Elemental mercury and mercury compounds occur naturally in geologic hydrocarbons including coal, natural gas, gas condensates, and crude oil. Coal contains trace amounts of mercury in the form of mercuric sulphate (known as cinnabar) that, when the coal is burned, is released into the environment as mercury vapor .
Mercury is extremely toxic. It is considered by the World Health Organisation as one of the top ten chemicals or groups of chemicals of major public health concern.
Mercury moves around the world in three key ways. First, it is actively traded as a global commodity, often for uses like artisanal and small-scale gold mining–where substantial mercury releases into the environment are routine. Second, airborne mercury, carried in the air pollution caused by the combustion of coal, is dissipated over very wide areas before being deposited on land and on water. Third, once mercury enters a river or ocean, natural bacteria may absorb it and convert it to a more toxic form: methylmercury.
Methylmercury accumulates in fish and shellfish. Large predatory fish are more likely to have high levels of mercury as a result of eating smaller fish that have acquired mercury through the ingestion of plankton. Large predatory fish at the top of the aquatic food chain may accumulate levels of methylmercury as high as 1 part per million. The mercury ‘bio-accumulates’ as it moves up the food chain. It also gets more concentrated–so larger predatory fish have higher concentrations of mercury in their tissue than smaller fish further down the food chain.
Global emissions are running at somewhere between 5000 and 8000 tonnes a year—the exact figure seems to be unclear. Although emissions from coal fired power plants have declined in North America and in many western European countries due to much-improved emission control technologies, globally, emission are on the rise. In 2018, UN agencies reported that global emissions had risen 20% between 2010 and 2015 .
In the US, mercury emissions in 2014 were about 55 tons, with coal-fired units responsible for about 42% of the total . That figure may be declining as coal fired power plants are gradually shut down—but the story is different in Poland. A Polish newspaper reported in June 2018 that emissions of mercury from coal power plants had jumped a whopping 87 percent in just one year. European data showed that that mercury pollution from the giant Belchatow power plant in central Poland was 18 times higher in 2016 than in the previous year.
The frequent flyer
Airborne mercury is a global traveller. Once in the air it can remain for up to a year and travel long distances. In Canada, 97% of mercury deposited on Canadian landscapes, roughly 47 tonnes a year, comes from other countries—mainly from Asia. The highest rate of deposition is in the Canadian Arctic where almost 9 tonnes/year of airborne mercury settles each year—where it is gradually absorbed into natural ecosystems, marine environments and fish. 
Two groups of people are more sensitive to the effects of mercury. Foetuses are most susceptible to the adverse developmental effects of the element. Methylmercury exposure in the womb can result from the mother’s consumption of fish and shellfish. It can adversely affect a baby’s growing brain and nervous system. The primary health effect of methylmercury is impaired neurological development. Cognitive thinking, memory, attention, language, and fine motor and visual spatial skills may be affected in children who were exposed to methylmercury as foetuses.
The second group is people who are regularly exposed to high levels of mercury—such as populations that rely on subsistence fishing. According to the World Health Organisation, among selected subsistence fishing populations, between 1.5 and 17 in 1000 children showed cognitive impairment and mild mental retardation caused by the consumption of fish containing mercury. These children were in populations in Brazil, Canada, China, Columbia and Greenland. The figure below shows how mercury moves through the environment and eventually into the human body .
In the US, a nationwide study of blood samples in 1999-2000 showed that more than 15 % of women of childbearing age had blood mercury levels that would cause them to give birth to children with mercury levels exceeding the EPA’s maximum acceptable dose for the element. This dose was established to limit the number of children with mercury-related neurological and developmental impairments. Researchers have estimated that as many as 630,000 children are born in the US each year with blood mercury levels high enough to impair performance on neurodevelopmental tests and cause lifelong loss of intelligence .
The Minamata convention
The Minamata Convention on Mercury entered in force in 2017. Signed by 128 countries and ratified by 100 countries and the European Union, the Convention takes its name from the most severe mercury poisoning disaster in history, which came to light in Minamata, Japan, in May 1956, after sustained dumping of industrial wastewaters into Minamata Bay, beginning in the 1930s. Local villagers who ate fish and shellfish from the bay started suffering convulsions, psychosis, loss of consciousness and coma. In all, thousands of people were certified as having directly suffered from mercury poisoning, now known as Minamata disease. Both Canada and the US are signatories to the convention.
The convention requires parties to apply best available technology to curb mercury emissions from new fossil-fuel power plants. Coal-fired power plants are the worst offenders in terms of mercury emissions.
However, the convention does not apply to existing coal-fired power plants, which are likely to have older equipment and higher pollution levels. And in spite of the rapid penetration of utility-scale solar power and wind energy, globally, coal remains the predominant fuel for power generation. Moreover, the Minamata convention does not set any limit on the number of new coal-fired power plant that a party can construct. Even with best available technology, mercury emissions from coal-fired power plants are not completely eliminated, which means that emissions of mercury are likely to rise even when control technology is installed—as long as coal continues to be used to generate electricity.
In November 2018, as the second meeting of the Conference of Parties was getting underway in Geneva, it was reported that global emissions of mercury had risen by 20 percent from 2010 to 2015. An estimated 15 million miners are exposed to mercury through its use in artisanal and small-scale gold mining in over 70 countries .
Mercury vapour can be removed from coal fired power plants using a variety of pollution control technologies—but not completely. The mercury that is removed is then deposited in the solid waste products from the power plant. Impoundments, slag heaps, tailing ponds, and tips are never permanent repositories of toxic pollutants. Once coal is mined and burned, one way or another, the mercury will find its way into the environment–and eventually into the food chain.
For more information check out these sources:
 See: Mercury in petroleum and natural gas: estimation of emissions from production, processing and combustion. EPA report EPA-600/R-01-066, September 2001
 See: Countries meet to address mercury as global emissions rise by 20%. Accessed at: //www.thegef.org/news/countries-meet-address-mercury-global-emissions-rise-20
 See the EPA website : //www.epa.gov/mercury/basic-information-about-mercury#airemissions
 See: Poland’s coal-related mercury emissions revised upwards, significantly. At: //ieefa.org/plands-coal-related-mercury-emissions-revised-upward-significantly/
 See the report on mercury from the Canadian Council of Ministers of the Environment (CCME) at : //www.ccme.ca/en/resources/air/mercury.html
 See the World Health Organisation fact sheet: Mercury and health. Updated March 2017, at //www.who.int/mediacentre/factsheets/fs361/en/
 See the WHO Fact sheet: Mercury and health. Ibid.
 The chart is from Physicians for Social Responsibility report Op.cit.
 The data are from the Physicians for Social Responsibility report. Op. cit.
 See: Countries meet to address mercury as global emissions rise by 20%. Accessed at: //www.unenvironment.org/news-and-stories/press-release/countries-meet-address-mercury-global-emissions-rise