nium and cigarette smoking and the impact of exposure to other sources of radiation or other potential hazards.

The committee concludes that there is limited/suggestive evidence of no association between exposure to uranium and lung cancer at cumulative internal dose levels lower than 200 mSv or 25 cGy. However, there is inadequate/insufficient evidence to determine whether an association does or does not exist between exposure to uranium and lung cancer at higher levels of cumulative exposure.

Lymphatic Cancer

The lymphatic system is an important potential target for uranium radiation because inhaled insoluble uranium oxides can remain up to several years in the hilar lymph nodes of the lungs. Studying the effect of uranium exposure on lymphatic cancer is more difficult than studying lung cancer because lymphatic cancer is much less common. Thus, the small number of cases magnifies the effects of all of the methodological concerns cited in the foregoing discussion of lung cancer data. The small number of expected cases means wide confidence intervals for SMRs and far too few cases for the subgroup analyses that are necessary to establish useful dose–response relationships.

Lymphomas are neoplastic transformations of cells that reside principally in lymph nodes. In view of the localization of inhaled uranium oxides in lung lymph nodes, uranium should be associated with malignant lymphomas if it is associated with any lymphoid malignancy. Table 4.12 shows the results of the epidemiologic studies that provide information on the relationship between uranium exposure and lymphoid malignancy. Some studies provided results on specific types of lymphoid malignancies while others grouped the data.

Studies of mill workers on the Colorado Plateau found an increase in lymphoid cancer deaths. The Archer et al. (1973a) study found an SMR of 392 in a small sample of uranium millers (4 cases observed versus 1.02 expected). The four cases were in millers who had not worked in the furnace area with the highest levels of exposure to uranium and vanadium. The Waxweiler et al. (1983) study with a larger cohort of millers found 5 observed and 4.2 expected deaths (SMR = 119, 95% CI 21–217) due to lymphosarcoma, reticulosarcoma, or Hodgkin’s disease. However, four of the deaths due to lymphatic cancers were of employees with less than 5 years of employment, and the increase in the SMR was not statistically significant. Further, other potential exposures including vanadium and thorium could have caused the increased cases of lymphoid malignancy. In most of the studies listed in Table 4.12, the number of deaths due to lymphatic cancer has been small and the deviations from the expected number of deaths have been consistent with random variation. The Fernald cohort (Ritz, 1999) showed an increased SMR (129), but the confidence interval (91–177) included 100. By far the largest study was the Polednak and Frome (1981) report of the Oak Ridge experience, which included the period early in the nuclear industry in which workers were exposed to



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