22/12/2009

Mercury
Occurrence, exposure and dose
The mercury compound cinnabar (HgS), was used in pre-historic cave
paintings for red colours, and metallic mercury was known in ancient
Greece where it (as well as white lead) was used as a cosmetic to lighten
the skin. In medicine, apart from the previously mentioned use of mercury
as a cure for syphilis, mercury compounds have also been used as diuretics
Hazards of heavy metal contamination
British Medical Bulletin 2003;68 173
[calomel (Hg2Cl2)], and mercury amalgam is still used for filling teeth in
many countries26.
Metallic mercury is used in thermometers, barometers and instruments
for measuring blood pressure. A major use of mercury is in the chloralkali
industry, in the electrochemical process of manufacturing chlorine,
where mercury is used as an electrode.
The largest occupational group exposed to mercury is dental care staff.
During the 1970s, air concentrations in some dental surgeries reached
20 ìg/m3, but since then levels have generally fallen to about one-tenth
of those concentrations.
Inorganic mercury is converted to organic compounds, such as methyl
mercury, which is very stable and accumulates in the food chain. Until the
1970s, methyl mercury was commonly used for control of fungi on seed
grain.
The general population is primarily exposed to mercury via food, fish
being a major source of methyl mercury exposure27, and dental amalgam.
Several experimental studies have shown that mercury vapour is
released from amalgam fillings, and that the release rate may increase by
chewing28.
Mercury in urine is primarily related to (relatively recent) exposure to
inorganic compounds, whereas blood mercury may be used to identify exposure
to methyl mercury. A number of studies have correlated the number of
dental amalgam fillings or amalgam surfaces with the mercury content in
tissues from human autopsy, as well as in samples of blood, urine and
plasma26. Mercury in hair may be used to estimate long-term exposure, but
potential contamination may make interpretation difficult.
Health effects
Inorganic mercury
Acute mercury exposure may give rise to lung damage. Chronic poisoning is
characterized by neurological and psychological symptoms, such as tremor,
changes in personality, restlessness, anxiety, sleep disturbance and depression.
The symptoms are reversible after cessation of exposure. Because of
the blood–brain barrier there is no central nervous involvement related to
inorganic mercury exposure. Metallic mercury may cause kidney damage,
which is reversible after exposure has stopped. It has also been possible to
detect proteinuria at relatively low levels of occupational exposure.
Metallic mercury is an allergen, which may cause contact eczema, and
mercury from amalgam fillings may give rise to oral lichen. It has been
feared that mercury in amalgam may cause a variety of symptoms. This
so-called ‘amalgam disease’ is, however, controversial, and although some
Impact of environmental pollution on health: balancing risk
174 British Medical Bulletin 2003;68
authors claim proof of symptom relief after removal of dental amalgam
fillings29, there is no scientific evidence of this30.
Organic mercury
Methyl mercury poisoning has a latency of 1 month or longer after acute
exposure, and the main symptoms relate to nervous system damage31. The
earliest symptoms are parestesias and numbness in the hands and feet.
Later, coordination difficulties and concentric constriction of the visual
field may develop as well as auditory symptoms. High doses may lead to
death, usually 2–4 weeks after onset of symptoms. The Minamata catastrophe
in Japan in the 1950s was caused by methyl mercury poisoning
from fish contaminated by mercury discharges to the surrounding sea. In
the early 1970s, more than 10,000 persons in Iraq were poisoned by eating
bread baked from mercury-polluted grain, and several thousand people
died as a consequence of the poisoning. However, the general population
does not face significant health risks from methyl mercury exposure with
the exception of certain groups with high fish consumption.
A high dietary intake of mercury from consumption of fish has been
hypothesized to increase the risk of coronary heart disease32. In a recent
case-control study, the joint association of mercury levels in toenail clippings
and docosahexaenoic acid levels in adipose tissue with the risk of a
first myocardial infarction in men was evaluated33. Mercury levels in the
patients were 15% higher than those in controls (95% CI, 5–25%), and
the adjusted odds ratio for myocardial infarction associated with the
highest compared with the lowest quintile of mercury was 2.16 (95% CI,
1.09–4.29; P for trend = 0.006).
Another recent case-control study investigated the association between
mercury levels in toenails and the risk of coronary heart disease among male
health professionals with no previous history of cardiovascular disease.
Mercury levels were significantly correlated with fish consumption, and the
mean mercury level was higher in dentists than in non-dentists. When other
risk factors for coronary heart disease had been controlled for, mercury
levels were not significantly associated with the risk of coronary heart
disease34.
These intriguing contradictory findings need to be followed up by
more studies of other similarly exposed populations.