Mercury is a highly toxic element that enters the environment through both natural and human-linked (anthropogenic) pathways. Natural sources of mercury to air include the oceans, land, biomass burning and volcanoes.  Since mercury is an element and cannot be destroyed, one big challenge for mercury scientists is to understand how much of mercury in the environment is due to anthropogenic sources.  The major categories of anthropogenic sources of mercury to air are fossil fuel combustion (mainly coal), metal production, industrial and artisanal gold production, cement production, chlor-alkali industry and waste incineration.  Annual average anthropogenic mercury emissions to air are estimated to be approximately 2000 tonnes (2000 megagrams/year), but may be up to nearly 3000 tones.  Annual average natural emissions to air are estimated to be at least 2000 tonnes with the higher end estimates over 5000 tonnes.

The toxicity of mercury is largely dependent on the chemical form and the path of exposure. The most toxic form for dietary intake is methylmercury [CH₃Hg]. Exposure of animals to methylmercury affects the immune system, alters genetic and enzyme systems, and damages the nervous system. Methylmercury is also a teratogen, which means it is particularly damaging to developing embryos, which are 5 to 10 times more sensitive than adults.  The most toxic form for exposure through inhalation is elemental mercury (Hg⁰).  At sufficient levels, continued inhalation exposure to elemental mercury causes tremors, gingivitis, and excitability.  At very high levels, inhalation of elemental mercury can cause death.  High levels of elemental mercury are usually confined to industrial settings or mercury spill events.  Since elemental mercury is difficult to absorb in the digestive system, there is minimal effect from dietary intake.

Once present in aquatic ecosystems, elemental and inorganic mercury can undergo chemical transformations to methylated mercury species and enter the food web.  Because methylmercury is not easily purged from organisms, biomagnification of up to 10⁷ can occur in the aquatic food chain.  Harm to upper level predators such as birds, sea mammals and humans can result when consuming tissues with high levels of methyl mercury (mainly fish).  Acute exposure to high levels of methyl mercury contamination has been well documented in cases such as Minamata Bay, Japan in the 1950′s and Iraq in the 1970s.  Recent low-level exposure studies have been used to set government guidelines for methyl mercury consumption nationally and internationally.  New studies also suggest that methyl mercury concentrations in rice are a potential concern.

Considerable effort has been spent trying to understand how mercury cycles through the environment.  A major advancement in understanding began in the early 1990s when scientists began using new ultra-trace level mercury detection instruments, speciation methods and ultra-clean sampling techniques. Ice and sediment core records indicate that environmental mercury levels have increased substantially over pre-industrial levels.  Direct release of mercury to aquatic ecosystems has been dramatically reduced in Europe and North America, but continues to be an area of concern for emerging and developing countries.  However, research has shown that atmospheric emission, dispersal, transformation and subsequent wet and dry deposition is the primary route for mercury contamination in remote aquatic ecosystems far removed from direct sources of contamination. The best control strategy to prevent biomagnification is to limit mercury releases to the environment. While not practical to control natural sources such as volcanoes and geological deposits, it is possible to control anthropogenic releases from activities such as manufacturing, mining, and power production.  In the past 10 years, there has been an increased focus on developing mercury control policies and regulations at the national and international level.  The increased global emphasis reducing mercury emissions and use provides support new control technology development, new and better monitoring equipment and advance research studies.

Links for more information or contact Tekran^® directly at lab-air-info@tekran.com:
USGS: Mercury in the Environment
US EPA: Mercury
European Commission Environment: Mercury
Environment Canada: Mercury and the Environment
UNEP Mercury: Reducing Risk from Mercury
UNEP Report: Global Atmospheric Mercury Assessment
US ASTDR: ToxFAQ for Mercury
US EPA Office of Water: Methyl Mercury Fish Tissue Criterion
MercNet – National Monitoring Workshop Report
Mercury Fate and Transport in the Global Atmosphere