Dear collegues, in my understanding of the never-ceasing obligations as a former member of the ICRU and former chairman and member of the German Radiation Protection Commission I want to submit two proposals for amendment of the draft of the 2005 recommendations of the ICRP. Let me first say that I am quite happy about the general quality of the draft and not at least about the fact that some recent recommendations of the German Radiation Protection Commission, e.g. on the radiation weighting factor, have in principle been adopted. However, I have also two proposals for change: I. A new special name for the unit of the quantity “radiation weighted dose”? This comment refers to paragraphs S14 (last sentence) and 51 (last sentence before eq. 1). The idea to coin the new term “radiation weighted dose in an organ or tissue” in order to substitute it for the term “equivalent dose in an organ or tissue” appears to be a good solution for the terminological problem of ICRP 60 that “equivalent dose” (ICRP, weighting factor wR) is not the same as “dose equivalent” (ICRU, weighting factor Q). However, the ICRP is now in danger of stepping into another terminological problem, and that is the idea to coin “a new name for the unit of radiation weighted dose so as to avoid the use of the name sievert both for radiation weighted dose and for effective dose” (§ 51). A problem here arises because there is the worldwide accepted fundamental concept of the Système International (SI) of physical quantities and units that the special features of a quantity are expressed in the definition and name of the quantity itself, whereas the unit is unspecific and often used for a “family” of related quantities. For instance, the unit Volt is used for direct voltage as well as for alternating or pulsed voltage, the unit Joule is used for electrical energy as well as mechanical, thermal, chemical or radiation energy, the unit meter is used to express the wavelength of light as well as that of sound, the diameter of an atom as well as that of the sun etc. Medical doctors daily use that systolic pressure and diastolic pressure are distinct by definition, but have the same unit, the mm Hg. In this sense, generations of physicists, technicians and medical staff have now been educated to distinguish related quantities by their definitions and names and not by the names of the units. In our field of dosimetry and radiation protection, the ICRU, in 1975, has applied for and achieved an exemption from this rule, in that the Comité International des Poids et Mésures accepted that absorbed dose should have the “special unit name” gray, and dose equivalent should have the “special unit name” sievert, although the SI unit of both quantities is the J/kg. This decision meant - now for almost 30 years - that all the weighted doses which are used in radiation protection, the ambient dose equivalent, the directional dose equivalent, the personal dose equivalent, the effective dose and the equivalent dose in an organ or tissue, have got the same name sievert for the unit. This has found general acceptance by many thousands of people working in the field, and there is neither the necessity nor the possibility to change this practice. Therefore the idea to “avoid the use of the name sievert for radiation weighted dose” is an idea at odds with the state of the art, without real advantage in present-day practice and without chance of being internationally accepted. The appropriate way would be to carefully explain the meaning of the new quantity “radiation weighted dose” to the users of the new ICRP recommendation and to clarify its distinction from the quantity “effective dose”, while both quantities have the unit sievert. I also want to mention the similar idea of § 94 to introduce another new unit name at odds with the Système International, the “gray-equivalent”. One may argue that its practical use will be very limited, that it will not appear in legal dose limits, and that the experts who have to deal with deterministic effects at relatively high doses and with the special RBE values prevailing at this dose level will know how to handle the proper use of units. However, the ICRP should hesitate to lend its hand to solutions hard to understand by those who want a clear system of concepts in radiation protection and who desire that ICRP takes the lead towards a proper terminology. The clean as well as the realistic solution is to go the same way as described above, i.e. to make the distinction by the name and well-described meaning of the quantity - the “RBE-weighted absorbed dose” - but not to modify the unit, the gray. II. Limitation of the application range of the LNT model This comment concerns a general point - the desirable limitation of the application range of the LNT model - and especially refers to paragraphs 38 and 101. Epidemiological studies presently are not able to reveal whether ionizing radiation at the level of the average radiation background is able to produce cancer in the amount predicted by the “risk coefficients” (cancer mortality or morbidity per unit of dose) obtained by research at higher doses. Radiobiological arguments - if not only single-cell effects including the bystander effect, but also systemic effects, especially of the anticancer components of the immune system, are properly accounted for - presently neither support nor contradict the assumption that the average natural background is producing cancer in the amount predicted by the risk coefficients derived at higher doses. The publication of the German Radiation Protection Commission: A. Kaul et al. (1987) “Possibilities and limits of the concept of collective dose”, Health Physics 53, 9 - 10, clearly requires to admit that we do not know the values of the risk coefficients at very low exposures of the order of the average natural radiation background. The recent publication (Sept. 28, 2004) by the Académie des Sciences and the Académie nationale de Médicine, Paris “La relation dose-effets et l’estimation des effets cancérogenes des faibles doses de rayonnements ionisants”, emphasizes the importance of the immunosurveillance system and refutes the arguments in favour of the validity of the LNT model at very low doses. In the light of these insights the ICRP might consider to take the appropriate steps to set realistic limits to applications of the LNT model: (a) The ICRP might consider to communicate and explain again the original purpose of the LNT model as conceived in ICRP 27, namely for the rough estimation of detriment by cancer in the framework of planning radiation protection measures such as the setting of dose limits. (b) On the other hand, the ICRP might consider to discourage the unreflected use of the LNT model to numerically assess the expected number of cancer death cases or the individual probability of causation in defined exposure groups at dose levels comparable with the annual natural radiation background, because such numbers must be considered as not having a safe scientific basis. Moreover, it should become clear that the accuracy of the calculation would not really become better by the eventual use of organ-, gender- and age-specific risk coefficients as long as it remains a scientifically not supported extrapolation into an uninvestigated dose region. I have seen that the ICRP, in its published proposal for the 2005 recommendations, has already started some steps in this direction. E.g., paragraph 38 expresses ”that any increment of exposure above the natural background produces a linear increment of risk”. On the other hand, an equivalent consideration of the natural radiation background has not been made in paragraph 101. Would it then not be equivalent to say in paragraph 101 “that in general and for practical purposes, cancer risk will rise in direct proportion to the increment of absorbed dose in organs and tissues above the natural background”? But let me say that these changes again appear as having a hypothetical character as long as they are not based on observed facts. For the time being - not at all discouraging further research - it might perhaps be better to state very clearly that for exposures of the order of magnitude of the average annual natural radiation background we presently do not know the cancer risk coefficients. To conclude, I am of course among those who do not challenge gross estimations, based upon the LNT model, for purposes of practical radiation protection such as the setting of dose limits. But I propose that ICRP might as well take the occasion of objectively stating that at the present level of insight, epidemiological as well as experimental, we do not posses any precise values of risk coefficients at doses of the order of the average annual natural radiation background. In the hope of mutual understanding and with the expression of my high estimation, Prof. Dr. Dietrich Harder, Göttingen.