|First of all, let me congratulate for the new draft of the recommendations, which is much more in line with the previous ICRP documents (in particular justification principle is welcome back), took into account many of the comments made on the first draft and is also much clearer and understandable.
In the following some comments are reported.
Page 26 (80) “…The Commission recognises that there are particular uncertainties on the risk of radiation-induced solid cancers following in utero exposure. Nonetheless, the Commission considers that it is prudent to assume that life-time cancer risk following in utero exposure will be similar to that following irradiation in early childhood…” and (81) “The Commission recommends that in utero exposure should not be a specific protection case in prolonged exposure situations where the dose is well below about 100 mSv.” Comment: what is well below 100? Is it 80 or 50 or 10? It is an exposure situation of 100 mSv/y or 100 mSv during the pregnancy? It is not enough clear. How can we consider this figure compared with the 1 mSv for the remainder of pregnancy (see n.(177))?
Some of the conclusions of ICRP Publication 90 Biological Effects after Prenatal Irradiation (Embryo and Fetus) are the following. (409) “Low-LET radiation of 0.1 Gy or less can cause pre-implantation death at certain radiosensitive stages” (see (7): “…no observation in humans are available… therefore risk analysis can only be achieved on the basis of animal experiments which have been performed with mice and rats”). Comment: which are the uncertainties in applying these results to women? Moreover, it can be argued that in fixing this undefined level of “well below 100 mSv” pre-implantation death is not considered. Pre-implantation death could not be a problem for the society, but surely it is for the woman (and possibly the couple!) involved. (433) “There is pronounced experimental evidence of high radiosensitivity of the developing brain, and of thresholds for the numerous phenomena of damage… The lowest experimentally observed doses causing persistent damage at the anatomical and structural level are in the range of 0.1-0.3 Gy of acute low-LET radiation.” Comment: even if the risk may be lower for prolonged exposures, is “well below 100 mSv” enough conservative? (456) “Analyses of the atomic bomb data suggest that the magnitude of cancer risk in adulthood from in-utero exposure may be similar to that from radiation exposure in early childhood”. Comment: the high radiosensitivity of small children compared to adults is well known. Is the proposed maximum dose enough conservative, too? Moreover, at the end of ICRP 90 a list of open questions and needs for future research shows the still great and numerous uncertainties on these estimates. For example (463) “The risk of defects in brain development is still not clear in the low dose range at low dose rate…” (464) “…It cannot be excluded that individuals who received slight developmental defects of the brain from prenatal irradiation may suffer more strongly from mental deficiencies in old age than unexposed individuals. This is a completely open field that should be studied.” (466) “… It is clearly important to study the lifetime risk after prenatal irradiation instead of considering to age 15 years. Long-term follow-up of the in-utero irradiated atomic bomb survivors is therefore a high priority.” Conclusions: are the cited statements of 2006 ICRP Recommendations (in 80 and 81) enough prudent? Do they respect the precautionary principle?
Page 34 paragraph 4.3.4 Comment: the effective dose is defined in this document as in ICRP Publication 60, but many parameters of the calculation were changed: in the future it will not be possible to understand which type of assessment was made. Is not it a possibility of confusion and non comparability of data? In (153) it is concluded “Despite changes in dosimetric modelling, as well as differences in the computation of effective dose, previous assessments of equivalent dose or effective dose should be considered adequate” and this is understandable for the consistency of the system. But the problem remains, I imagine, reading scientific papers, too.
Page 58 (219). Comment: the sentence “It is responsibility of national authorities to select this constraint, which should not exceed 100 mSv per year and typically should not exceed around 20 mSv per year” is not convincing. First of all, these values are really impressive if compared with the limits for workers, considering that in the populations also infants, small children and pregnant women are present. Moreover, as it is well known, the exposure due to natural radioactivity is generally the highest received after the medical one. Looking at UNSCEAR Report 2000 page 139 in Table 30 the Distribution of population with respect to total exposure to natural sources is reported. The typical value is lower than 2 mSv/y (fraction of total 0.39 + 0.30= 0.69), whereas a fraction of total of only 0.0004 is exposed to doses higher than 10 mSv/y. Indeed, UNSCEAR concludes that “ On this basis, worldwide annual exposures to natural radiation sources would generally be expected to be in the range 1-10 mSv, with 2.4 mSv being the present estimate of the central value”.
