|A working group in the Central Research Institute of Electric Power Industry (CRIEPI) has discussed the draft of 2005 ICRP recommendation. Our comments are focused on 4 aspects: LNT hypothesis, Dose Constraints, Exclusion, and Dose Rate Effects/DDREF.
1. LNT hypothesis
(1) In the new recommendation ICRP adopted the linear no-threshold (LNT) hypothesis for the estimation of cancer risk from low dose radiation. For these years, however, a lot of scientific data, both experimental and epidemiological, have accumulated to suggest that the cancer risk from low dose/dose-rate irradiation should be lower than that predicted by the LNT hypothesis. On the other hand, it is also true that due to the uncertainty associated with the risk assessment, the existence or absence of threshold cannot be proved. Under these circumstances it is understandable that the ICRP takes a practical approach regarding the radiation effects, i.e., the LNT hypothesis with some cautions as in Paragraph 54. At the same time, we must remember that the general public tends to believe that LNT hypothesis proves that radiation is harmful no matter how low is the dose. The fear about radiation among the general public hinders the use of radiation for our benefit. It is therefore necessary to be very cautious when mentioning the effects of low dose exposure.
(2) Therefore, we suggest some modifications in several paragraphs:
1) The parts in paragraph S6 shown below in parentheses should be deleted since these explanations are too strong to express the effect due to tiny exposure.
gIn all situations the constraints are complemented by the requirement to optimise the level of protection achieved. This is because there is presumed to be some probability of health effects (even at small increments) of exposure to radiation above the natural background. (The Commission therefore recommends that further, more stringent, measures should be considered for each individual source.)h
2) With the practical approach in mind, the last sentence of paragraph 38 seems exaggerated. The word ganyh should be deleted.
3) The word, gscientificallyh in paragraph 101 should be deleted since the LNT hypothesis is adapted for practical purposes rather than scientifically.
4) In the last sentence in paragraph 187 the phrase, geven at small doses above the natural backgroundh should be deleted. It is not necessary to emphasize the risk due to small doses.
(3) As the recommendations of the ICRP have a great influence not only on regulatory bodies and radiological protection experts, but also on our society, the ICRP should make more efforts to clear up the misunderstanding of the general public on low dose exposure.
Therefore, it might be useful to create a new section g4.3 Basic concepts of risk estimationh after the section 4.2. The new text explains the LNT hypothesis and its usage in some more detail.
1) Exposure of a few tens of mSv has no significant health effects, as can be judged from the fact that the world average annual dose due to natural radiation is estimated to be 2.4 mSv and some places have more than ten times higher than the average. Strict control of radiation even at low dose levels is required simply to monitor the effectiveness of radiation protection, not because low dose levels pose any danger to workers or to the public.
2) The ICRP adopts the LNT hypothesis for practical reasons: it is simple, easy to use, agrees relatively well with scientific assumptions and allows us to sum doses.
3) The ICRP recognises that the health effects due to exposure of a few mSv could not be detected although the risk could be estimated using the LNT hypothesis.
4) The LNT hypothesis should not be used to assess risks of stochastic effects in retrospective situations for exposures in identified individuals, nor should it be used in epidemiological evaluations of human exposure since it is defined for radiation protection purposes.
2. Dose constraints
The purpose of the radiological protection is to protect INDIVIDUAL from excess unnecessary exposure to ionizing radiation. Therefore, it is not necessary to apply the dose constraints for radiation workers whose exposure is monitored INDIVIDUALLY by personal dosimeters.
(1) The description on 0.01mSv from Table S1 and Table 7 should be deleted.
TableS1 and Table 7 give maximum dose constraints. It is not appropriate to show a minimum dose constraint in the same table.
(2) A value of 0.01 Bq/g recommended as an exclusion activity concentration for artificial alpha-emitters is not consistent with 0.1 Bq/g reported in IAEA RS-G-1.7. The difference beyond an order between ICRP and IAEA has not well-discussed internationally yet. Such an exclusion level should be essentially given as a dose criterion. There should be a further discussion on the irrationality due to using the different dose criteria between naturally occurred and artificial radionuclides. Therefore, some tables and paragraphs listed below regarding the level of radioactivity for exclusion from the recommendations should be deleted.
1) Paragraph S12: gThe Commission has concluded that the values in Table S2 provide a practical definition of what is to be considered radioactive and therefore the levels at which materials are to be within the scope of its recommendations. It now recommends the figures in Table S2 as the basis of exclusion from the scope of its recommendations.h
2) Table S2 that shows the recommended Exclusion Levels.
3) Paragraph 24, the last two sentences, to delete gThe Commission considers that numerical criteria for exclusion would assist in the consistent application of the concept. Its recommendations are found in Chapter 8. h
4) Paragraph 204-212 and Table 10 in Chapter 8.
4. Dose Rate Effects/DDREF
It is well known that biological effects, including carcinogenic effects, of ionizing
radiation highly depend on dose rate as well as dose. The dose rate effects have been taken into consideration in the ICRR recommendation as the Dose and Dose Rate Effectiveness Factor. The value of 2 was given for any dose below 0.2 Gy and any dose rate below 0.1 Gy/hr (paragraph 74 in Publication 90); however, it is hard to believe, from biological point of view, that there is a discrete change in radiosensitivity of living organisms at 0.2 Gy and 0.1 Gy/hr and that the sensitivity is constant below these dose and dose-rate. For these years, experimental and epidemiological data have accumulated on carcinogenic effects at much lower dose rate. Many of them suggested that the DDREF should be larger than 2. These data should be extensively examined to re-evaluate the DDREF. Furthermore, in the future, radiological protection system should be constructed on the basis of both dose and dose-rate.
The working group on comments on ICRP recommendation, CRIEPI.