Health Risks of Ionizing Radiation: - Clark University

108 Uranium Miners
were selected 12 . Two studies indicated that excess
lung cancer was associated with exposures as low
as 40-90 and 80 WLM (in Ontario and Sweden,
respectively). The concluding estimates of ERR
were 0.009/WLM to 0.014/WLM depending on
exposure rate.
The National Institute of Health (NIH)
combined data from eleven studies, including most
of the studies described above, for a total cohort of
68,000 miners (Lubin et al. 1994). The inverse doserate
effect noted in many of the studies above was
evident in the combined cohort. Risk was also found
to depend significantly on attained age and time
since exposure. A summary estimate of ERR was
presented as 0.005/WLM (0.002-0.01), although
more complex models were presented to account for
the strong influence of non-radon factors. Lubin et al.
(1995, 1997) also presented more focused analyses
of the inverse dose-rate effect and risk estimates at
the low end of miner exposures. These models were
updated and used in the BEIR VI report on the health
effects of radon (NRC 1999) 13 .
8.5 Discussion
Studies of uranium miners and other miners have
consistently demonstrated a linear relationship
between radon exposure and lung cancer risk and
risk estimates generated by these studies have
been remarkably similar. All studies have noted an
apparent inverse dose-rate effect, where the total
dose received by the lung tissue is more damaging
if it is spread out over time. The theoretical basis
for this effect lies in the number of alpha particles
crossing a cell. High dose rates are more likely to
result in more than one alpha particle crossing a
cell. The possibility that a cell might develop into a
cancer decreases after the first ‘hit’ because, among
other possibilities, it becomes more likely that the
cell will simply be destroyed. The inverse dose-rate
effect is not expected to be important at very low
doses because the probability that any one cell will
be hit twice is incredibly small (the probability that
a cell will be hit even once is small). Lubin et al.
(1995) observed that the inverse dose-rate effect
appeared to diminish below 50 WLM.
13 The BEIR VI report derived a summary estimate ERR of 0.0076/WLM (NRC 1999).
The effects of time and age have been consistently
apparent in these studies as well. The final BEIR VI
report (NRC 1999) chose not to emphasize a single
risk coefficient and instead presented two alternative
models where risk estimates could be generated
according to the characteristics of a hypothetical
exposure scenario. Although this is inconvenient in
some ways, making it much harder for a lay reader
to understand what the risks of radon might be, for
example, it is in many ways a more honest approach
to the complexities of the interaction between
radiation and the human body.
Comparisons with residential radon studies.
The NIH analysis of 11 miner studies projected
risks to residential radon exposures and estimated
that roughly one-third of lung cancer deaths among
nonsmokers could be attributed to radon (Lubin et al.
1994). This kind of estimate is not straightforward
because we know that the relationship between radon
and lung cancer risk depends on dose rate, attained
age, and time since exposure. Figure 8-4 shows the
estimated ERR for different ranges of cumulative
exposure. It can be seen here that the risk coefficient
is probably higher at low levels of total exposure;
Lubin et al. (1997) assessed the ERR for all workers
with cumulative doses less than 50 WLM and derived
an estimate of 0.012/WLM (0.002-0.025). This
might be more informative for residential exposures
where most people accumulate less than 20 WLM.
On the other hand, the dose rate in the miners with
Figure 8-4 Dependence of lung cancer risk estimate on
dose based on a combined analysis of the lung cancer
mortality risk in 11 cohorts of miners (Lubin et al. 1994).