RE: Ancient radiation levels were higher than today's levels

["Karam, Andrew" <Andrew_Karam@URMC.Rochester.edu>]

From: Bradshaw, Keith [mailto:Keith.Bradshaw@nnc.co.uk]

	So my question is, what proportion of DNA lesions are caused by
background radiation?  
==========================
Dear Keith,

According to "The Handbook of Radiobiology" (Prasad, 1995, CRC Press)
radiation currently accounts for 1-6% of background mutations.  My
understanding is that lesion is damage to the DNA while a mutation is
unrepaired damage to the DNA that is carried forward to the next generation
of cells.

If we assume that radiation is currently 6% of overall mutation rates, then
raising background rad levels by a factor of 8 would mean that radiation
would account for about one third of all mutations.  This is because, of an
arbitrary 100 mutations at current levels, 6 would be radiogenic.
Multiplying this by 8 gives 48 radiogenic mutations of a total of 142 that
would then exist.  For the lesser case, mutations would increase from 100 to
107, of which 8 would be radiogenic.  

It's also important to bear in mind that the overall level of environmental
mutagens (such as free oxygen concentrations) has changed with time.  This
may mean that radiogenic mutations were much more significant in the distant
past than they are now, just because they had less "competition".

Sincerely,

Andy

Andrew_Karam@URMC.Rochester.edu
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