Biomedical Engineering Reference
In-Depth Information
ditionally defines collective dose commitments as the integrals over infinite time
of the average individual
H
T
and
E
due to a specified event, either for a critical
population group or for the world population.
Limits on Intake
NCRP Report No. 116 introduces the annual reference level of intake (ARLI). It is
defined (p. 59) as “the activity of a radionuclide that, taken into the body during a
year, would provide a committed effective dose to a person, represented by Refer-
ence Man, equal to 20 mSv. The ARLI is expressed in Becquerels (Bq).” (Reference
Man, the ICRP model for dose calculations from the intake of radionuclides, will
be described in Chapter 16.) As mentioned after Eq. (14.7), a radionuclide will be
retained with an effective half-life in the body that depends on both the radiologi-
cal half-life and the metabolic turnover rate. By definition, the committed effective
dose is delivered over the 50 y following an intake. If the radionuclide has an ef-
fective half-life that is short compared with 1 y, then an intake of 1 ARLI during a
year will result in an effective dose close to 20 mSv during that year. That is, the
effective dose in the year of the intake will be about the same as the committed ef-
fective dose. On the other hand, if the effective half-life is comparable to or longer
than 1 y, then the effective dose during the year of intake will be less than 20 mSv.
For a very long effective half-life, the effective dose each year will be considerably
less than 20 mSv, averaging
(20 mSv)/(50 y)
0.40
mSv y
-1
. The ARLI is keyed to a
cumulated effective dose of 20 mSv over the next 50 y after an intake. ARLI values
are computed for both inhalation and ingestion of a radionuclide. The NCRP and
ICRP specify that the annual effective
dose limit
(given below in Table 14.4) applies
to the sum of the effective dose from external radiation and the committed effective
dose from intakes during the year.
Theoretically, continuous intake annually at the level of the ARLI should main-
tain a worker's exposure within the basic annual and cumulative limits given in
Table 14.4. If several radionuclides are involved or if external radiation is a factor,
then appropriate reductions among the different ARLIs are to be made to assure
compliance with the basic recommendations set forth in the table. The limitation
of annual effective dose through application of the ARLI will also protect individual
tissues against the likelihood of deterministic effects.
The same quantity, also based on a committed effective dose of 20 mSv, is called
the annual limit on intake (ALI) in ICRP Publication 60. Values of the ALI are
given in ICRP Publication 68. Prior to ICRP Publication 60 and NCRP Report No.
116, the term ALI was used by both organizations. However, it was then based
on a 50-mSv committed effective dose equivalent, rather than the 20-mSv com-
mitted effective dose, and other criteria for deterministic effects, as described in
Section 14.7.
The NCRP defines the derived reference air concentration (DRAC) as “that con-
centration of a radionuclide which, if breathed by Reference Man, inspiring 0.02 m
3
per min for a working year, would result in an intake of one ARLI.” The working
=
