Learn about mercury

Mercury Sources

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Natural Mercury

Mercury is released to the environment naturally in several ways, such as from volcanoes, forest fires, weathering of rocks, and under‑sea vents.

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Human‑Made Mercury

Mercury is emitted from human actions such as burning of fossil fuels and municipal waste. Coal‑fired power plants and independent gold mining operations are the two largest anthropogenic (human‑based) sources globally.

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Recycled Mercury

Previously deposited mercury can be re‑emitted to the environment through natural processes such as the cycling of mercury between the oceans and atmosphere. The original source of this recycled mercury can be natural or human‑made.

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Atmospheric Transport

Once mercury is exposed to the Earth’s ecosystem it can flow between living and non‑living entities and it can be transported long distances through the atmosphere. Almost all mercury in the atmosphere is in the gaseous elemental form (Hg0). In this form, mercury is insoluble and readily transported and distributed throughout the global environment. It remains in the atmosphere for 1 to 1.5 years.

Gaseous elemental mercury can be transformed (possibly with assistance from other atmospheric pollutants such as ozone, sulfur dioxide, and soot) to a more reactive, water‑soluble, gaseous oxidized form (Hg2+). This form of mercury may travel tens to hundreds of kilometers and can remain in the atmosphere for hours to days. The transformation of Hg0 to Hg2+ is thought to cause elemental mercury to be deposited on land and in water.

Particulate mercury (Hg[p]) is mercury adsorbed onto other particulate matter. It tends to fall out relatively close to the source of emissions, with larger particles falling out faster than smaller ones.

Mercury Deposition

Mercury in the atmosphere can deposit locally near sources or can be transported over long distances before depositing to the Earth in rain and snow (wet deposition) or as gases and particles (dry deposition).

It can deposit directly onto the ocean surface, or deposit to watersheds and eventually enter coastal waters via rivers. The timing and location of mercury deposition is determined by its chemical form, the location of emission sources, and environmental conditions.

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Mercury is a toxic pollutant that
persists in the environment

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Mercury is a naturally occurring element yet industrial activities, such as power generation from coal‑fired power plants, emit mercury to the atmosphere where it can be transported and deposited locally, regionally, and globally.

Within watersheds and lakes, natural processes convert inorganic mercury to highly toxic methylmercury, which increases in concentration through food webs. In the surface ocean, mercury concentrations have increased four‑fold over the past 500 years, with a two‑fold increase over the last century concurrent with industrialization and energy production.

According to leading mercury scientists, fish consumption is the main way people are exposed to mercury. The consumption of estuarine and marine fish account for more than 90% of the methylmercury exposure in the U.S. and other regions.

Methylmercury production

After mercury is released to the environment, it is transformed into forms that are increasingly dangerous to human health. Most mercury is released to the environment as inorganic mercury; it is then converted by bacteria to the organic form, methylmercury, usually in aquatic environments.

Methylmercury can be produced within upland watersheds and transported to downstream coastal waters. It can also be produced in coastal wetlands or sediments or in the water column of the open ocean. Methylmercury combined with inorganic forms of mercury constitute “total mercury.”

Methylmercury in fish is 100 million times greater than in water.

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Prey Fish
1‑10 million x
Predator Fish
10‑100 million x

How does mercury bioaccumulate?

Methylmercury is absorbed by organisms and stored in living tissues. It is taken up by microscopic algae that are eaten by zooplankton (such as small crustaceans), which are consumed by small fish, which are in turn eaten by large fish.

Methylmercury is stored in muscle tissue where it is not easily eliminated. As predator eats prey larger fish ingest and store larger amounts of methylmercury. This results in biomagnification along the food chain. The transfer of methylmercury through the food chain is influenced by factors such as the number of links in the chain and the nature of the food chain, organism growth rate, and supply of nutrients.

Organisms at the top of the food chain can have high methylmercury concentrations. Long‑lived, predatory fish, such as swordfish and tuna, can have methylmercury levels as much as 10 to 100 million times higher than methylmercury concentrations in the surrounding ocean waters.

How does APMMN measure
mercury deposition?

The APMMN began measuring total mercury in rain and snow (wet deposition) in 2013. Automated wet deposition samplers and precipitation gages measure mercury concentrations following the APMMN standard operating procedures.

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Taiwan-style MIC

Chemical Analysis

Cold vapor atomic fluorescence spectroscopy (CVAFS) at National Central University (NCU), Taiwan

Mercury Forms

Total mercury wet deposition and precipitation concentrations


Precipitation depth measured onsite (best) or as close as possible to the sampler (within 5 km)

Sample Frequency

One Week

Sampling Schedule

Tuesday ‑ Tuesday

All APMMN samples are sent to the mercury lab at National Central University (NCU) in Taiwan, which analyzes all forms of mercury in single measurements and reports this as total mercury concentrations.

The NCU team reviews field and laboratory data for completeness and accuracy, and flags samples that were mishandled, compromised by precipitation collector failures, or grossly contaminated. NCU delivers all data and information to the APMMN Program Office for final checks and resolution of remaining discrepancies. Data are then made available through this website.

APMMN is working with partners, including Japan’s National Institutes of Minamata Disease and Environmental Studies and NADP, to develop standardized capabilities for estimating mercury dry/total deposition in select locations using a combination of technologies, including continuous atmospheric mercury analyzers, manual sampling approaches, and passive measurement devices.

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