Health Risks of Ionizing Radiation: - Clark University

When we look at an analysis of SMRs, for example,
we should consider each estimate in the context
of the overall SMR for the cohort. If workers at a
facility have an SMR of 0.8 overall then they are a
healthy cohort with mortality rate 20% lower than
average. Judging SMRs for specific diseases by
whether or not they are significantly greater than 1 is
misleading in this case, because they should in fact
be lower than 1 in a healthy cohort. Any SMRs that
are significantly greater than 1 become very strong
evidence of an effect.
Another common factor involves time;
researchers often represent workplace exposure with
a proxy variable like years of employment. This is
affected by the fact that sick people are weeded out
of employment over the years. A person with lung
cancer, for example, is unlikely to remain working
for very long. This results in a situation where the
people who have been working the longest can be
expected to be unusually healthy. This is known
as the healthy survivor effect. Baillargeon and
Wilkinson (1999) studied the healthy survivor effect
among workers at Hanford; they found that there
was a stepwise decline in the standardized mortality
ratios of workers with increasing employment length
and that workers in the highest employment duration
category showed a significant survival advantage
over other workers.
Other methodological limitations associated
with occupational exposure studies include:
• relatively low numbers of excess deaths, reducing
precision in risk estimation
• missing or incomplete workplace employment
and exposure records (this is especially true for
older data)
• confounding variables such as smoking that are
difficult to control
• poor records of disease incidence leading to a
reliance on mortality data
Despite these limitations, many researchers
look to studies of occupational exposure as a key
resource for data on health effects of chronic, lowlevel
radiation exposure. These workers constitute
a large population for whom exposure monitoring,
although far from ideal, is relatively thorough.
Furthermore, the majority of doses received by
these workers are relatively low. The mean dose
Radiation Workers 73
in a study of workers from three countries was 40
mSv and 80% of workers had doses below 0.05
Sv (Cardis et al. 1995). In a large Canadian study
the mean dose was even lower (6.6 mSv) because
it included exposure in medical professions; in this
case 97% of workers had doses below 0.05 Sv (Sont
et al. 2001).
Sections 6.2 and 6.3 discuss studies of worker
health by facility. Examining facilities individually
can reveal information and risks that “a common
estimator” from a multi-site study may obscure (Ritz
1999). On the other hand, many facility-specific
studies have been inconclusive because the cohorts
are small and because the healthy worker effect
limits meaningful comparisons to analyses within
the cohort. Because of this, a number of large pooled
analyses have been undertaken, some general and
some focused on specific disease endpoints. These
are addressed in Section 6.4. Section 6.5 presents a
summary and discussion of health effects in nuclear
workers.
6.2 US facilities
Hanford. Hanford, managed by the DOE, is
located in Richland, Washington and covers 560
square miles. From 1944 to the late 1980s, Hanford
operated as a plutonium production facility for
the nuclear weapons complex. More than 100,000
people have worked at Hanford over the years. In
our literature review for Hanford it became clear that
there had been something of an academic battle over
interpretation of the occupational exposure research
for this facility. Stewart and Kneale (1991) provide
some background as follows: In the early years of
standard setting for occupational radiation exposure
there was a lot of fluctuation and disagreement.
The Atomic Energy Commission (AEC) lowered
its exposure limits as risk awareness increased.
However, after hearing that significant risk was
difficult to detect in low-dose survivors of the
atomic bomb, the AEC wished to relax its standards.
In 1964 the AEC agreed to sponsor a lifetime health
mortality study of all of its employees in order to
gather more definitive information by which to set
standards. They hired Thomas Mancuso, of the
University of Pittsburgh School of Public Health,
to lead the study and Mancuso decided to focus on
data from Hanford. Shortly after Mancuso began his