relatively high amounts of inhaled uranium. In that study there were 37 deaths compared to 61 expected (SMR = 61).

Conclusions on Lymphatic Cancer

The number of cases is too small and the confidence intervals for SMRs are too wide to draw any conclusions about the association between uranium exposure and lymphoid malignancy. In particular, it is not possible to do subgroup analyses linking different levels of uranium exposure to the death rate from lymphoid malignancy. In this instance, where the evidence is all epidemiological in nature, concluding that an association may not exist would require some evidence that the incidence of lymphoid malignancy remains the same as the level of exposure increases.

The committee concludes that there is inadequate/insufficient evidence to determine whether an association does or does not exist between exposure to uranium and lymphatic cancer.

Bone Cancer

Like the lymphatic system, bone is an important potential target for the effects of uranium because uranium is distributed to the bone, replaces calcium in bone matrix, and may remain in the bone for several years. Studying the effects of uranium exposure on bone cancer is even more difficult than studying lymphoid malignancy because bone cancer is rarer, which means wide confidence intervals for the SMRs and far too few cases to establish useful dose–response relationships. The studies of bone cancer are listed in Table 4.13. According to the BEIR IV report (NRC, 1988), if there were carcinogenic effects in humans from exposure to uranium it would most likely result in increased risk of bone sarcoma (i.e., there is biological plausibility due to the deposition of uranium in bone). However, studies to date have not found an increase in bone cancers.

Conclusions on Bone Cancer

Bone cancer is rare; thus, the number of cases in all studies is small. For this reason, there is insufficient evidence to determine whether an association exists between acute or chronic exposure to uranium and bone cancer. Nevertheless, the large size of the Oak Ridge cohort (Polednak and Frome, 1981) does provide some evidence that exposure to uranium is not associated with a large excess risk of bone cancer (e.g., a relative risk of 3.0 or greater).

The committee concludes that there is inadequate/insufficient evidence to determine whether an association does or does not exist between exposure to uranium and bone cancer.



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