A short explanation of
the Mercury Issue
Mercury is a natural element. Due to its unique properties (shiny metal that is liquid at room
temperature), mercury has been used in a variety of applications. Pure mercury is rarely found
in nature, mercury is extracted mainly from cinnabar. Mercury and most of its compounds are
highly toxic to humans, ecosystems and wildlife. Even relatively low doses can have serious
adverse neurodevelopmental impacts. Mercury is persistent and bioaccumulative.
In recent years, environmental mercury levels have risen substantially; human activities have
contributed significantly to the amount of mercury in the environment and in the food chain.
Mercury is used in a number of industrial processes (e.g. chlor-alkali, plastics industry, etc)
and in products (thermometers, dental amalgam, batteries, light bulbs, etc). It is also released
unintentionally through the burning of fossil fuels (particularly in coal-fired power plants).
About half of the mercury currently released into the atmosphere comes from human activity.
Elemental mercury released in the atmosphere is eventually deposited in aquatic
environments, where it can be converted to methylmercury, its most toxic form, through
bacteria. Depending on its form, mercury can stay in the air up to a year and can be
transported throughout the hemisphere before it is deposited; therefore the mercury issue is a
global problem and cannot be addressed at the national level.
Methylmercury bioaccumulates in fish and enters the food chain. As larger fish eat smaller
ones, methylmercury is concentrated up the food chain, a process known as biomagnification.
As a result, top level predators in aquatic systems can have levels of methylmercury built up
in their systems that are 100,000 times higher than methylmercury levels in the waters where
they live.
Fish in and downstream of mercury hotspots (small scale gold mining operations, mercury
cell chlor alkali plants, coal-fired power plants etc) can contain high mercury concentrations,
but fish in other regions (even in the Arctic where there is little or no local mercury pollution)
are affected as well. Elevated mercury levels have been measured in many freshwater and
marine species throughout the world. Factors that influence mercury levels in the fish include
age, size, weight and length of the fish, as well the characteristics of the body of water (e.g.
local contamination, pH, etc.). Mercury concentrations in fish generally range from about
0,005 to 1,4milligram of mercury per kilogram of tissue (mg/kg). Smaller, younger or nonpredatory fish will tend to have lower mercury levels than large, older predatory fish. Large
predatory fish are often migratory and thus fish from particular waters with high mercury
contamination can be found anywhere.
Ingesting fish or aquatic/marine mammals that have built up high levels of methylmercury
passes their toxic burden to those who consume them, including humans. More than 90% of
the mercury/methylmercury in fish ingested is readily absorbed into the body through the
gastrointestinal tract.
As methylmercury in fish is bound to tissue proteins rather than fatty deposits, there is little
one can do to reduce exposure when eating fish (trimming/skinning of the fish does not
reduce the mercury content of the fillet portion and no cleaning or cooking methods can
reduce the amounts of mercury intake either). Fish remains an important source of protein,
vitamins and micronutrients and is an important constituent of a balanced diet. Mercury
intake depends obviously not only on the level of mercury and the type of fish, but also on the
amount consumed and the frequency of consumption.
The reference intake levels for methylmercury exposures range from 0.7 to 2 μg methylmercury per
kilogram body weight (μg/kg body weight) per week. The Joint FAO/WHO Expert Committee on
Food Additives (JECFA) has established provisional tolerable weekly intakes (PTWIs)1 for
total mercury at 5 μg/kg body weight and for methylmercury at 1.6 μg/kg body weight.
EU legislation setting maximum levels for mercury in fishery products is already in place2.
Limits are set at 0,50 or 1,00mg/kg wet weight depending on the type of fish. In view of the
actual levels of mercury detected in fish, there is very limited scope to reasonably reduce the
levels. Alternative solutions for protecting vulnerable groups (e.g. targeted consumer advice)
are necessary.
An information note3 on methylmercury in fish and fishery products issued by the European
Commission in 2008 suggests that women who might become pregnant, women who are
pregnant or breastfeeding and young children should not eat more than one small portion
(<100g) per week of large predatory fish, such as swordfish, shark, marlin and pike. Also they
should not eat tuna more than twice per week.
Detailed guidance on identifying populations at risk from mercury exposure has recently been
issued by UNEP Chemicals in cooperation with WHO4.
While an accurate analysis of potential risks from fish consumption would require detailed
information on fish species consumption patterns as well as methylmercury levels in the
consumed fish, the following table gives a potentially useful indicator of the risk estimate of
mercury exposure from ingestion of fish.
Date: 12/07/2012
1
The PTWI is the amount of a substance that can be consumed weekly over an entire lifetime without an
appreciable risk to health.
2
Commission Regulation (EC) No 1881/2006 of 19 December 2006 setting maximum levels for certain
contaminants in foodstuffs
3
Information Note on Methylmercury in fish and fishery products, CEC/DG SANCO, 21 April 2008
4
Guidance for Identifying Populations at Risk from Mercury Exposure, UNEP Chemicals/WHO, August 2008
2