Is Mercury in Food Bad for You? Exposure Effects and Toxic Limits

Traces of mercury can be found in everyday products. Older thermometers, batteries, and even skin care products may contain mercury in small amounts. This is also true for some of the foods that we eat. 

Fish and other seafood are known to have the highest levels of mercury when it comes to food. But only specific types of fish pose a possible health risk. Most fish contain very low levels of mercury. Pregnant people and children need to be careful about their mercury exposure. 

In this article, we will review why mercury can be harmful, how mercury poisoning can happen, and what you need to know to lower your risk of exposure.

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What is mercury?

Mercury is a naturally occurring element. Like carbon, oxide, and hydrogen, it’s one of the elements that’s part of the earth’s makeup. Mercury exists in three forms.

Elemental (metallic) mercury

This is the type of mercury most people picture when they think of this element. Elemental mercury is a silver liquid that used to be part of thermometers and is still used in manufacturing processes. Most people aren’t exposed to high levels of this type of mercury on a regular basis, unless they work with it as part of their job. But this is also the type of mercury that naturally exists on the Earth’s surface, so it’s found in coal and fossil fuels. When coal fossil fuels are burned, mercury is released into the atmosphere. So everyone is exposed to a small amount of elemental mercury. 

Inorganic mercury compounds

Inorganic mercury compounds form when mercury binds to other elements. They can occur naturally and be released into the air during mining and the burning of fossil fuels. They can also be human-made. Inorganic mercury compounds are used in industrial processes. Again, most people aren’t exposed to this type of mercury on a regular basis, unless they work in metal or chemical processing. But inorganic mercury compounds are found in some skin-lightening creams and in certain remedies used in traditional forms of medicine. When people use these items, they get exposed to inorganic mercury. 

Organic mercury (methylmercury) compounds

Organic mercury compounds form when mercury attaches to carbon. This is the type of mercury that can be found in food. Mercury that’s released into the air by burning fossil fuels eventually falls back to the earth’s surface via rain and snow. Once deposited on land, the mercury can combine with carbon through a complicated process that scientists still don’t fully understand. From there, it gets absorbed by plants and then eaten by animals (and eventually by humans). 

Most of the concern around mercury’s effects on human health is related to organic mercury compounds. This is the type of mercury anyone can be exposed to, since it’s the type found in food.

What does mercury do to the body?

Mercury can be toxic to humans. When someone is suddenly exposed to high levels of mercury, particularly, it can be toxic to the body in many different ways. It can lead to adverse health effects like:

• Severe lung damage that can affect their ability to breathe

• Severe vomiting and diarrhea

• Skin rashes and inflammation of the eyes

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But when someone is exposed to lower levels of mercury over a long period of time, it tends to affect the body a little differently. This includes:

• Nervous system damage, which can lead to tremors, insomnia, memory loss, and mood and personality changes

• Kidney damage

The good news is that most people do not develop mercury toxicity from their food. The level of mercury in food isn’t very high. And people usually don’t eat enough of these high-mercury foods to lead to health issues. Instead, mercury toxicity more commonly happens when someone is exposed to some sort of mercury spill or leakage in their workplace, rather than from food. 

But there are two groups of people who do need to be careful about how much mercury they’re exposed to from their food: young children and anyone who is pregnant. 

Mercury — in any form — can impact the developing brain, even in smaller amounts. This can lead to many different health problems later on in childhood, like:

• Changes in vision

• Blindness

• Seizures

• Tingling of the fingers and toes

• Numbness around the mouth

• Irritability

• Memory problems

• Hearing problems

Newborns can also be exposed to mercury through breastmilk. Young infants who are exposed may develop problems with their nervous system, digestive system, kidneys, and liver. 

So, it’s important to limit mercury exposure during: 

• Pregnancy

• Breastfeeding

• Infancy

• Early childhood

How much mercury is toxic?

It’s not clear. 

When it comes to mercury toxicity in adults, blood levels above 100 ng/mL have been associated with clear signs of mercury poisoning. But children can develop mercury-related injury at lower levels. Some scientists say that levels between 30 ng/mL and 40 ng/mL can lead to delayed development in children. 

But that doesn’t mean that levels lower than 30 ng/mL are safe. It’s not yet clear if any mercury level is safe. But the Environmental Protection Agency (EPA) states that people’s blood mercury levels should be below 5.8 ng/mL.

Foods high in mercury

Some foods are higher in mercury than others. Fish have the highest levels of organic mercury. This has to do with the fact that inorganic mercury can be transformed to organic mercury in water. 

Fish that contain the highest levels of mercury include:

• King mackerel

• Marlin

• Orange roughy

• Shark

• Swordfish

• Tilefish (caught in the Gulf of Mexico)

• Tuna, albacore

• Bass

• Cod

The fish that contain the most mercury are shark, swordfish, tilefish, and king mackerel. 

