As per the ICRP (2007), the threshold dose is defined as
the “estimated dose for incidence (EDI) of a specific observable effect in 1%
of individuals exposed to radiation”. Tolerance dose is used to denote the maximum
amount of radiation a tissue can withstand without developing clinical signs of
injury in more than a few percents of individuals. “Clinically significant” term
is used to denote the level of severity which is detectable and is associated
with noticeable symptoms or sign of impairment of function.
“Cell death” term is used to denote the loss of the cell’s
reproductive integrity, without necessarily losing other cell functions. “Cell
survival” can be defined as the ability of a cell to proliferate indefinitely to
form a colony of its daughter cells.
Humans can tolerate a higher total dose of chronic,
low-dose-rate irradiation than an acute single dose. Chronic exposure provides
enough time to repair a sub-lethal injury to the cell. Injury to the cell is
repaired by the inherent repair mechanism in the body. In addition to this, there
are “adaptive reactions” at the cellular, organ, and whole-body level. That is
the reason why under chronic exposure situations, a higher total dose is required
to develop clinical signs of cell injury as compared to the acute single dose.
It is desirable that the dose limit (100 mSv in 5 years)
for occupational exposures is permitted under chronic exposure conditions.
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