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Is
cadmium worse than lead?
Reprinted
from Toxic. March 1994 with permission.
Cadmium
is a heavy metal with wide use and distribution in industrial
processes. Sources include mining and metallurgy; manufacturing and
processing industries; metal finishing; sewage disposal; combustion
of fuels and wastes; pesticides and fertilisers. Cadmium is a
by-product of zinc and lead smelting. The toxicology of cadmium is
comprehensively reviewed by Friberg et al 1986, and Robards and
Worsfold 1991.
Along
with lead and mercury it is a "heavy" metal of
environmental significance because of documented adverse effects in
animal studies at low doses and in human epidemiology - and because
of widespread environmental distribution.
Environmental
sources
Cadmium is present in all mediums. Airborne cadmium in urban
air is in the range up to 0.060 µg/m3. Meat, fish and fruit contain
1 to 50 µg/kg and grains from 10 to 150 µg/kg. The greatest
concentration is in the liver and kidney of animals, and in
shellfish such as muscles and oysters which concentrate cadmium up
to levels of 100 to 1000 µg/kg. In developed countries the total
daily cadmium intake from all mediums may be in the range 10 to 40µg/kg.
Cadmium
is more readily taken up by plants than other metals such as lead.
In this context, the cadmium content of phosphate fertiliser is a
significant source. Cadmium content of Aust. fertilisers is in the
range 2060 ppm, and is derived mainly from rock phosphate.
Williams has reported the problems of trace metals in
superphosphate. Tobacco. tomatoes and brassicas take up cadmium more
efficiently from soils than some other crops. Studies have shown a
slow but steady increase in the cadmium content of vegetables over
the years, and consequent increase in body burdens.
Toxicology
Cadmium is more efficiently absorbed from the lungs than the
gastrointestinal tract. Calcium and iron deficiencies may enhance
the uptake of cadmium. Cadmium, like zinc, has a predisposition for
the male reproductive organs, particularly the prostate. Dietary
zinc decreases cadmium absorption. Blood cadmium in
non-occupationally exposed persons is usually less than 1 µg/dl
(micrograms per decilitre).
Although
high levels of cadmium can cause acute effects, the principal
long-term effects of low-level exposure to cadmium are chronic lung
disease, emphysema and chronic kidney disease. Effects on the
cardiovascular and the skeletal system have also been reported.
People heavily exposed in Japan developed Itai-Itai disease which is
characterised by bone pain, osteomalacia and osteoporosis.
Kidney
and skeletal effects appear not to be reversible and may be
progressive according to follow-up studies of exposed populations.
As with lead, cadmium is associated with increased blood pressure in
several studies of occupationally-exposed workers. Electrical and
biochemical heart disturbances have also been reported in
experimental animals.
The
IARC rates cadmium as a probable human carcinogen, category 2A,
based on animal studies and human occupational epidemiology. This
evidence is much more developed than for lead. Human studies have
found increases in lung cancer and to a lesser extent, cancer of the
prostate.
Much
recent interest centres on the spermatogenic effects of low levels
of cadmium exposure. A recent paper by Kok-War Hew et al shows that
a single dose of 1 mg/kg of cadmium chloride resulted in failure of
spermiation in experimental rats. Poor sperm characteristics have
been correlated with high cadmium blood levels (mean 1.35 µg/dl) in
human studies. (Archives of Andrology 29/2, 177-183, 1992,
abstract).
Cadmium
is a toxic, persistent and bioaccumulating metal which is increasing
in the environment and human tissue.
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