Fish and shellfish that are lower in mercury include:

• Salmon

• Trout

• Tilapia

• Sole

• Sardines

• Shrimp

• Oysters

Can you develop mercury toxicity from food?

All of this may have you wondering if you should avoid eating fish. Rest assured, the answer is no. There are very few instances of mercury poisoning from eating contaminated fish, and these are related to chemical spills like:

• Large amounts of mercury were spilled into the sea by a chemical company in a small town in Japan called Minamata. The residents there developed mercury poisoning after eating the fish from the bay.

• A similar chemical spill occurred along the Agano River in Japan and led to another Minamata disease epidemic in the 1960s.

• Later in the 1970s, a large outbreak of mercury poisoning happened in Iraq after people ate contaminated grain. The grain was never intended for eating. It was treated with mercury to prevent fungus from growing, and it was meant to be used by farmers. 

These isolated episodes were the result of people eating high amounts of mercury due to contamination. They would not happen from eating fish normally.

In fact, the American Heart Association recommends eating 2 servings of heart-healthy fish per week for adults who aren’t planning to become pregnant, are pregnant, or are breastfeeding. Most fish and shellfish contain trace amounts of methylmercury. And if eaten in moderation, they pose little to no health risk. 

Other sources of mercury exposure

Toxic mercury exposures typically happen through the environment. Most exposures to mercury happen when a product that contains mercury breaks. If a mercury spill or leakage is not immediately contained, it can evaporate and turn into an odorless, toxic gas. In this case, it may be inhaled. Other times, mercury may be accidentally swallowed.

Examples of possible sources of exposure include:

• Thermometers and other consumer products: Older thermometers contain mercury. If broken, the mercury may leak or be accidentally swallowed. Electric switches, barometers, and thermostats also may contain mercury.

• Dental fillings: Specific dental fillings — called dental amalgams, or silver fillings — contain mercury. Data shows that the risk of exposure from these fillings is extremely small.

• Gold mining: In some mining locations, mercury is combined with gold. This mixture is then heated to remove the mercury as a vapor, which can be very dangerous. If miners work in areas where this was done in the past, they may be exposed to leftover mercury.

Symptoms of mercury toxicity

Since methylmercury can affect the brain, symptoms can look like those you may see with a stroke. Mercury toxicity can also cause lung damage and breathing problems. Symptoms include:

• Loss of vision

• Muscle weakness

• Numbness of the hands and/or feet

• Impaired speech

• Difficulty walking or a lack of coordination

• Impaired hearing

• Difficulty breathing

• Coughing

• Bleeding gums

If you are at higher risk of mercury exposure and develop any of the above symptoms, ask your provider to get tested right away for mercury poisoning. 

What you need to know about mercury if you are pregnant

If you are pregnant, it can be confusing to know what to do when it comes to eating fish. 

Fish is a healthy part of your diet during childbearing years. It contains omega-3 fatty acids, as well as iron, iodine, and choline. All have been shown to have positive effects on a developing child’s brain, spinal cord, and immune system.

Thankfully, comprehensive guidelines exist to help eliminate the confusion. It is recommended that those who are pregnant or breastfeeding eat between 8 oz and 12 oz per week (or 2 to 3 palm-size servings) of a mix of seafood that is lower in mercury. Popular choices include:

• Catfish

• Pollock

• Salmon

• Shrimp

• Tuna (canned, light) 

It is worth noting that there are different types of tuna that vary in their levels of mercury. This is because some tuna live longer or are larger than others, allowing them to collect higher levels of mercury. For people who may become pregnant, are pregnant, or are breastfeeding, and for children between 1 and 11 years old:

• Canned, light tuna is the best choice.

• Canned, fresh, or frozen albacore, white tuna, and yellowfin tuna are also good choices.

• Bigeye tuna should be avoided.

How to limit mercury intake

The thought of mercury poisoning can be scary. But the odds of it happening are low. Here are some ways you can limit your mercury intake to avoid issues:

• Follow the weekly recommended dietary intake of fish. Limit eating a lot of fish that have high mercury levels. Instead, choose fish and shellfish that have little to no mercury.

• Be aware of your surroundings. You can help limit environmental exposure to mercury by keeping an eye out for any signs of spills or leakages.

• Avoid products that may contain higher levels of mercury. You can do this by replacing old thermometers with newer, mercury-free options.

Why trust our experts?

Written by:

Cherilyn Davis, MD

Cherilyn Davis, MD, is a board-certified pediatrician in New York City. She has held local and national roles at the American Medical Women’s Association including board member of the physician division and physician chair of social media.

Edited by:

Patricia Pinto-Garcia, MD, MPH

Patricia Pinto-Garcia, MD, MPH, is a medical editor at GoodRx. She is a licensed, board-certified pediatrician with more than a decade of experience in academic medicine.

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