Moreover, high contamination situations as the Chernobyl zone or the atomic test sites cannot be considered typical.
Therefore, in my opinion both the two constraints suggested for existing exposure situation should be much lower and in particular the typical one should not exceed around 2 mSv/y.
Page 67 (260) “Prenatal doses from most correctly performed diagnostic procedures present no measurably increased risk of prenatal death, developmental damage including malformation, or impairment of mental development over the background incidence of these entities”. Comment: with the uncertainties underlined above, can this be stated without any doubt? It seems it is not the case. For example probably prenatal deaths can occur, even if we cannot detect them, and in any case can not this be interpreted as a suggestion to radiologists to act without any prudence on pregnant women?
Page 67 (261) Comment: the sentence “Almost always, if a diagnostic radiology examination is medically indicated, the risk to the mother of not doing the procedure is greater than the risk of potential harm to the embryo/fetus” seems to be too optimistic. It is well known that in many countries there is a large abuse in the use of diagnostic procedures with ionising radiation and the competence and experience of the medical practitioner or the radiologist on the effects of ionising radiation, the justification principle and doses to the patient are not so deep. As above: is not preferable to apply the precautionary principle and recommend to limit exposures of pregnant women? In any case, foetus and embryo should be protected as much as possible, therefore a general recommendation that exposure to ionising radiation of pregnant women should be avoided or be kept as low as possible seems to be necessary.
Page 67 (266) Comment: The sentence “This definition includes the exposures of families and friends of patient discharged from hospital after diagnostic or therapeutic nuclear medicine procedures” read carefully is clear, but in any case in order to avoid ambiguity, it is suggested to add a specification to clarify that friends in this case are not visitors, but people helping the patient.
Page 68 (269) Comment: medical exposures requested by insurance companies are heavily under discussion. Are they justified? The principle of justification states that (see n. 30) “… a new exposure situation should do more good than harm, i.e. yield an individual or societal benefit that is higher than the detriment it causes”. Which is the societal benefit from exposures requested by insurance companies? Is not it only an economical benefit for the insurance company? The statement weakens or definitely cancels the possibility of the citizen to refuse an exposure of this type.
Moreover, as far as other types of legal exposures are concerned (for security reasons, immigration, etc.) a recommendation to national authorities to analyse their justification and optimization seems to be needed.
Page 73 (294). Comments: the paragraph is unclear. In radiation protection exclusion has always been considered for exposures to ionising radiation (like cosmic radiation at ground level, K-40 in the body, etc.) which are unavoidable. Here the term exclusion is also used for exposures to ionising radiations - due to material with activity concentration lower than some fixed values – which are largely avoidable, defining them uncontrollable. Several comments can be made to this choice. First of all this is confusing, because the former type of exposures is completely different from the latter one. Mixing the two concepts does not work for clarity. Secondly, not all the exposures to activity concentrations below the stated values are uncontrollable, on the contrary some are easily controllable (e.g. the use of byproducts in building material: it is enough to reduce the percentage of byproducts allowed to be used). Moreover, natural radionuclides have much different radiotoxicity per unit activity concentration, in themselves and depending also on the scenarios of exposure (indoors or outdoors, external or internal, inhalation or ingestion, etc.). Therefore, the choice of only one value for all the radionuclides in the natural chains, could exclude high exposures from regulations. Indeed, sources below 1000 Bq kg-1 for the heads of uranium and thorium series and 10,000 Bq kg-1 for 40K, can expose population to doses of some tens of mSv. Building material is one (see in annex 1, the calculation already sent as comments to the first draft of new ICRP Recommendations, now completed with an assessment of doses due to radon concentration), but not the only, example. Therefore the statement in The Scope of radiological Protection Regulation (see n.94) that “..such levels are coherent with the 1 mSv in a year criterion”, is simply not true. Indeed, it is already denied in note n.54 of the same document, where it is said that “…in the case of building materials having a high concentration of several natural radionuclides may result in doses up to 15 mSv”.
