Biomedical Engineering Reference
In-Depth Information
Table 14.5
Exposure limits from NCRP Report No. 91
NCRP-91
Occupational Exposure
Effective Dose Equivalent
Annual
50 mSv
Cumulative
10 mSv
×
age (y) guidance
Dose Equivalent
Annual
150 mSv lens of eye;
500 mSv all other tissues and organs
14.7
Occupational Limits in the Dose-Equivalent System
ICRP Publication 60 (1991) and NCRP Report No. 116 (1993) appeared at a time
during which an extended and intense review of radiation-protection regulations was
being conducted by federal agencies in the United States. Prior to these two new
publications, radiation-protection practices were generally administered under the
system based on dose equivalent (Section 12.2).
Table 14.5 shows the earlier recommendations for occupational exposures given in
NCRP Report No. 91 (1987), the predecessor of Report No. 116. Compared with Ta-
ble 14.4, the effective dose equivalent in Table 14.5 was superseded in Report No. 116
by the effective dose,
E
. The respective numerical values for the annual and cumula-
tive limits are the same in both reports. The effective dose equivalent is defined like
E
in Eq. (14.4), with
H
T
then representing the dose equivalent instead of the equivalent
dose.
4)
The restrictions on effective dose and effective dose equivalent are employed
in both systems in order to limit stochastic effects of radiation. The equivalent-dose
limits for individual tissues and organs in Table 14.4 and the corresponding dose-
equivalent limits in Table 14.5 were made in order to prevent deterministic effects
from occurring.
There is a significant difference in the recommendations of Reports No. 91 and
116 for the limitation of annual intakes. Under Report No. 91 (p. 19), “The Annual
Limit on Intake (ALI) is the maximum quantity of a radionuclide that can be taken
into the body based on ICRP Reference Man
each year without the committed
effective dose equivalent being in excess of the annual effective dose equivalent limit
...
...
or the committed dose equivalent to any tissue being in excess of the nonstochas-
tic [deterministic] limit.” In both systems, the deterministic limits apply whether an
individual tissue or organ is exposed selectively or together with other tissues and
organs. As we saw in Section 14.6, values of the ARLI in Report No. 116 are deter-
mined from an annual 50-y committed effective-dose limit of 20 mSv. The analogous
ALI in Report No. 91 are based on a limit of 50 mSv. As a result of this reduction,
there are only a few radionuclides that could approach lifetime doses of concern for
4
In addition to the conceptual differences
between these two quantities (Section 14.4),
some radiation and tissue weighting factors
were also revised in Report No. 116.
