Mercury is emitted from a range of natural sources such as volcanoes, soils, under sea vents and mercury-rich geological zones, as well as fresh waters and the oceans, plants, forest fires, sea salt spray and meteoric dust. Although natural emissions occur mainly as elemental mercury vapor (Hg0); particulate and vaporous oxides, sulphides and halides as well as methylmercury vapor may also be released.
It is estimated that annual, natural emissions from continental sources are approximately 1,000 tons. In pre-industrial times, evasion from the oceans is thought to have been in the area of 600 tons. Today, however, evasion from the oceans has increased to approximately 2,000 tons due to the re-emission of mercury deposited as a result of human activities.
Although humans have extracted and utilized mercury for centuries, mining and industrial applications for the metal have increased significantly since the industrial revolution. Despite mercury's toxic nature, humans have taken advantage its unique properties to conduct electricity, measure temperature and pressure, act as a biocide, preservative and disinfectant and catalyze reactions. Consequently, the amount of mercury mobilized and released due to human activities has greatly increased, leading to elevated concentrations in air, water, soil, sediments, and living organisms.
Historically, mercury has been used by many cultures for a variety of symbolic purposes, such as in good luck charms and to ward off evil. Mercury played a predominant role in alchemy, and was thought to have medicinal uses such as curing syphilis in the 19th century. While the majority of these practices are no longer followed, some peoples still use mercury in artisanal jewelry and trinkets for tourist sales.
Mercury emitted to the atmosphere is considered a serious environmental threat to both animals and humans. Today, a variety of industrial and combustion processes contribute to mercury releases. The largest sources are from metal mining and smelting, municipal waste incineration, sewage and medical waste incineration, coal-fired power plants and cement manufacturing. Other sources of mercury arise from chloralkali plants (used to produce chlorine, primarily for bleaching in the pulp and paper industry), mineral ores processing, steel manufacturing, petroleum refining and fossil fuel combustion.
In addition, mercury may enter the environment during the life cycle of a range of consumer, medical and industrial products. The largest use of mercury is in electrical products such as fluorescent lamps, thermometers, thermostats and electrical switches. These products are found in residences, office, commercial and industrial buildings and cars. Other mercury-containing products containing mercury include pressure sensing devices, blood pressure reading devices, thermometers and dental amalgams.
The following list itemizes additional products that have been known to contain mercury. Although most of these products are no longer manufactured with mercury, older items containing mercury may still be in circulation. Cumulatively, these products may represent a significant reservoir of the toxin that can potentially enter the environment.
Mercury can become a part of the global mercury cycle when mercury-containing products are broken and the spilled mercury poured down the drain, or when these products are disposed of in landfill sites. Mercury releases from products, as they break down in both active and closed landfills, may represent a significant pathway for the transport and eventual deposition of the toxin in various terrestrial and aquatic ecosystems. In general, mercury may be released from municipal solid waste landfills as a trace component of landfill gas, which is generated during the decomposition of waste under anaerobic conditions, or in the liquid leachate flowing from the site.
Because the improper disposal of mercury-containing products can lead to environmental and health effects, it is important that individuals reduce their consumption of these products and dispose of them properly.