Page 73 (296). Comments: 1) the European pooled analysis showed that lung cancer risk for residential exposures to radon is statistically significant even at concentration lower than 200 Bq m-3. This is an important conclusion - which is not cited and - which should suggest to avoid to propose - in The scope of Radiological Protection Regulation n.(113) -”… a minimum exemption level of 200 Bq m-3 for exposure situations to radon in dwellings."; 2) notwithstanding the cited strong influence of the smoking or non-smoking status on the cancer risk, no comment or application of this difference is made in all the paragraph; 3) an absolute cancer risk is highly dependent on age of the exposed person and the cohort considered. For this reason it is difficult to understand how the absolute risk cited could be taken into account for estimating the risk from domestic exposure as suggested in (297), without any proposal or suggestion given by the Commission. 4) as regards the sentence “The current available epidemiological evidence indicates that risks other than lung cancer from exposure to radon (and decay products) are likely to be small.” it should be stressed that very few and limited studies were carried out on radon and pathologies other than lung cancer, and these have nothing to do with the recent pooled analysis. The sentence at the end of the paragraph, which begins with the pooled analysis, could be misinterpreted.
Page 74 (301) and table 6. “Even though the nominal risk per Sv has changed slightly, the Commission… retains the relationship between the constraint of 10 mSv given in Publication 65 and the recommended corresponding activity concentration.” Comment: the change was not only in the nominal risk per Sv, but also (see in (297)) in the risk per unit exposure of radon, and therefore with the “conversion convention” the relevant dose. Therefore, even if for the sake of continuity the ICRP 65 choices are reaffirmed, this change should be quoted, too.
The two proposed value for the constraints for Rn-222 are really too high. For domestic dwellings 600 Bq m-3 is three time the value under which the risk of lung cancer was showed to be significant (see above). This is not a problem of precaution, but of preventing from shown effects. Moreover, nothing is said about new buildings, for which a goal more ambitious than for existing building could be chosen. ICRP 65, even if abandons the two levels, devotes a paragraph (4.2.3.) to stress the possibility of preventing radon in new building, whereas here nothing is said. Finally, there are no news about the risk of infants and small children due to radon indoors (lower breathing capacity, but higher breathing rate and radiosensitivity). Therefore, it is suggested to repeat in these recommendations the concepts of paragraph 5.1.3 of ICRP 65 - regarding workplaces used by members of the public - which maintains that schools (in particular, but not only) “…should be treated as dwellings for the purpose of setting an action level for remedial measures”.
1500 Bq m-3 for workplaces is too high for two reasons: first of all it is easy and not much expensive to remediate an indoor environment with such a radon concentration. Secondly, at a radon concentration between 1000 and 1500 Bq m-3 the risk is really high and exposes the worker to a dose of category A workers, which could possibly be summed with other doses (gamma from building material and/or ionising radiation due to the work itself). Moreover, this exposure is higher than that already chosen by IAEA (even if with a different meaning), suggested by EU Commission in the document Radiation Protection 88 and foreseen in most of national legislation.
Finally, I am really doubtful if the term constraint (or, as proposed recently planned exposure) is correct for radon. In (302) it is said that “It is the responsibility of the appropriate national authorities… to establish their own constraints … All reasonable efforts should be made to reduce radon-222 exposure at home and at work to below the constraints that are set…” It is not better to leave the old term of action level? A level at which remedial action are in any case justified and needed? It is difficult to think at a planned exposure of 10 mSv/y and in any case these levels have a complete different meaning from the constraints for artificial radioactivity.
