tions in one generation in a species. It is calculated as a ratio of the average
spontaneous and induced mutation rates in a set of genes. A small doubling
dose indicates a large relative mutation risk and vice versa. In humans, it is
considered to be of the order of 100 rad (100 cGy), but it depends on the
dose rate, the gender, and the type of species (BEIR V, 1990). Based on this
doubling dose, the ICRP has given an estimate of the probability of induced
hereditary disorders to be 0.6 × 10 −^2 per sievert (0.6 × 10 −^4 per rem) for the
working population (ICRP 60, 1991).
Genetically Significant Dose
The genetically significant dose (GSD) is the dose that, if received by every-
one of the entire population, would cause the same genetic damage as the
gonadal dose now being received by a limited number of individuals of the
population through medical procedures, natural radiations, TV viewing,
flying at high altitudes, and so forth. The GSD is an index of the expected
genetic damage on the whole population, and is calculated as an average
value from the gonadal doses received from all exposures by the exposed
personnel with proper weighting with respect to the chances of their having
offspring. Thus, the GSD depends on the total number of individuals irra-
diated and the relative expectancy of their having children. The weighting
factor is needed because older people have lesser probability of having off-
spring than younger people.
The contributions of various sources of radiation to GSD are given in
Table 15.2. The GSD values from natural radiation sources are considered
to be equal to the gonadal dose, because natural radiation exposes the
entire population of all ages uniformly. Of all medical procedures, diag-
nostic x-rays contribute most to the GSD. It is, therefore, essential that strict
protective measures are taken to avoid unnecessary gonadal exposure.
Long-Term Effects of Radiation 257Table15.2. Annual genetically significant dose (GSD) in the U.S. population about
1980–1982.
Source Contributions to GSD in mrems (mSv)
Natural sources
Radon 10 (0.1)
Other 90 (0.9)
Medical
Diagnostic x-rays 20–30 (0.2–0.3)
Nuclear medicine 2 (0.02)
Consumer products 5 (0.05)
Occupational ~0.6 (0.006)
Nuclear fuel cycle <0.05 (0.0005)
Miscellaneous environmental sources <0.1 (0.001)
Total ~132 (1.32)
Adapted with permission from Table 8.2 in NCRP report No. 93, 1987.