22/12/2009

Arsenic
Occurrence, exposure and dose
Arsenic is a widely distributed metalloid, occurring in rock, soil, water
and air. Inorganic arsenic is present in groundwater used for drinking in
several countries all over the world (e.g. Bangladesh, Chile and China),
whereas organic arsenic compounds (such as arsenobetaine) are primarily
found in fish, which thus may give rise to human exposure40.
Smelting of non-ferrous metals and the production of energy from fossil
fuel are the two major industrial processes that lead to arsenic contamination
of air, water and soil, smelting activities being the largest single anthropogenic
source of atmospheric pollution41. Other sources of contamination
are the manufacture and use of arsenical pesticides and wood preservatives.
The working group of the EU DG Environment concluded that there
were large reductions in the emissions of arsenic to air in several member
countries of the European Union in the 1980s. In 1990, the total emissions
of arsenic to the air in the member states were estimated to be 575 tonnes.
In 1996, the estimated total releases of arsenic to the air in the UK were
50 tonnes42.
Concentrations in air in rural areas range from <1 to 4 ng/m3, whereas
concentrations in cities may be as high as 200 ng/m3. Much higher concentrations
(>1000 ng/m3) have been measured near industrial sources.
Water concentrations are usually <10 ìg/l, although higher concentrations
may occur near anthropogenic sources. Levels in soils usually range from
1 to 40 mg/kg, but pesticide application and waste disposal can result in
much higher concentrations40.
General population exposure to arsenic is mainly via intake of food
and drinking water. Food is the most important source, but in some areas,
arsenic in drinking water is a significant source of exposure to inorganic
arsenic. Contaminated soils such as mine-tailings are also a potential
source of arsenic exposure40.
Absorption of arsenic in inhaled airborne particles is highly dependent on
the solubility and the size of particles. Soluble arsenic compounds are easily
absorbed from the gastrointestinal tract. However, inorganic arsenic is
extensively methylated in humans and the metabolites are excreted in the
urine40.
Arsenic (or metabolites) concentrations in blood, hair, nails and urine
have been used as biomarkers of exposure. Arsenic in hair and nails can
be useful indicators of past arsenic exposure, if care is taken to avoid
external arsenic contamination of the samples. Speciated metabolites in
urine expressed as either inorganic arsenic or the sum of metabolites
(inorganic arsenic + MMA + DMA) is generally the best estimate of recent
arsenic dose. However, consumption of certain seafood may confound
Hazards of heavy metal contamination
British Medical Bulletin 2003;68 179
estimation of inorganic arsenic exposure, and should thus be avoided
before urine sampling40.
Health effects
Inorganic arsenic is acutely toxic and intake of large quantities leads to
gastrointestinal symptoms, severe disturbances of the cardiovascular and
central nervous systems, and eventually death. In survivors, bone marrow
depression, haemolysis, hepatomegaly, melanosis, polyneuropathy and
encephalopathy may be observed. Ingestion of inorganic arsenic may
induce peripheral vascular disease, which in its extreme form leads to
gangrenous changes (black foot disease, only reported in Taiwan).
Populations exposed to arsenic via drinking water show excess risk of
mortality from lung, bladder and kidney cancer, the risk increasing with
increasing exposure. There is also an increased risk of skin cancer and
other skin lesions, such as hyperkeratosis and pigmentation changes.
Studies on various populations exposed to arsenic by inhalation, such
as smelter workers, pesticide manufacturers and miners in many different
countries consistently demonstrate an excess lung cancer. Although all
these groups are exposed to other chemicals in addition to arsenic, there
is no other common factor that could explain the findings. The lung cancer
risk increases with increasing arsenic exposure in all relevant studies,
and confounding by smoking does not explain the findings.
The latest WHO evaluation40 concludes that arsenic exposure via
drinking water is causally related to cancer in the lungs, kidney, bladder
and skin, the last of which is preceded by directly observable precancerous
lesions. Uncertainties in the estimation of past exposures are important
when assessing the exposure–response relationships, but it would seem
that drinking water arsenic concentrations of approximately 100 ìg/l have
led to cancer at these sites, and that precursors of skin cancer have been
associated with levels of 50–100 ìg/l.
The relationships between arsenic exposure and other health effects
are less clear. There is relatively strong evidence for hypertension and
cardiovascular disease, but the evidence is only suggestive for diabetes and
reproductive effects and weak for cerebrovascular disease, long-term neurological
effects, and cancer at sites other than lung, bladder, kidney and skin40.