We have heard a great deal about dose reconstruction and dose assessment this morning. I should like to start by trying to put these results into some sort of context.
Yesterday we heard about environmental monitoring, corroboration of monitoring results, laboratory inter-comparisons and sampling as well as some concern about future needs regarding 'hot particles', resuspen-sion, migration, quality assurance and uncertainties. All this information will ultimately be a major database for exposure and dose estimates. Why do we want dose
esti-mates? I can think of several reasons and I would like to mention two or three. First, in order to provide a basis for the IAEA's response to the request from the USSR, there is a need to determine the exposures and doses to persons residing in particular locations, with particular life-styles, under certain conditions, i.e. the controlled areas. It is also necessary to estimate anticipated exposures and doses to people who were to relocate else-where. This is probably the primary reason for attempt-ing to undertake the assessment of exposure and dose
Session 3
estimates. However, there are other reasons why one would wish to estimate exposure or dose. One is able to identify those individuals, subgroups or populations which might need or wish to be more closely monitored and followed for possible future health effects on the basis of their past or anticipated exposures. Finally, if there are to be reasonable, rigorous epidemiological studies in the future, there must be individual doses that can be calculated and assessed, perhaps even doses to specific organs such as the thyroid, bone marrow, and so on.
It is clear that since these reasons focus increasingly on the individual, there is an increasing commensurate need for individual, and therefore more difficult, dose estimates. The dose and consequent risk of living in a certain area can often be approximated by using average values and coefficients, but by the time we get to the other end of the spectrum with large scale epidemio-logical studies, every effort must be made to estimate individual doses. This point was emphasized yesterday by our distinguished Chairman in referring to the exten-sive efforts that have been made to address dosimetry studies in Japan. It is difficult to overestimate the impor-tance of dosimetry and dose reconstruction.
Let us review briefly what we have heard this morn-ing. (If anyone here is not familiar with the processes involved in dose reconstruction, I suggest reading the first three paragraphs of the Introduction to Part E of the Technical Report.) We heard this morning a review of dose methodologies carried out in the USSR, including both external and internal dosimetry. These dose recon-struction methodologies were based upon data obtained from both direct measurements (e.g. whole body counts for caesium and thyroid measurements from iodine) and indirect measurements (e.g. dosimeter readings, expo-sure rate meaexpo-surements, ground deposition values of caesium and strontium). Some of these were not always as clear as one might wish, but there were obviously sufficient bases for reviewing and assessing them.
Specific dose assessments in seven settlements were carried out by the Task Group, whose methodologies were based upon both measurements and modelling, and included both external and internal exposures. The results of these independent assessments were compared with similar assessments made by Soviet scientists for the seven settlements. Considerable time was spent com-paring the two assessments, and we have heard several times that there were factors of approximately two to three difference between them (i.e. the Task Group dose assessments were usually lower than the Soviet
assess-ments by a factor of two to three). I would like to re-emphasize what Dr. Anspaugh mentioned earlier: con-sidering all the uncertainties and all the parameters that must go into these calculations, I think the comparison is very close. I doubt that it would be much closer even if carried out under more desirable circumstances. I can say that in the USA we are engaged in retrospective thyroid dose calculations due to iodine resulting from the atmospheric nuclear weapons tests at the Nevada test site; we are going back nearly 40 years in some of these calculations, and it is not an easy thing to do. I think we can all appreciate the complexities and the difficulties that have been encountered, and I think it is reassuring that the two independent approaches should result gener-ally in comparable findings.
The issue of 'hot particles' was raised again and there seem to be differing opinions about their relative impor-tance. I think all this points to is the need for further studies to identify how prevalent such particles are, what they are made of, and whether or not they are biologi-cally significant. The issue of caesium transfer in soil was touched on and responded to. The issue of data availability was raised, and I am sure that the Task Group would welcome any additional data that perhaps, being unfamiliar with the language and circumstances, they did not ask for in an appropriate manner and or at an appropriate time. I am sure that they would welcome any additional information that anyone can give them.
The issue of short lived isotopes was raised and was addressed by various people, some more precisely than others. Finally, one or two questions were asked about ICRP methodologies and philosophies, but I don't think this is the time to go into that subject.
