Reply to: Observed biological effects vs regs

[BYRPV@ccmail.ceco.com]

The linear model is NOT out-of-date!  The BEIR V Committee's report
(1990) recommended the linear extrapolation.  There are uncertainties
associated with all of the studies purporting much lower risks or
hormetic effects.  For example, the studies of populations subject to
high natural backgrounds are hindered by the difficulty in establishing
appropriate control populations.  Groups such as BEIR, ICRP, and NCRP have
not ignored the studies showing no risk or even hormesis; they have not
found them to be convincing.  All these groups recognise the uncertainties
in extrapolating from high dose/high dose-rate data.  For example, the
BEIR V report states, "The committee recognizes that its risk estimates
become more uncertain when applied to very low doses.  Departures from a
linear model at low doses, however, could either increase or decrease the
risk per unit dose."  and later in the report, "Moreover, epidemiologic
data cannot rigorously exclude the existence of a threshold in the
millisievert dose range.  Thus the possibility that there may be no risks
from exposures comparable to external natural background cannot be ruled
out."  The ICRP and NCRP have recently endorsed the linear extrapolation,
with a reduction by a factor of two to account for lesser risk at lower
doses.
Moreover, there are studies purporting to show that the risks at low doses
are greater than predicted by BEIR V (e.g., the study of Oak Ridge workers
by Wing et al.)  However, these studies have also have flaws,
such as the failure to control for chemical carcinogens.
The problem is, no one knows what shape the dose-response curve should
have for small doses and low dose-rates.  We therefore use the linear
model until we have STRONG evidence to support a better one.  It is better
to be somewhat overly conservative than to later discover that we have
caused significant harm.  Failure to be sufficiently conservative in early
years doomed many early physicists, radiologists, radium dial painters,
and thorotrast patients.
In defense of the NRC (and there are many issues on which I don't defend
them), their current dose limits for workers are much less conservative
than those recommended by the ICRP and NCRP.  On the other hand,
I believe the
EPA's limits to be absurdly low and much too costly to society.
 - Ed Leidholdt
   San Francisco
The above view are my own and certainly do not reflect those of my
employer.