Re: doubling dose/"low level" -Reply

[Charles Willis <CAW@nrc.gov>]

Good Morning,

The low dose, low dose rate expression has developed in interesting ways.  In
some contexts, we tend to think of low level radiation as being well below the
average levels from natural sources.  On the other hand, when the ICRP and
NCRP addressed the "dose and dose rate effectiveness factor" (DDREF), they
defined low dose and low dose rate as being quite high.  The reason, of course, is
that they were looking for biological effects for which the effectiveness of high and
low dose rates could be compared and few effects are seen at low dose rates.  For
example, in the famed "induced pink events" in Tradescantia, where the effect
seems linear with dose down to about 250 mrad, the effectiveness of radiation
decreases as the dose rate is decreased.  However, the data do not extend to dose
rates below 72 rad/day.  Similarly, when experiments were done with Sr-90 in mice,
when the dose rate during the first 100 hours was dropped to 15.3 rad/day, the
irradiated mice lived longer than the controls (and had about the same number of
bone sarcomas).   Of course, effects have been seen from lower dose rates; in
Charlie Meinhold's popular presentation on the biological basis for the current
recommendations, he presents data on biological effects from exposure at rates as
low as 8.3 rad/day!

So what is meant by "low level radiation"?  It seems to depend on the context.

Charlie Willis
caw@nrc.gov