- 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 onefourth as effective in producing mutations as the same
dose 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 jonizing radiations to which
we are exposed in nature.
Irradiation 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.
isotopes x40 and ol4,
Internal emitters include the radiowhich exist as a small percentage of
these elements and are normal constituents of the body,
and