Mercury is a naturally occurring element found in air, water and soil. It exists in three different forms, elemental or metallic mercury, inorganic mercury compounds, and organic mercury compounds. Coal-burning power plants are the largest source of mercury emissions to the air caused by humans in the United States, accounting for more than 40% of all domestic anthropogenic mercury emissions.1 Additional mercury emission sources include the burning of hazardous waste, chlorine production, mercury product breakage, mercury spillage, metal processing, and cement production.
Mercury travels in the air and is eventually deposited into water where microrganisms and abiotic reactions convert it to methyl mercury. Methyl mercury builds up in fish and shellfish, which constitute the main sources of human exposure to this highly toxic element. Exposure level depends on the fish consumption rate, the individual’s body weight, and the level of methyl mercury found in specific fish species consumed. Methyl mercury can reach extremely high levels in predatory fish such as swordfish, king mackerel, and shark.
Methyl mercury exposure at high levels can harm the brain, heart, kidneys, lungs, and immune system of people of all ages. Methyl mercury is acutely toxic to humans because of its ability to cross the blood/brain barrier. It has also been demonstrated that high levels of methyl mercury in the bloodstream of unborn babies and young children may harm their developing nervous systems, having a negative impact upon learning and cognitive abilities.
In response, U.S. regulatory bodies have enforced strict legislation to specify maximum allowable concentrations of methyl mercury in fish and to monitor mercury emissions to the air.
The Food and Drug Administration (FDA) has established action levels for poisonous or harmful substances in human food and animal feed.3 The regulation specifies a maximum level of 1 mg/kg methyl mercury in edible portions of fresh, frozen or processed fish, shellfish, crustaceans and other aquatic animals. This action level represents the limit at or above which the FDA will take legal action to prohibit imports and remove products from the market. The Environmental Protection Agency (EPA) has introduced a methyl mercury guideline that recommends a limit on mercury consumption based on body weight, more specifically 0.1 mg/kg body weight per day.
The EPA has also issued several stringent regulations to reduce mercury pollution. The final regulations for municipal waste combustors substantially reduced mercury emissions to the air by as much as 91% between 1990 and 2005. Information on compliance with the emission standards for medical waste incinerators (MWIs) indicates that mercury emissions from MWIs are 95% lower than in 1990. The final standards for mercury from chlor-alkali production, introduced in 2003, were projected to cut mercury emissions from point sources at these facilities by 74% and total mercury emissions by about 11% compared with 1999 levels.
In 2005, the EPA issued the Clean Air Mercury Rule to permanently cap and reduce mercury emissions from coal-fired power plants.5 This rule makes the United States the first country in the world to regulate mercury emissions from utilities. In its most recent regulation, the EPA aims to reduce mercury emissions from industrial boilers by 17% from 12 tons to 10 tons per year.
Due to these stringent regulations and the growing public food safety concerns, raw materials suppliers, food manufacturers and processors, government organizations and private food testing laboratories must analyze mercury in fish. The most established method for total mercury analysis is vapor generation atomic absorption (AA) spectrometry, which has emerged as the method of choice for routine, reliable total mercury screening. For samples contravening the legislative value of 1 mg/kg for total mercury, a speciated determination is likely to be required to determine the methyl mercury concentration.