Page 75 (304). Comment: the few statements on thoron appear too weak and not very useful. In particular, the sentence “…it is usually sufficient to control the intake of the decay product, lead 212…” seems not adequate. Moreover, the following sentence about the non applicability of the conversion convention suggested in Publication 65 seems of scarce utility: what should be done?
A short document - prepared at the beginning of this year for the art.31 group of experts – on the state of the art about thoron monitoring and dosimetry is enclosed as Annex 2.
Exclusion level for natural radionuclides applied to building materials
C.Nuccetelli, S.Risica, Istituto Superiore di Sanità
The proposed exclusion activity concentrations (see 294) of “…1000 Bq/kg for the heads of uranium and thorium series and of 10,000 Bq/kg for 40K…”, were used to assess the possible exposure of inhabitants due to building materials.
Gamma exposure due to building materials
The EU publication Radiation Protection 112, Radiological protection principles concerning the natural radioactivity of building materials (1999), using well-established results of room models, states a dose criterion. This is made
1) deriving an activity concentration index
I = CRa/ 300 Bq kg-1+ CTh/ 200 Bq kg-1+ CK/ 3000 Bq kg-1,
where CRa, CTh an CK are the radium, thorium and potassium concentration in Bq kg-1 and
2) assessing that for material used in bulk amounts (e.g. concrete) if I 1 the indoor gamma dose – excess to the dose received outdoors - is 1 mSv y-1.
If this condition for the index is verified, the radon concentration due to that building material is estimated to be less than 200 Bq m-3.
With the above cited exclusion activity concentrations proposed by ICRP a calculation made with the model and hypotheses of the document Radiation Protection 112 gives an annual effective dose to the inhabitants due to external exposure to gamma of (10-14) mSv y-1, as it is shown in the following.
With the RP 112 hypotheses
Dimensions of the model room 4 m x 5 m x 2.8 m
Thickness and density of the structures 20 cm, 2350 kg m-3
Annual exposure time 7000 h
Dose conversion 0.7 Sv Gy-1
Background 50 nGy h-1
The results of the calculations made with the model per unit activity are
Specific dose rate, nGy h-1 per Bq kg-1
Ra-226 Th-232 K-40
Floor, ceiling and walls (all structures) 0.92 1.1 0.08
Floor and walls (wooden ceiling) 0.67 0.78 0.057
Floor only (wooden house with 0.24 0.28 0.02
And with the ICRP proposed activity concentrations of
Ra-226 1000 Bq/kg
Th-232 1000 Bq/kg
K-40 10000 Bq/kg
the results are the following
nGy h-1 nGy h-1
Ra-226 Th-232 K-40 total
Floor, ceiling and walls (all structures) 920 1100 800 2820
Floor and walls (wooden ceiling) 670 780 570 2020
Floor only (wooden house with concrete floor) 240 280 200 720
this means the following effective doses
mSv y-1 mSv y-1
Ra-226 Th-232 K-40
Floor, ceiling and walls (all structures) 4.5 5.4 3.9 13.8
Floor and walls (wooden ceiling) 3.2 3.8 2.8 9.8
Floor only (wooden house with concrete floor) 1.2 1.4 1 3.6
In conclusion, due to the fact that the third type of building is rare in many countries, the annual effective dose would be in the range (10 – 14) mSv y-1.
Radon concentration in indoor air due to building material
Radon concentration can be assessed in the following way
CRn = (E S) / (V V)
where E = CRa Rn d/2 (exhalation rate)
With the Radiation Protection 112 hypotheses
S = (4 x 2.8 x 2 + 5 x 2.8 x 2 + 4 x 5 x 2) m2 = 90.4 m2
V = 4 x 5 x 2.8 m3 = 56 m3
V = 1 h-1
CRa = 1000 Bq kg-1
d = 0.2 m
Rn = 0.0076 h-1
= 2350 kg m-3 (typical for concrete)
resulting exhalation rate, radon concentration indoors and relevant effective doses to inhabitants depend on the value of the emanation coefficient e in the following way
typical min max
ε 0.05 0.005 0.3
E (Bq m-2 h-1) 89.1 8.91 534
CRa (Bq m-3) 144 14.4 863
Effective dose (mSv y-1) 2.4 0.2 14.4
These doses, to be added to the external ones, do not take into account the possible thoron contribution.
