- 19 repair could be complete.
For genetic tissue it is gener-
ally believed that there is no repair of damage once
incurred,
but in a recent experiment by Russell with mice,
" ..low dose rates of radiation turned out to be only one-
fourth as effective in producing mutations as the same
dese given at high dose rates." (Joint Committee on Atomic
Energy 1959).
At the present time experimental data on
the biological effects from very low radiation doses are
not available;
therefore,
it cannot be said positively
that there is or is not a threshold dose.
Before considering the hazard from radioisotopes
that have been added to our environment by man,
consider-
ation will be given to the lonizing radiations to which
we are exposed in nature.
[rradiation by natural sources
is relatively constant in any one area but varies from area
to area with local geological conditions.
External emitters
from natural sources include cosmic rays and the radioactive
isotopes present in the crust of the earth and in the air.
Cosmic rays account for about one quarter of the natural
background radiations.
Internal emitters include the radio-
isotopes K4O and ol4, which exist as a small percentage of
these elements and are normal constituents of the body,
and