can be more than one million times higher than the levels in the surrounding water. This process, called bioaccumulation, is why the biggest, longest-lived predatory fish carry the highest risk. Eating these species regularly, especially in large portions, can push your mercury intake well above safe levels.
The EPA’s reference dose for methylmercury is set at 0.1 micrograms per kilogram of body weight per day, based on the risk of developmental brain damage in children. That threshold is low enough that even moderate consumption of high-mercury fish a few times a week can exceed it, particularly for pregnant women and young children.
Breathing Mercury Vapor
Elemental mercury, the silvery liquid you may have seen in old thermometers, is relatively harmless if swallowed because your gut absorbs very little of it. But when it evaporates into a gas, it becomes dangerous. Mercury vapor passes through the lungs, enters the bloodstream quickly through the thin membranes in your airways, and crosses into the brain and kidneys. Once there, it can be converted into a form that stays trapped in tissue for years.
This route of exposure is a serious concern in certain workplaces. Small-scale gold mining operations, which use liquid mercury to bind with gold particles and then burn off the mercury to recover the gold, are the single largest source of mercury air pollution globally. Miners who inhale these fumes regularly develop kidney and lung damage, along with progressive nervous system problems.
Outside of mining, people who work in industries that process mercury, manufacture certain electronics, or handle mercury-containing equipment face similar risks from repeated low-level vapor exposure. Even dentists and dental assistants can carry slightly elevated blood mercury levels (up to 15 ng/mL, compared to the general population’s normal level below 10 ng/mL) from working with dental amalgam, which is roughly 50% mercury by weight.
Broken Thermometers and Household Spills
A broken mercury thermometer is the most common household exposure scenario. The small silver beads that spill out release vapor at room temperature, and the risk increases in warm or poorly ventilated spaces. The amount of mercury in a single thermometer is small, but improper cleanup can make the problem worse.
If you break a mercury thermometer, the EPA recommends getting everyone, including pets, out of the room immediately. Open windows to the outside and close doors to the rest of the house. Never vacuum mercury, because a vacuum breaks the beads into smaller droplets and launches vapor into the air. Never sweep it with a broom for the same reason. Instead, use stiff cardboard or a squeegee to gather the beads, then pick up smaller droplets with an eyedropper or sticky tape like duct tape. A flashlight held at a low angle in a darkened room will reveal any remaining beads as tiny reflective spots. Seal everything in zip-lock bags and contact your local health department for disposal instructions.
If mercury spills on carpet, upholstery, or curtains, those materials need to be cut out and discarded. Mercury can settle into fibers and continue releasing vapor for a long time.
Skin-Lightening Creams and Cosmetics
Mercury is still used as an active ingredient in some skin-lightening and anti-aging creams, particularly products manufactured outside the United States or sold through informal markets online. It may appear on labels as “mercurous chloride,” “calomel,” “mercuric,” or “mercurio.” Many contaminated products don’t list mercury at all.
Mercury absorbs through the skin and can build up in the body with repeated use. The FDA has linked these products to a range of symptoms: tremors, numbness and tingling in the hands and feet, irritability, memory problems, vision and hearing changes, and depression. Because these creams are often used daily for weeks or months, exposure accumulates gradually. People sometimes don’t connect their symptoms to the product they’ve been applying. Babies and young children in the household can also be exposed by touching treated skin or surfaces, and their developing brains are especially vulnerable.
Dental Amalgam Fillings
Silver dental fillings contain about 50% mercury, and they do release small amounts of mercury vapor over time. Activities like chewing, grinding your teeth, and even drinking hot liquids can increase vapor release slightly. People with several amalgam fillings tend to have somewhat higher mercury levels in their blood and urine than people without them.
That said, studies on people with amalgam fillings have not shown conclusive evidence of harmful health effects in the general population. The levels typically remain within what’s considered safe. The FDA does, however, recommend that certain higher-risk groups, including pregnant women, nursing mothers, children under six, and people with kidney problems, avoid amalgam fillings when possible.
Thimerosal in Vaccines
Thimerosal, a preservative used in some vaccines, contains ethylmercury, which is chemically distinct from the methylmercury that accumulates from fish. The difference matters. Ethylmercury has a half-life in the blood of just 3 to 7 days and is efficiently excreted through the stool. It does not accumulate in the body. Blood levels return to baseline within about 30 days of vaccination. In studies comparing the two forms, brain concentrations of mercury were three times lower with thimerosal exposure than with equivalent methylmercury exposure. Safety guidelines developed for methylmercury do not apply to the trace amounts of ethylmercury in vaccines.
How Mercury Poisoning Is Detected
Mercury poisoning is diagnosed through blood or urine testing. Normal whole blood mercury falls below 10 ng/mL. Levels above 50 ng/mL indicate significant exposure to organic mercury (the kind from fish), while levels above 200 ng/mL indicate significant exposure to inorganic mercury. These thresholds apply to all ages.
Symptoms of mercury poisoning develop gradually with chronic exposure and can be easy to mistake for other conditions. The hallmarks include tremors (often starting in the hands), numbness or tingling around the mouth, hands, and feet, memory and concentration problems, mood changes like irritability or depression, and changes in vision or hearing. In severe cases, particularly with high methylmercury exposure, coordination problems, muscle weakness, and cognitive decline can become permanent. Children exposed in the womb or during early development face the highest risk of lasting neurological damage, even at exposure levels that might not produce obvious symptoms in adults.