Radiation protection aspects of thoron exposure
1. Thoron (Rn-220) is much different from radon (Rn-222) due to its very short half life (T½ = 55.6 s), which does not allow it to move away significantly from its source.
2. Generally thoron is scarcely present in indoor environment, except in case building material (e.g. in Italy) or soil (e.g. in USA) have high concentrations of Th-232 and high porosity (see references in ref.1). In these situations sometimes the presence of thoron can expose the population to a not negligible dose.
3. For these reasons in international literature thoron has been studied and discussed much less than its isotope Rn-222. The same can be said for ICRP: ICRP more recent publications (n. 60 and 65), where radon is widely analysed and discussed, do not consider Rn-220.
2. Monitoring of thoron exposure
1. As regards Rn-222, it is not its concentration indoors the quantity significant for dose, but the concentration of its decay products. However, radon is much easier to be measured than its decay products. Therefore, for assessing indoor exposure to radon decay products, passive long duration measurements of radon concentration are generally used. This choice is based on the following facts: a) an equilibrium factor of radon decay products to radon indoors can be assessed; b) the effective dose corresponding to a radon concentration is quite constant independently on the variability of aerosol concentration – that influences the decay products – and the dimensions of particles to which decay products attach. Conventional dose conversion factors (based on epidemiology) are available directly from radon concentration to effective dose (ICRP assessments, with the hypothesis of an annual occupancy of 7000 hours and an equilibrium factor of 0.4).
2. Measurement of thoron concentration is generally of low significance, due to the very short half life of thoron, which therefore can be rarely detected at some distance from the source. However, TnDP have a half life of some hours and are therefore generally quite homogeneously mixed in an indoor environment. In any case, for comparability of thoron measurements it is necessary for the measurement protocol to include a precise statement on measuring the thoron concentration at some specific distance from the source (1, 2).
3. Since the correlation between thoron and its decay products (mainly Pb-212) cannot always be ensured, a correct estimate of the dose may even require measurement of both thoron itself and its decay products (2). In general, however, in order to assess the dose relevant to an exposure to TnDP a measurement of TnDP should be made and a dose conversion factor from TnDP to effective dose must be used (1).
4. However, a complete assessment of population exposure should not forget that people during sleep generally breathe air close to walls and in this case the lung dose due to the thoron itself may even be greater than that from inhalation of thoron decay products. In these cases thoron concentration should be measured as close as practicable to the relevant wall (2).
5. For thoron some passive long duration detectors have been designed up to now (2) but few measurements are available in literature in comparison with those available for radon. As regards TnDP the lack of knowledge (in both passive measurement techniques and measurements) is much heavier. In particular no survey carried out up to now is comparable in terms of duration, number and representativeness of the measurements with those carried out for radon (1, 2). This scarcity of data is also confirmed by UNSCEAR 2000 (see Annex A, paragraphs 120, 121).
6. As regards the calibration of detectors for thoron/thoron decay product measurements it should be stressed 1) no primary standard source is available at present, differently from radon 2) there is a severe deficit worldwide of high quality reference chambers/facilities and only some national organisations (Physicalisch Technischen Bundesanstalt in Germany, National Institute of Radiological Sciences in Japan, New York University in USA) have one available, but only now some intercomparisons of these facilities begin to be carried out (3); 3) no intercomparison run of detectors has been organised in order to guarantee the quality of measurements.
