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Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2
quantify systematic errors related to the dosimetry technology and radiation fields as well as errors related to laboratory practices (Fix and others 1994, 1997; Gilbert 1998). When these errors were taken into account in the risk estimation process, it resulted in a widening of the confidence intervals around the ERR (Gilbert and Fix 1995) as shown in Table 8-6.
Doses from Neutrons, Low- and Very-High-Energy Photons, and Internal Contamination
In the three-country study, efforts were also made to identify workers with substantial doses from radiations other than high-energy photons (mainly from neutrons, low-energy radiation, and contamination with radionuclides, particularly plutonium), for whom recorded dose estimates may be in error. Although it was not possible to identify all such workers, risk estimates based on restricted dosimetry analyses, which excluded all such workers who could be identified, did not differ greatly from those based on the standard approach (-0.04 and 2.05 Sv−1 respectively, for all cancers excluding leukemia and for leukemia excluding chronic lymphocytic leukemia (CLL) compared to -0.07 and 2.18 Sv−1 in the standard population). In addition, the estimate of risk for all cancers excluding leukemia and lung cancer (the organ that would receive the majority of the dose from plutonium contamination) was identical to that of all cancers excluding leukemia (−0.07 Sv−1; 90% CI −0.39, 0.30). It is therefore unlikely that the risk estimates in this study are substantially biased by inclusion in the analyses of a minority of workers with dose from neutrons, low-energy photons, and internal contamination (Cardis and others 1995).
Possible Confounding and Modifying Factors
As in most occupational cohort studies, information on life-style factors such as smoking habits, diet, and other oc
TABLE 8-6 Estimates of the ERR per Sievert with 90% CIs for the Hanford Worker Study Based on Recorded Doses and Based on Estimated Organ Doses
All Cancers Excluding Leukemia
Leukemia Excluding CLL
(90% CI <0, 1.5)
(90% CI <0, 2.7)
Organ doses (corrected for systematic errors related to radiation fields)
(90%CI <0, 1.7)
(90% CI <0, 3.6)
NOTE: CLL = chronic lymphocytic leukemia.
cupational exposures could not be obtained retrospectively for all members of the cohort. In the three-country combined analyses, there was little indirect evidence for an association between cumulative dose and mortality from smoking-related cancers, respiratory diseases, or liver cirrhosis; thus, it is unlikely that smoking or alcohol consumption are strongly correlated with radiation dose (Cardis and others 1995). This is supported by the observation that the risk estimates for all cancers excluding leukemia and all cancers excluding both leukemia and lung cancer were nearly identical (Cardis and others 1995): also, the results of two studies, carried out within the Hanford (Petersen and others 1990) and AEA (Carpenter and others 1989) cohorts, respectively, showed little evidence for an association between smoking and occupational radiation dose. A positive association between smoking and occupational radiation dose was found in the AECL cohort in Canada (Howe and others 1987).
A positive association between radiation dose and mortality from circulatory disease was observed in the four cohorts included in the three-country study in which information on SES was least detailed (Rocky Flats, Sellafield, AECL, Canadian NDR). It may reflect residual confounding by life-style factors for which the SES variable is an inadequate proxy.
Radionuclides in the Working Environment
At uranium fuel production facilities, inhalation of airborne uranium dust may represent an important potential source of radiation exposure. Workers in these facilities have two main possible sources of radiological exposure to tissues of the whole body: external γ-ray exposure and internal depositions that deliver radiation doses (mainly from α-particles) primarily to the lung and lymphatic system. If the uranium dust is soluble, exposure of other tissues may also occur such as liver, kidney, and bone, although organ doses would be expected to be small. Low-LET radiation risk estimates for tumors in these organs are possibly confounded by high-LET radiation exposure for workers at uranium production facilities, since workers with a significant dose from internal contamination are often persons with substantial external exposure. A number of studies of such workers have been reviewed (Cardis and Richardson 2000; NRC 2000).
Comparison of findings among uranium-processing facilities is complicated by the fact that processes and historical periods of operation have differed among facilities, leading to differences in exposure conditions and follow-up among cohorts. Further, assessment of past internal uranium exposure of nuclear workers is complicated by the methodological difficulties of internal dosimetry, as well as by inadequate historical information with which to quantify internal radiation doses accurately. These exposure measurement