Comments on the ICRP foundation document draft
"Health Risks Attributable to Radiation" has been reviewed in the Low Dose Radiation Research Center, Central Research Institute of Electric Power Industry.
1. The LNT hypothesis (p.5, Lines.188-190 / pp.58-60, Chapter4, 4.4.5)
It should be appreciated that the Committee 1 clearly states that the LNT hypothesis is for the purposes of radiological protection, due to the uncertainty associated with the estimation of the risk from low dose/dose-rate irradiation. A fear or unreasonable concern about low level radiation/radioactivity of general public is based on the LNT hypothesis; the statement of ICRP that the LNT hypothesis is only for the purpose of radiological protection and does not necessarily reflect the actual effect of very low dose would reduce the anxiety of the public. However, in this regard, the statement that the use of LNT hypothesis is a "scientifically plausible assumption" (p.5, ll.188-190) may be misleading. "Practical" rather than "scientifically plausible" should be used.
2. Dose related to tissue injury (p.28 Chapter 3, Section 3.1.7)
In Chapter 3, Section 3.1.7, there is a discussion whether dose equivalent (Sv) or radiation weighted dose (Gy) should be used for the dose limits for the eye and the skin, and the use of equivalent dose is recommended. There seems, however, some confusion in the discussion.
(1) "Equivalent Dose" is recommended to be replaced with "Radiation Weighted Dose" in the draft of Recommendation 2005(S14)
(2) Equivalent dose is given by multiplying the absorbed dose with the radiation weighting factor. The radiation weighting factor is defined for the stochastic effects at low dose region; therefore, it is not reasonable to use the equivalent dose for tissue injury.
(3) Although the use of equivalent dose is recommended (Lines 1174-1175), it seems to contradict the recommendation of the Committee 2 to use RBE-weighted dose (p8 in "Basis for Dosimetric Quantities Used in Radiological Protection").
(4) It is very confusing to use the same name of the unit "Gy" for both RBE-weighted dose and absorbed dose. "Gy-Eq" recommended in the draft 2005 recommendations (Paragraph 94) may be appropriate for this purpose.
(5) The term "Tissue Reaction" seems not appropriate as a counterpart of the term "Stochastic Effect"; more reasonable term should be used.
3. Source of Epidemiological Data for estimating the human risk (pp.40-, Chapter 4, Section 4.4)
It is appreciated that the FD considered epidemiological data on medical exposure, occupational exposure, and environmental exposure as well as A-bomb survivors. However, we have another set of data which should be taken into consideration from research on health effects of those living in high natural background radiation area. The strength of these data are as follows; (1) they cover both males and females, (2) they cover all ages, (3) the exposure is protracted one at low dose rate, (4) the exposure was under non-stressful situation compared to medical exposures and A-bomb survivors. These data have accumulated and a number of qualified papers have been published to demonstrate that there was no increase in cancer mortality in those area where the natural background radiation is 3 times as high as the average (J. Radiat. Res., 41 Suppl., 31-41, 2000).
4. Change in the tissue weighting factors (p.56, Chapter 4, Table 4.3)
The tissue weighting factor has been changed from 0.2 to 0.08 and 0.05 to 0.08 for the gonad and the breast, respectively. In the draft 2005 recommendation, however, it was 0.05 for the gonad and 0.12 for the breast. The explanation for the change given in the draft 2005 recommendation was reasonable: the decreased contribution of hereditary effects and the increased contribution of breast cancer in the detriment. Detailed explanation should be necessary for the further change of the tissue weighting factors.
5. Hereditary Effect (p.76, Chapter 6)
Historically, the hereditary effects were one of the greatest concerns in radiological protection. However, under the situation that no hereditary effect has been detected in human beings, it should not be reasonable to estimate the risk based on the spontaneous mutation rate in humans and the doubling dose obtained from animal experiments. Assessing the hereditary risk in this way seem not in accordance with the conclusion that "Even in the absence of a true dose-threshold, any effects on IQ following in-utero doses of a few tens of mGy would be undetectable and therefore of no practical significance" (Lines 1211-1213, p29). The Committee could mention that there is "no practical significance" in the hereditary effects for low dose/dose-rate radiation, while the historical review on the hereditary effects would be very informative.
6. The equation on the line 2505 should be read as follows;
DD = (Average spontaneous mutation rate/average induced mutation rata)* (the dose at which the induced mutation rate is obtained)
7. Table7.1 (pp.102-103)
This table is a concise summary of the present FD. If the differences in the conclusions or numerical judgements from those in the previous recommendations are listed, it would be more informative.