3. How to assess the dose from exposure to thoron decay products
1. As far as radon risk estimate is concerned it can be said.
In 1981 ICRP, in its Publication 31, based it on the dosimetric approach, calculating the dose per unit of exposure through dosimetric models and multiplying it by the risk per dose unit, obtained mainly from epidemiological studies on Hiroshima and Nagasaki survivors.
In 1987, with its Publication 50, ICRP preferred the epidemiological approach based on underground miner cohorts, but continued to use the dosimetric approach.
In 1993, with its Publication 65, ICRP abandoned the dosimetric approach and calculated a conventional Dose Conversion Factor (DCF) dividing the risk/exposure factor by the risk/effective-dose factor. This is the reason why a different DCF for workers (1.4 Sv per J h m-3) and for general population (1.1 Sv per J h m-3) was adopted for an identical risk/exposure factor but different risk/dose factors.
2. A similar approach for thoron decay products cannot be used, due to the lack of epidemiological studies, so that only the dosimetric approach is applicable (1). However, not only some years ago (1), but also very recently (2) it has been stated that even if new study were presented on lung dosimetry of radon and thoron decay products "…Up to now the information required to obtain reliable Dose Conversion Factors (DFCs) for Thoron and Radon gas and their progeny is not sufficient. Improved information on DCF dependency on both air quality conditions… and physiological parameters… are needed…" (2).
3. Moreover, the last published ICRP document on thoron (Publication n.50, issued in 1987, which assumes a DFC of 0.52 Sv per J h m-3) is quite old and is based on dosimetric models available in 1983 (1). This value refers to adult exposure indoors, whereas for children the value was estimated to be up to twice as high (1).
4. The value of DFC adopted by the Council Directive 96/29 EURATOM of 0.5 Sv per J h m-3 comes presumably from the latest available ICRP assessment.
5. F.Bochicchio and C.Nuccetelli, in their already cited paper of 1988 (1), made the following suggestion, "A conventional (with a similar meaning of ICRP 65) DCF for TnDP could be obtained by dividing the ICRP65 conventional DFCs for RnDP by the 'R' ratio of the ICRP 50 DCFs for RnDP and TnDP (R= 1.8/0.52=3.5), yielding 0.3 and 0.4 Sv per J h m-3 for adults in dwellings and workplaces, respectively. A similar procedure was used in ICRP50 to assess the lung cancer risk due to TnDp exposure from the risk estimated from RnDp exposure of miners”.
6. Two colleagues of NRPB supported this proposal having calculated effective doses relevant to TnDP using the recent Human Respiratory Tract Model (see ref.4).
7. The same “comparative dosimetric approach” was adopted by UNSCEAR in its 2000 Publication to assess the world exposure to thoron.
Research is still needed on a) measurement techniques for TnDp; b) dosimetric models for both thoron and TnDp. In any case, because of the wide experience gained on thoron exposures and dosimetric calculations after the last ICRP Publication on thoron (Publication 50, 1987), art.31 group of experts could suggest to ICRP Commission to deal with thoron in its new recommendation, in particular the dosimetric quantities needed for a correct assessment of dose and risk, and dose conversion factors.
This short note was prepared thanks also to the useful suggestions of my colleagues C.Nuccetelli and F.Bochicchio, expert on the matter.
1. C.Nuccetelli and F.Bochicchio. The thoron issue: monitoring activities, measuring techniques and dose conversion factors. Radiation Protection Dosimetry 78 (1), 59-61, 1998.
2. Summary of the ECE II Second International Workshop "New Perspectives of Thoron Surveys and Dosimetry, Niska Banja, Serbia, June 6-10, 2005
3. S.Tokonami et al. An intercomparison exercise of thoron concentration between NYU and NIRS, ECE II Second International Workshop "New Perspectives of Thoron Surveys and Dosimetry, Niska Banja, Serbia, June 6-10, 2005
4. J.W.Marshall and A.Birchall, Letter to the editor, Radiation Protection Dosimetry, 81 (4) 311-312, 1999.