Mercury is a naturally occurring element. It can be released to the environment from natural sources – such as weathering of mercury-containing rocks, forest fires, volcanic eruptions or geothermal activities – but also from human activities. An estimated 5500-8900 tons of mercury is currently emitted and re-emitted each year to the atmosphere1 , with much of the re-emitted mercury considered to be related to human activity, as are the direct releases.
Due to its unique properties, mercury has been used in various products and processes for hundreds of years. Currently, it is mostly utilised in industrial processes that produce chlorine (mercury chlor-alkali plants) or vinyl chloride monomer for polyvinyl chloride (PVC) production, and polyurethane elastomers. It is extensively used to extract gold from ore in artisanal and small scale gold mining. It is contained in products such as electrical switches (including thermostats), relays, measuring and control equipment, energy-efficient fluorescent light bulbs, batteries and dental amalgam. It is also used in laboratories, cosmetics, pharmaceuticals, including in vaccines as a preservative, paints, and jewellery. Mercury is also released unintentionally from some industrial processes, such as coal-fired power and heat generation, cement production, mining and other metallurgic activities such as non-ferrous metals production, as well as from incineration of many types of waste.
Once released, mercury persists in the environment where it circulates between air, water, sediments, soil and biota in various forms. Mercury can be transported long distances in the atmosphere. It can also be incorporated by microorganisms and converted to methylmercury, and then concentrated up the food chain. Exposure to mercury occurs mainly through ingestion of fish and other marine species contaminated with methylmercury, its most toxic and bioaccumulative form. People may also be exposed to elemental or inorganic mercury through inhalation of mercury vapour during occupational activities or spills or through direct contact from mercury use.
Source : http://www.mercuryconvention.org