2. Relevance to Public Health 2.1 Background and Environmental Exposures to Uranium in the United States

Uranium is an alpha-emitting, radioactive, heavy metal that occurs naturally in nearly all rocks and soils. Twenty-two isotopic forms of uranium have been identified, mainly associated with nuclear reactor operations or high-energy physics experiments; the most prevalent isotopes found in the environment are the three naturally-occurring isotopes: U, U, and U. Most uranium isotopes undergo decay by alpha emission, a few undergo beta emission, and several isotopes, including U, can also undergo spontaneous fission. U decays through 16 radioactive progeny, including U, to reach stable lead-206 (Pb), while U decays through 13 radioactive progeny to reach stable Pb. The rate of decay (or half-life) for most uranium isotopes is long; the half-lives of U, U and U are 4.5x10, 7.0x10 and 2.5x10 years, respectively. Since the activity of a given mass of uranium depends on the mass and halflife of each isotope present, the greater the relative abundance of the more rapidly decaying U and U, the higher the specific activity. Naturally occurring uranium is an isotopic mixture containing 99.284% U, 0.711% U, and 0.005% U by mass (49% U, 2% U, and 49% U by radioactivity) and has a specific activity of 0.68 μCi/g. The industrial process of enrichment separates natural uranium into enriched uranium (increased percentage of U) and depleted uranium (decreased percentage of U). The process of enrichment also increases the percentage of U (thus, enriched uranium is more reactive); depleted uranium has a decreased percentage of U and is less radioactive. Uranium enrichment for commercial nuclear energy produces uranium that contains about 3% U by activity. Uranium enrichment for other purposes, including nuclear weapons production, can produce uranium containing as much as 97.3% U and having a higher specific activity (~50 μCi/g). Depleted uranium is the byproduct of the enrichment process. Depleted uranium has even less specific activity (0.33 μCi/g) than natural uranium.