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Thorium (Th)
Because most
thorium salts are excreted via urine, a high urine thorium level
indicates exposure and probably increased body burden of this
element. Thorium is considered mildly toxic for two reasons,
low-level radioactivity and slight biochemical toxicity.
Thorium is a radioactive element having 7 isotopes with half
lives that exceed one hour. Thorium 232 constitutes 99% of the naturally
occurring thorium and this is the isotope measured. Thorium 232 has a
halflife of 1.4x10 to the tenth years. It decays by alpha emission
to produce radon, Radon 228. In turn Radon 228 (half life 6.7 years) decays
to other radioactive isotopes, eventually reaching lead. This
radioactive decay process produces alpha, beta and gamma emissions.
Several decades ago, a thoria (Thorium O2) suspension (”Thorotrast”) was
used diagnostically as a radiopaque agent. After a long period of
latency, an unusually high proportion of individuals who received
this procedure have developed leukemias, granulomas, and malignant
liver tumors. These are slowly-developed diseases often with 20-30
year periods before onset or definite diagnosis.
The biochemical effects of thorium are mild. Reactive thorium
salts at high levels may inhibit amylase and phosphatase enzymes.
Most orally ingested thorium, if not excreted in urine, binds to
bone tissue where it has a long biological half-life (years). There
is a literature report for abnormal lymphocytes in animals following
a thorium challenge.
Thorium has about the same abundance in the earth as does lead and
is encountered in mining activities for titanium and rare earth
elements. Commercially, thorium is used in incandescent gas lantern
mantles, refractory materials (thorium melts at 3300 degrees C), and as a
coating for tungsten in electronic applications. It is present in
nuclear fuels (Uranium 235 decays to Thorium 231).
Thorium may also be present in tungsten-inert-gas (”TIG”)
welding electrodes.