Health Risks of Ionizing Radiation: - Clark University

Figure 6-4. Plutonium is handled through a glovebox at the
Plutonium Finishing Plant at the Hanford site. Department
of Energy, Office of Environmental Management 1996.
1986 6 . In the 1981 analysis the dose-response pattern
appeared supralinear, although this suggestion did
not hold with further follow-up (Stewart and Kneale
1993, Kneale and Stewart 1993). The 1993 analysis
suggested that age at exposure was a very important
factor and that exposure at older ages (60 and older)
can carry a particularly high risk. The overall risk
was described with a doubling dose of ~260 mSv;
this is equivalent to an ERR of roughly 4/Sv (again,
this can be compared to the atomic bomb survivor
estimate of 0.47/Sv; Preston et al. 2003).
Oak Ridge. Oak Ridge, a 58 square mile
reservation located near Knoxville, Tennessee,
was established in 1943 as a laboratory to produce
enriched uranium for the Manhattan Project. During
the 1950s and 1960s Oak Ridge was a center for
the study of nuclear energy and related research. In
the 1970s, with the creation of the Department of
Energy, Oak Ridge expanded research to include
energy production, transmission and conservation.
It is now the location of the K-25 and Y-12 plants as
well as the Oak Ridge National Laboratory (ORNL),
also known as X-10.
The Oak Ridge studies are a good example of
how cohort selection can affect a study. Although
all researchers were drawing from a list of Oak
Radiation Workers 75
Ridge employees, there are a variety of ways in
which characteristics such as date of hire, job
description, race, gender, follow-up time, and other
factors were used to define cohorts. Some studies,
for example, have focused specifically on the Y-12
and/or X-10 plants in Oak Ridge since these had
higher documented exposures than the other Oak
Ridge facilities. Other studies exclude or analyze
separately the years during which Y-12 was used
for uranium enrichment. Researchers have also
used different interpretations of cause of death. The
variety of cohort definitions in Oak Ridge studies
have contributed to the variety of results you will
see below.
A retrospective cohort study of white male
employees (Checkoway et al. 1985) generally
found low SMRs, predictably demonstrating the
healthy worker effect. For prostate cancer, leukemia
and lymphoma, however, SMRs were positive 7 .
Leukemia mortality appeared to be related to dose,
although this was not quantified. In a 1988 paper
Checkoway et al. restricted their analysis to workers
at the Y-12 plant employed during 1947-1974.
Elevated SMRs were found for cancers of the lung 8
(1.36; 1.09-1.67), brain and CNS (1.80; 0.98-3.02)
and ‘other lymphatic cancers 9 ’ (1.86; 0.85-3.53),
but not leukemia (SMR 0.50; 0.14-1.28). US rates
were not available for multiple myeloma but based
on Tennessee rates an SMR of 1.43 (0.39-3.66) was
estimated. The authors note that although these risk
estimates lack statistical power they do suggest risks
at relatively low doses. A positive dose-response
relationship for lung cancer was apparent, but this
relationship was weakened when a 10 year latency
was assumed and ten years is considered a minimum
for radiation-induced lung cancer.
The association with CNS cancers was revisited
by Carpenter et al. (1987) in a case-control study
that included white males and white females who
were employed 1943-1979. This was a very small
study (27 cases with external and 47 with internal
dose estimates) and the results were uncertain. In
a comparison of workers with doses greater or less
7 Prostate cancer (1.16), Hodgkin’s disease (1.10), leukemia (1.48). These were not significantly positive, but this
should be taken with a grain of salt since the overall SMR was low (0.73).
8 Polednak and Frome (1981) calculated SMRs for workers employed at the Y-12 plant between 1943 and 1947 and
also found excess lung cancer (SMR 1.51 95% CI 1.01-2.31).
9 This category includes codes 202, 203, and 208 of the International Classification of Diseases, 8 th revision.