The Committee considered a brief report of the Workshop together with written comments from participants and other persons. In general terms, the Workshop was considered to have been very successful by most delegates.


The Committee decided to receive papers on non-cancer effects and further papers on tritium and microdosimetry. The Committee also decided to place on a 'B list' other matters (variability, synergism between chemicals and radiation, upper bounds on risks). It would return to these items if time allowed as these were relevant in assessing risks from internal exposures. The Committee also decided that it had not have time to deal with other issues (depleted uranium, test veterans). On the Committee's research priorities, the Committee agreed to press ahead with its existing requests to the CCRG (UK infant leukaemia rates after arrival of Chernobyl cloud in UK, and UK health effects following test bomb fallout) and the ONS (cancer rates near Bradwell from the estuarine hypothesis, and UK cancers in second generation following test bomb fallout).


The Committee decided that the Final Report would have to be more transparent than the preliminary one in recording the balance of views within the Committee on issues, and would have to be clearer on the conclusions reached on each issue. Detailed views would be contained in the technical appendices rather than the main text.


The Committee considered a paper on tritium's RBE and on the microdosimetric considerations of the beta particle from tritium's decay. A low radiation weighting factor of unity continued to be used for tritium, contrary to abundant RBE evidence indicating higher values. Possible tritium bioaccumulation in the environment could occur near chronic tritium discharges. On RBE, the low energy of the beta particle from tritium's decay resulted in high levels of ionisation within its average track length of 0.5 m -smaller than the average diameter of cell nuclei. From theoretical considerations, tritium's RBE, which came only from densely ionising track ends, was estimated to be a factor of two to three higher than gamma rays, and this was borne out by much experimental RBE evidence. Tritium's distribution was important as tritium decays in or near water molecules alone might lead to little effect, whereas tritium decays from OBT, especially OBT bound to DNA, would have a greater effect. Tritiated thymidine, a DNA precursor, and other tritiated proteins and precursor metabolites were discharged from Nycomed Amersham's Cardiff facility. The Environment Agency had reported tritium concentration factors of 104 and higher in flounder and plaice at Cardiff. Sellafield discharges were of HTO rather than OBT.

Radiation theory and RBE experiments suggested that tritium's RBE was greater than 1, with reported values between about 1.5 and 3. The recent draft ICRP Report 92 on RBE and wR had not recommended any change in tritium's wR. The draft had recommended that the reference radiation in RBE studies should be 250 kVp X-rays. Much experimental evidence with 250 kVp X-rays indicated RBEs of 2 to 3 for tritium which suggested that its wR should be increased to 2 or 3. As regards tritium's OBT form, reported high OBT concentrations in the environment would result in increased doses where human uptakes occurred. If tritium exposures resulted in increased concentrations in the cell nucleus, this would result in further enhanced doses to the DNA, as tritium anywhere in the cell nucleus would be within striking distance of DNA.


The Committee considered a paper which stated that a NRPB analysis had concluded that red bone marrow doses were within a factor of 2 of those used in the Darby et al Nordic study for one year olds. However, data on Sr 90 in bone for new-born infants suggested that foetal doses used in that study were low by a factor of about 2 for most of the period of interest. Overall, given the uncertainties involved, the dose estimates used in Darby et al were regarded as reasonably reliable.


The Committee considered a note of a meeting with the Childhood Cancer Research Group. This discussed the Committee's request for data on its proposed studies on UK infant leukaemia rates after arrival of Chernobyl cloud in GB, and GB health effects following test bomb fallout. Data on these studies was to be sent by the CCRG for the Committee to analyze.


The Committee further considered uncertainties in biokinetic and dosimetric models. US and EU dose experts had attempted to quantify uncertainties in the biokinetic models currently used to estimate nuclide distributions in organs following uptakes, and in dosimetric models to estimate organ dose coefficients of various radionuclides. The uncertainties, expressed as the ratio of the 95th to 5th percentiles of the calculated distributions, were very large in some cases, particularly in long-term concentrations of actinides in lung and bone, and in the resulting organ dose coefficients. Although the central values of the uncertainty distributions were usually close to ICRP values, in a few cases the uncertainty ranges for dose coefficients spanned three to four orders of magnitude.

The wide uncertainty ranges were partly due to differences of opinion on the chemical form of the intake, differences among experts' familiarity with uncertainty analysis, and some peculiarities in the ways in which uncertainties were combined. Also the models used by the study to derive doses and their uncertainties were not necessarily the same as ICRP models. Some contributing experts had different ideas about what they were analysing; others had estimated substantially smaller uncertainty ranges.

Concern was expressed about the reliability of dose coefficients for internal nuclides used in retrospective dose estimations in UK geographical areas. The current practices of calculating from central values only and omitting realistic estimates of uncertainties could result in false precision and incorrect conclusions. In particular, some argued that the magnitudes of the uncertainties in dose estimates for Seascale inhabitants might be comparable with the 200 to 300 fold shortfall in the doses identified by COMARE, for the excess leukemias to be ascribed solely to radiation exposure. The Committee agreed to recommend in the final report that the, sometimes large, collective uncertainties in dose estimations should be evaluated and included whenever doses were calculated for specific geographical areas or specific purposes.

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