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At low doses, it is considered that the effect of ionising radiation on living matter depends
upon the radiation type, R, and on whether the dose is to the whole body or to particular parts. This is
incorporated in the notion of effective dose to the organism, E (Sv) which is related to the dose absorbed
in a tissue or an organ exposed to radiation R, D TR (Gy) by:
E W W D
R
= ∑ T∑
T R
where W T is a weighting factor for tissue T, and W R is a quality factor, which is nevertheless called a
weighting factor, for radiation R. Values of W T and W R are recommended by the ICRP Publication 61
[164], The equivalent dose H T is expressed as:
H
R
= ∑W D
T R TR
R
and represents the threshold dose to a particular tissue. This weighted dose is expressed in J/kg, but this
unit is given the name of sievert (Sv).
Thus the gray must be used in speaking of deterministic somatic effects, and the sievert must
be used in speaking of stochastic effects.
In a given set of exposure conditions, each becquerel of a given radionuclide will cause an
effective dose E N (Sv/Bq) which depends on its nuclear properties (see some examples for internal
exposure in Tables 2.16 and 2.17). The conversion factor from activity to dose is thus E N (Sv/Bq).
Exposure due to a set of radionuclides of given activities, A RN , thus leads to a value of the
effective dose to the organism, E:
∑
E( Sv) = E ( SvBq − 1
) × A ( Bq)
RN
We shall return to these points in discussing incorporated radionuclides.
RN
TR
RN
4.1.2 Exposure related risk
The risk associated with exposure can be called Radiological Risk, RR, (not to be confused
with Relative Risk also sometimes abbreviated as RR), and can be expressed by a general formula of
the type:
RR (time -1 ) = [probability of a detriment per unit dose(dose -1 )]
× Σ [probability of occurrence of an event i (time -1 )]
× [dose delivered by event i (dose)]
Hence:
RR (time -1 ) =
p (dose -1 ) × Σ P i × (dose) i
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