I shall close with one or two observations. First of all, with regard to the reasons I gave as to why one would do dosimetry in the first place, it is clear from the Report and discussions that there is a reasonable basis for esti-mates of average exposures and doses in terms of com-munity locations and populations. It is also clear that the data currently available are not yet detailed enough to give the individual exposures that would ultimately be necessary for longer term epidemiological studies. The issue of uncertainty was raised again, with its com-panion, quality assurance, but I don't think we need dis-cuss them any longer. Nevertheless, continued dose reconstruction efforts are anticipated, and reference has been made to studies commencing in July of this year as well as some forthcoming meetings, so I have no doubt that we shall be hearing more about dose reconstruction of exposed populations in the USSR.
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Session 4 HEALTH IMPACT
Objectives and approach F.A. Mettler (USA)
INDEPENDENT CLINICAL HEALTH STUDIES General health situation
Cardiovascular disorders H.D. Royal (USA)
Haematology A. Kuramoto (Japan)
DISCUSSION
L.A. Buldakov (USSR): I would like to ask about the range in the ten year old age group. Does this mean exactly ten years, or is it 10-12 or 10-14 years? I would also like to ask about the range in other age groups.
F.A. Mettler: For practical purposes it is exactly two years, exactly five years, exactly ten years, exactly forty years. It is not a range; every one of those children and people was born in that year. In some cases we actually have it down to three months of that year (e.g. January, February, March of 1985), but they are not age ranges, they are specific slices in that year, and they are exactly that age.
Question from a Japanese participant: Did you also examine the mitotic connected anomalies quantita-tively? This was a very useful examination, for example,
for the victims of the Oak Ridge criticality accident many years after exposure.
F.A. Mettler: We did not examine that. You have to remember that, in many cases, we were hundreds of kilometres from an airport. You will also see later that we had some difficulties in doing cytogenetics because of the time it took to get to the laboratories where this analysis had to be done. For immune studies we res-tricted ourselves to fairly routine things that could be done in the field, and I think that helps us to understand Dr. Kuramoto's remark that there are very sophisticated immunological tests, but these require laser cell sorters and very competent people with superb access to sup-plies. These tests might be very interesting to do in the future, but we just could not do them on this trip. That is why we have to say that we cannot rule out subtle immunological abnormalities, but there are no huge abnormalities.
Sessions 4 and 5
INDEPENDENT CLINICAL HEALTH STUDIES (cont.)
Thyroid M.C. Sheppard (UK)
Cancer K. Mabuchi (Japan)
DISCUSSION
A.K. Gus'kova (USSR): Are you acquainted with the Osechenskij and Ivanov data for the BSSR? This material has been published and it fully confirms your view that there was only an increased frequency of chronic lympholeukosis and a general but slight ten-dency for the disease rate to rise even before the acci-dent. Our BSSR colleagues have these data and they could provide you with a copy.
F.A. Mettler (USA): Perhaps I should answer the question. Yes, we actually saw some data like that, but I think the conclusion of our expert epidemiologists was that the tumour registry system was not good enough for firm conclusions to be made. We did see a lot of data in the BSSR and UkrSSR and we looked at everything as a whole. On one hand, there is no evidence to suggest an increase; on the other hand, one might ask whether there could be an increase that was not detected by the system. We think that is possible. We don't know the answer.
A.J. Gonzalez (IAEA): I presume that your health effect numbers from the Hiroshima and Nagasaki data correspond to doses which are about two orders of
mag-nitude higher than the doses that the dosimetry group has given for the Chernobyl general population living in contaminated areas. To obtain the same statistical power, if the Chernobyl doses are two orders of mag-nitude lower than in Japan, the number of people in the sample would have to be four orders of magnitude higher, and obviously we do not have so many people in the contaminated areas. In addition, we have all the demographic problems that you have indicated relative to studying Chernobyl: problems with the registries, lack of data, old data, etc. Given these constraints, do you believe that there is really a chance of any late effects being detectable in the future by any epidemio-logical study? I am not suggesting that we should not undertake epidemiological studies, but it is convenient to put them into a realistic perspective.
K. Mabuchi: That is the point; I think we should.
There are many problems, but none of them is insoluble.
I think we can solve them.
F.A. Mettler: There may be subgroups of children where thyroid cancers are very easy to see, so the answer to Dr. Gonzalez' question is, yes, it is possible to design studies where you can clearly see differences between control and specific exposed populations.
34
Health Impact: Discussion