Mercury is a heavy metal that has significant impacts on human health. Mercury comes in three forms— metallic, inorganic, and organic—each with its own degree of toxicity and particular exposure pathways. Metallic mercury is the pure elemental form of the metal, and is extracted from cinnabar ore. After being heated to above 1,000 degrees Fahrenheit, mercury is refined into its liquid form, which is used in products such as thermometers, electricity switches, dental fillings; in the production of caustic soda and chlorine gas; and is used to extract gold from ores containing gold. Another primary form of the metal is organic mercury, most commonly known as methylmercury, and produced when elemental mercury combines with carbon. This is the form of the pollutant that can contaminate food chains.
Mercury naturally enters the environment through the breakdown of minerals into soil, which is then dispersed through the movement of air and water. Since the start of the industrial revolution in the 18th century, the release of mercury into the environment has been heavily amplified. Currently, the anthropogenic release of mercury accounts for up to two-thirds of the total mercury in the environment. 
Other common sources of mercury pollution in these countries include industrial mining, chemical manufacturing, solid waste disposal, and metals smelting. Most of these activities involve the heating of mercury, which releases it into the air in vapor form. The mercury is then transported in dust by wind. Mercury can settle into soil or surface waters through rainfall, and often is washed away from these sites along with tailings and sediments into local water bodies. Once in an aquatic ecosystem, elemental mercury can be transformed into methylmercury—a powerful neurotoxin—by bacteria, and can bioaccumulate and move up the food chain.
People are commonly exposed to mercury through the inhalation of vapors produced through the burning of mercury in these various industrial activities; however, many are also exposed to methylmercury through the consumption of contaminated food products such as fish. Once mercury enters the human body, it can permanently damage the brain, kidneys, and the development of a fetus. Exposure to methylmercury can cause arthritis, miscarriages, respiratory failure, neurological damage, and even death. Children are most at risk of mercury exposure, especially in regions adjacent to small- and large-scale gold mines.