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In considering the potential toxicity of depleted uranium, it is important to distinguish between radiologic and chemical toxic mechanisms. It is also important to note that the radiologic and chemical properties of uranium could act synergistically to cause health outcomes.

Radiologic Considerations

As discussed above, depleted uranium and naturally occurring uranium have different abundances of the three isotopes. The most notable difference is a decrease in the abundance of 235U from 0.72% to 0.20% in depleted uranium, which reduces overall radioactivity by about 40% (Harley et al., 1999).

The radioactivity of a source is based on the number of radioactive atoms undergoing radioactive decay in a given period. Radioactive decay is the attempt of any atom to rearrange or transform the constituent protons and neutrons of its nucleus in such a way that the atom ends up having lower inherent energy. Radioactive decay occurs spontaneously because energy is given off, rather than consumed, in the process. The result of radioactive decay is an atom (the daughter) with less inherent energy than that which preceded it (the radioactive parent atom). Uranium isotopes decay to other radioactive elements that eventually decay to stable isotopes of lead (ATSDR, 1999b).

The term radioactivity describes how many radioactive atoms are undergoing radioactive decay every second. It does not reflect what type of radiation is being emitted or the energy of that radiation. The traditional unit of radioactivity is the curie (Ci); 1 Ci equals 3.7 × 1010 disintegrations per second (dps). A disintegration occurs when an atom undergoes radioactive decay. The International System unit of radioactivity is the Becquerel (Bq); 1 Bq is equivalent to l dps. Common units of measurement are summarized in Box 2-1.

Radioactive half-life is the amount of time it takes for radioactivity to decrease by half, that is, for half the radioactive atoms to undergo radioactive decay. The half-life of 238U is 4.47 × 109 years, and the half-life of 235U 7.04 × 108 years (Bleise et al., 2003); radioactivity never reaches zero but only keeps fractionally reducing.

The isotopes of uranium emit alpha particles. Alpha particles are positively charged ions composed of two protons and two neutrons. Because of their size and charge, alpha particles lose their kinetic energy quickly and have little penetrating power. The range of an alpha particle is about 4 cm in air and considerably less (25-80 μm) in tissue (ATSDR, 1999a). As a result, uranium is a radiation hazard mainly when uranium atoms are in the body. As noted above, uranium isotopes decay to other radioactive elements that eventually decay to stable isotopes of lead. In the decay process, beta particles and gamma rays are

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