For these reasons, natural uranium is considered a low-level internal radiation hazard. Thus, the focus of the discussion on radiation in this chapter is on the effects of radiation from uranium in the body, rather than from external radiation. Since the specific activity (a measure of radioactivity) of depleted uranium is 40 percent lower than that of natural uranium (and much lower than enriched uranium), any health effects that are a consequence of the radioactive nature of uranium would be expected to be less prevalent in people exposed to DU than in people exposed by the same route to the same amount by weight of natural or enriched uranium.
The committee examined studies of health effects in uranium miners, but concluded that these studies have limited relevance because the primary disease-causing exposures were not to uranium but to radon decay products. The principal form of radiation exposure of uranium miners in underground mines has been to inhalation of alpha particles emitted by radon decay products in poorly ventilated mines (NRC, 1999). Radon progeny are known to increase the risk of lung cancer (NRC, 1999). In addition, miners were exposed to other possibly toxic dusts and, potentially, to diesel gas fumes, which might cause cancer and other diseases of the lung (NRC, 1999). Another serious limitation of most studies of uranium miners is the lack of information on cigarette smoking. The experience of uranium miners with diseases other than those of the respiratory tract could inform our knowledge of the consequences of uranium mining in organ systems other than lung. However, the literature on uranium miners has focused largely on lung and other cancers, and most publications have used mortality rather than morbidity as the outcome measure.
Radon is a radioactive decay product of uranium. In a confined, poorly ventilated area, such as a mine shaft, radon gas diffuses from the surrounding uranium-containing rock and accumulates in the atmosphere within the mine shaft, where miners inhale it. Radon progeny (polonium-218, lead-214, bismuth-214, and polonium-214) decay rapidly (with half-lives of 30 minutes or less) by emitting alpha particles. These isotopes attach to dust particles, are inhaled and deposited on the bronchial epithelium, and decay before natural clearance mechanisms can remove them. However, in relatively well-ventilated work spaces, such as uranium mills or uranium fabricating plants, radon gas is present in low concentrations. Furthermore, some uranium refining processes remove radium, the immediate parent of radon in the uranium decay series. Exposure to radon decay products is known to be associated with increased risk of lung cancer (NRC, 1999).
The Committee on the Biological Effects of Ionizing Radiation (BEIR) of the National Research Council has extensively studied the published literature on the health effects of radon and other internally deposited alpha-particle emitters such as uranium (NRC, 1988, 1990, 1999). The following section briefly summarizes some of the literature on health effects in uranium mine workers.