CADMIUM SOURCES, UPTAKE,
AND METABOLISM
Cadmium occurs within zinc, copper,
and lead ores and its concentration in
soil varies widely (typically 0.01–
7.0 ppm). This influences the amount
in local drinking water and the amount
delivered into tobacco leaves and other
plants. In the absence of occupational
exposure, cadmium enters the body in
trace amounts within drinking water
and foodstuffs, and within tobacco smoke. Food intake of cadmium
averages 10–25 mg/day, but may exceed
this considerably if shellfish is prominent
in the diet. Up to 30 mg of
cadmium contaminates a pack of cigarettes.
However, only a small proportion
of this (median 2.74 mg per pack in one
study, as Mannino and colleagues point
out) is transferred to mainstream
smoke, and only 20–50% of the amount
inhaled is absorbed. An even smaller
proportion (2–6%) of ingested cadmium
is absorbed. The net outcome is that
current smokers have roughly twice the
body burden of never smokers.
Cadmium entering the blood is
retained chiefly within the liver and
kidneys, where most becomes complexed
with metallothionein. This
makes it relatively innocuous, but that
which is not complexed is potentially
toxic—especially to the kidneys, but also
to the lungs and other organs. The
complexed cadmium has a long half life
(many years) and the body’s store
generally increases until late middle
age to a normal total of 10–30 mg. The
little that is re-released into the blood
may become re-absorbed temporarily
within tubular cells of the kidney and
then dissociated, allowing the potential
for delayed and ongoing toxicity. It is
unclear whether a similar mechanism
operates in the lungs. The daily excretion
of cadmium in urine is a useful, if
crude, marker of the total body burden.
Normal excretion averages 1–2 mg/day
at most, but the range can be wide. By
contrast, the blood cadmium level is a
poor reflection of the total burden and
relates more closely to recent exposure.