effects, including chromosomal breaks and additional effects in DNA that may result in genetic damage. All isotopes of uranium undergo decay by the emission of alpha particles from the nucleus with photons of x and gamma radiation. Most of the energy released by the radioactive decay of a uranium nucleus is in the form of kinetic energy imparted to the alpha particle, typically about 4.2 MeV. Despite the large amount of energy, alpha particles have a limited range in soft tissue—about 30 μm—and so are unable to penetrate the superficial dead layer of skin. Thus, alpha particles pose a hazard only if taken into the body. Photons, however, are able to penetrate the body, depositing relatively small amounts of energy as they traverse tissues, and may pose a hazard both internally and externally. Beta particles, which are emitted by some uranium decay products, have a variable range in tissue that depends on their kinetic energy, which is typically a fraction of that of an alpha particle. The most energetic beta particles have a range of only about 1 cm in soft tissue.

Biologic effects of radiation are typically classified as deterministic or stochastic. A deterministic effect is one for which there is a clearly defined threshold and that increases in severity as the dose increases above the threshold. An example of radiation-induced deterministic effect (in this case, nonionizing-radiation-induced) is ordinary sunburn, which requires a minimal dose and increases in severity as the dose increases. A stochastic effect has a probability of occurrence that increases in proportion to the dose, but its severity is unrelated to the dose. An example of stochastic effects is the increased probability of skin cancer caused by exposure to sunlight. Stochastic risks posed by exposure to ionizing radiation include radiogenic cancers and genetic mutations. Deterministic effects are usually associated with high doses and typically occur relatively soon after exposure; thus, they are said to have a short latent period (time between exposure and manifestation of the effect). Stochastic effects, such as carcinogenesis, may not manifest themselves for many years and thus have a long latent period.

Although compelling evidence to the contrary exists for some cancers, stochastic risks are generally assumed to follow a linear-no-threshold (LNT) dose-response curve, at least for the purposes of determining potential health effects and establishing radiation-protection standards. Thus, doubling the dose is assumed to double the incremental risk, tripling the dose triples the incremental risk, and so on. However, even if the risk coefficients (see discussion below) are accurately and precisely known and the LNT hypothesis is an accurate characterization of the dose-response relationship, the risk is most likely overstated because an exposed person could die from other causes before the radiogenic cancer would be manifested, particularly if the cancer has a long latent period. Furthermore, the simplistic LNT model of response does not consider possible other low-dose effects now under study, such as bystander effects and adaptive responses, that may or may not be significant.

The primary radiologic concern related to chronic low-level exposure to DU, as might result from intake of DU and deposition in tissues or from external exposure due to living in a contaminated environment, is the development of a



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