Civil Engineering Reference
In-Depth Information
4.5.1 Stochastic Effect
Assessing the stochastic effect quantitatively is not easy, as it is impossible, at the
present state of knowledge, to recognize whether a tumor developed as a result of an
exposure to ionizing radiation or for some other reason. Epidemiological studies of
large populations (atomic bomb victims of Hiroshima and Nagasaki) have been
used so far in attempts to determine the number of deaths from cancer exceeding the
number of deaths from cancer occurring from natural background radiation. The
results of the evaluation of the data from Hiroshima and Nagasaki can be related to
the dose of the preceding radiation exposure, and result in the dose/effect relation-
ship. However, the data calculated in this way do not constitute an immutable
quantity. For instance, the number of deaths from cancer increases with the age of
the Hiroshima-Nagasaki population under study. The International Commission on
Radiological Protection, in its publication ICRP 60 [
5
], for the first time took into
account data of the Hiroshima-Nagasaki population more than 40 years after the
atomic bombs were dropped, and extrapolated these data to the entire lifespan [
9
,
15
].
For determination of the risk associated with low doses, the roughly linear
dependence found at high radiation doses is now extrapolated back to the range
of low radiation doses. In this way, the entire cancer risk is rather overestimated in a
conservative sense.
Figure
4.2
reflects the hypotheses which can be extrapolated back from the data
of Nagasaki-Hiroshima. Curve A, which is favored by most scientists, is based on a
threshold level. Curve B, for which also a number of scientists argue, is based even
on positive healing health effects (hormesis) of a low radiation dose [
9
,
15
,
16
].
The linear dose-effect relation is recommended in ICRP 60 and ICRP 103 as a
conservative proposal.
4.5.2 Deterministic Effects of Radiation
Deterministic effects of radiation arise when a large number of cells are damaged
considerably by a high radiation dose such that regeneration is not possible or the
cells die. There are various threshold doses of radiation for the deterministic effect
in various organs of the body.
At a radiation dose of less than 1 Gray, most tissues produce no clinical
symptoms of disease (ICRP 60 [
7
]). However, there are exceptions:
- the male gonads:
>
0.15 Gray causes temporary sterility,
permanent sterility results at
3 Gray;
- the bone marrow reacts with disorders of the blood at
>
>
0.5 Gray.
