11.6. Biological Effects of Radiation 657
Now, a cell is composed of a very large number of atoms. Not all of them have the
same importance in terms of cell functioning. The most important part of the cell
are the chromosomes, which carry the DNA. A damage to the chromosome has the
potential of causing cell mutations leading to genetic effects and cancer development.
Unfortunately these effects occur very slowly and take a long time, on the order of
years, to become noticeable. All types of ionizing radiation, no matter how intense,
are capable of causing such damages. However the probability of their occurrence is
seen to decrease with decreasing radiation flux or intensity.
Cell mutation is not the only type of damage caused by radiation. A high level
of dose can, in fact, cause so much ionization that the cell dies. If a large number of
cells in an organ die, they can cause the organ to malfunction or even lead to total
failure. The gravity of the situation can be appreciated by noting that, according
to different studies, it is believed that receiving a whole body dose of approximately
500 remfor a few minutes can cause death to the 50% of the population. However
such a high dose is expected to be delivered only in extreme radiation environments,
such as atomic explosions or accidents of nuclear power reactors.
Fig.11.6.1 shows the possible effects of radiation on biological cells. As shown
in the figure, one possibility is that the cell simply dies after irradiation. This is
of not much concern unless a large number of cells in an organ die, causing it to
malfunction or fail. The other possibility is that the cell does not die but gets
damaged. By damage we mean that the bonds between atoms get weakened and
as a result new chemicals are formed. The natural repair mechanisms of the cells,
for most part, take care of such localized damages. The probability that the repair
is successful depends on the extent and locality of the damage. However it should
be mentioned that any part of cell can get repaired including the chromosomes.
In fact, the cell damage and repair occur constantly in biological organisms even
without irradiation. The damages caused by low level radiation are therefore not
much different than the damages caused by other sources, such as foreign chemicals
and autoimmune disorders.
Unfortunately not all repair mechanisms always function perfectly. For example,
if at the time of irradiation a cell is in the middle of performing a certain task, the
repair may not go through successfully. This might cause the cell to start operating
abnormally. Such a cell is the one that can eventually cease to reproduce or start
reproducing uncontrollably. The latter is the source of the development and spread
of cancer or genetic disorders. If the cell does not reproduce itself, it can still cause
operational problems and genetic effects. However such effects will be localized
unless a very large number of cells start behaving in like.
A point to note is that all cells are not equally prone to radiation damage. For
example, the cells that produce blood have been found to be highly susceptible to
radiation damage. In general, the more reproductively active a cell is, the more
sensitive it is to the damage.
Now that we understand the basic radiation damage mechanisms to individual
cells, let us discuss the macroscopic effects of radiation dose.
11.6.AAcuteandChronicRadiationExposure
It has been found that both high instantaneous and high integrated doses are harmful
to health. A high instantaneous dose leads to the so calledacute exposure, while a low