2. What are the impacts of mercury on human health?
- 2.1 What are the potential health effects of mercury?
- 2.2 How are we exposed to mercury?
- 2.3 What levels of mercury might cause harm?
- 2.4 How great are the risks from mercury today?
2.1 What are the potential health effects of mercury?
The toxicity of mercury
depends on the form of mercury to which people are
Although mercury and its compounds are
toxic substances, there is
ongoing debate about exactly how toxic they are. Toxic effects,
especially in the case of
methylmercury, may be
taking place at lower
previously thought, but this is proving difficult to establish
because the suspected toxic effects are subtle and their
mechanisms complex. Methylmercury is of particular concern
because it can accumulate
in the food chain to reach high concentrations
Methylmercury is special
organic mercury compounds
because large numbers of people are
exposed to it and its
toxicity is better
understood. Methylmercury in food, such as fish, is a particular
health hazard because it is easily
taken up into the body
through the stomach and intestines.
It is a poison for the nervous system.
Exposure during pregnancy is
of most concern, because it may
development of the unborn
baby’s brain. Some studies suggest that small increases in
exposure may affect the
heart and circulatory system.
Moreover, there is some evidence at present that
methylmercury can cause
cancer in humans, but it is
far from conclusive: the International Agency for Research on
has classified methylmercury as
"possibly carcinogenic to humans"
Methylmercury’s poisonous potential was highlighted by an
incident in Minamata (Japan), in the 1950s, where wastes from a
chemical factory using mercury were discharged into the local
Elemental mercury is also
poisonous to the nervous system. Humans are mainly
inhaling vapours. These are
absorbed into the body via
the lungs and move easily from the bloodstream into the brain.
However, when elemental mercury is
ingested, little is absorbed
into the body.
The inhalation of
elemental mercury vapours
neurological and behavioural
disorders, such as tremors,
insomnia, memory loss,
neuromuscular changes and
headaches. They can also
harm the kidneys and
exposures have also led to
deaths. However, there is no evidence at present that elemental
mercury causes cancer in
humans and it has been classified by
into Group 3
"unclassifiable as to carcinogenicity in humans"
2.2 How are we exposed to mercury?
The main source of
elemental mercury vapour is
dental amalgam (a tooth
Diet, particularly fish, is generally the main source of both
Methylmercury is by far the
most common organic form, and is especially found in fish and
For some people, the workplace may also be an important source
of exposure. Examples include
mercury mines, thermometer factories, refineries and dental
clinics, as well as the mining and manufacturing of gold
extracted with mercury.
People can also receive extra
doses in specific
situations, such as when mercury compounds are used in
skin-lightening creams, soaps and traditional medicine.
Exposure may also arise from
localised pollution through air and water, and from mercury
spills at home or work (e.g. from certain old gas meters
2.3 What levels of mercury might cause harm?
For methylmercury, the US
Environmental Protection Agency
has estimated a safe daily
intake level of 0.1
µg/kg body weight per day.
This was based on a study in the Faroe Islands, where fish
containing significant levels of mercury form a large part of
the diet. The study compared
development test scores for
children whose mothers had been
A European Union scientific review, in 2001, has supported this
safe daily intake level.
For elemental mercury
vapour, several studies show that long-term workplace exposures–
at around 20 µg/m3 of air or higher – have subtle
toxic effects on the central
Other adverse effects of various forms of mercury have been
seen in humans, but either the findings are less consistent or
the doses involved are much
The Working Group that prepared this assessment, in line with
its mandate, did not assess the potential effects of
elemental mercury vapour
from dental amalgams or
reach any conclusions about whether or not dental amalgams cause
adverse effects. This remains a matter of scientific
2.4 How great are the risks from mercury today?
Whilst the diet and
amalgam fillings in teeth
are respectively the main sources of
methylmercury and mercury
vapour exposure for most
people, sources such as local pollution, exposure at work,
cultural practices and traditional medicines are important in
Assessments of mercury
exposure have been made in
various parts of the world. For example, a recent study of 1700
women in the USA found that about 8% of them had mercury
concentrations in their
blood and hair exceeding the levels that correspond to the
estimated safe dose.
Data indicate that
exposures in Greenland, Japan
and some other areas are generally higher than in the USA. On
the other hand, measures have been taken in recent decades to
reduce emissions of mercury in various countries
Fish is the main food source in many parts of the world and
provides nutrients that are not easily replaced. Mercury
contamination adds health
risks to this important
Many countries, international organizations and scientific
investigations have reported mercury
concentrations in fish
between about 0.05 and 1.4
mg/kg of fish
tissue, depending on the
water and the fish.
Predator fish and marine mammals that eat other fish tend to
have higher levels of mercury because mercury
bioaccumulates in fish and
is biomagnified up the food
Mercury levels are thus higher in such fish as king mackerel,
pike, shark, swordfish, walleye, barracuda, large tuna (as
opposed to the small tuna usually used for canned tuna),
scabbard and marlin, as well as in seals and toothed
Moderate consumption of fish with low mercury levels is not
likely to result in worrying levels of
exposure for humans. However,
people who consume higher amounts of contaminated fish or marine
mammals may be highly exposed
to mercury and are, therefore, at
risk. Indeed, high
concentrations of mercury in
fish have led governments in a number of countries to give
warnings to consumers. These advise people, especially sensitive
groups (such as pregnant women and young children), to limit or
avoid consumption of certain types of fish from specific