Repair & DDREF

[FRAMEP@ORAU.GOV]

Howdy:

Regarding the linear no-threshold approach to risk estimates,
anonymous wrote:

>Have they forgotten repair and regeneration ?

The answer is absolutely not. Let me make 3 points.

1. Repair can be assumed to have occured at the high doses (i.e
greater than 20 rad) used to estimate the risks. In this sense,
nothing new is going on at low doses.

2. Repair mechanisms (DNA or otherwise) are not perfect. My car
doesn't come back from the repair shop completely fixed every time I
take it in (most times it does, but not always). Within limits,
sending twice as many damaged cars in for repair results in twice as
many unrepaired cars coming out. In fact, we sometimes refer to DNA
repair/misrepair mechanisms. The last time I took my car to the shop 
was for radiator work. There was nothing wrong with my parking brake
but they decided to repair it anyway. The brake doesn't work properly
now.

3. The current risk estimates (ICRP based) actually assume that the
risks are lower at low doses and low dose rates than they are at high
doses. In other words, we assume that the repair mechanisms are more
effective at low doses (this is perfectly compatible with points 1
and 2). A simple linear extrapolation from the risks seen at
Hiroshima and Nagasaki  would give a risk of approximately 10 in
10,000 per rem. This is divided by 2 to account for the lower risk at
low doses. That is why the risk to the general population is
currently 5 in 10,000 per rem. This factor of two is the dose and
dose rate effectiveness factor (DDREF). 

Anyone interested in this issue, whether or not you agree with the
results, should read ICRP 60. Of course, if you are a fan of
hormesis, you can argue that the repair mechanisms for some reason
are normally operating at a suboptimal rate and that radiation
stimulates repair. This might then be imagined to reduce the total
amount of unrepaired DNA damage, damage brought about by radiation
and everything else.

Hope this is of help

Best wishes

Paul Frame
Oak Ridge Institute for Science and Education
Professional Training Programs
framep@orau.gov