Mercury is a toxic heavy metal that's found naturally in air, water and soil.
It's released into the environment in several ways, including through industrial work like burning coal or natural occurrences like volcanoes.
Three main forms exist: elemental (metallic), inorganic and organic (1).
People can be exposed to mercury in a number of ways, such as breathing in mercury vapors during mining and industrial work.
You can also be exposed by eating fish and shellfish. That's because fish and shellfish are exposed to low concentrations of mercury due to water pollution.
Over time, it can concentrate in their bodies. This is usually in the organic form, which is known as methylmercury.
This is a highly toxic form that can cause serious health problems when it reaches certain levels in the body.
BOTTOM LINE: Mercury is a naturally occurring heavy metal. It can build up in the bodies of fish in the form of methylmercury, which is highly toxic.
Many types of fish contain mercury.
One study found that 25% of fish from 291 streams around the US contained more than the recommended limit (2).
Another study found that one-third of fish caught on the New Jersey shore had mercury levels higher than 0.5 parts per million, a level that could cause health problems for people who eat this fish regularly (3).
Overall, larger and longer-lived fish tend to contain the most mercury (4).
These include shark, swordfish, fresh tuna, marlin, king mackerel, tilefish from the Gulf of Mexico, northern pike and more (5).
Larger fish tend to eat many smaller fish, which contain small amounts of mercury. This is not easily excreted from their bodies, so levels accumulate over time. This process is known as bioaccumulation (6).
Mercury levels in fish are measured as parts per million (ppm). Below are the average levels in different types of fish and seafood, from highest to lowest (5):
BOTTOM LINE: Different types of fish contain varying amounts of mercury. Larger and longer-lived fish usually contain higher amounts.