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.

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.

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.

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 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 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.