Natural radioactivity in building material in the European Union: robustness of the activity concentration index I and comparison with a room model

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1 IOP PUBLISHING JOURNAL OF RADIOLOGICAL PROTECTION J. Radiol. Prot. 32 (2012) doi: / /32/3/349 Natural radioactivity in building material in the European Union: robustness of the activity concentration index I and comparison with a room model C Nuccetelli 1, S Risica 1, M D Alessandro 2 and R Trevisi 3 1 Italian National Institute of Health (Istituto Superiore di Sanità), Technology and Health Department, Viale Regina Elena 299, I Rome, Italy 2 Institute for the Complex Systems, National Research Council, Piazzale Aldo Moro 7, I Rome, Italy 3 Italian National Workers Compensation Authority (INAIL, formerly ISPESL), Department of Occupational Hygiene, Via Fontana Candida 1, I Monteporzio Catone (Rome), Italy cristina.nuccetelli@iss.it Received 24 January 2012, in final form 18 June 2012, accepted for publication 5 July 2012 Published 2 August 2012 Online at stacks.iop.org/jrp/32/349 Abstract Using a wide database collected in the last 10 years, the authors have calculated the activity concentration index I for many building materials in the European Union. Suggested by a European technical guidance document, the index I has recently been adopted as a screening tool in the proposal for the new Euratom basic safety standards directive. The paper analyses the possible implications of the choice of different parameters for the computation of index I, i.e. background to be subtracted, dose criteria, etc. With the collected data an independent assessment of gamma doses was also made with an ISS room model, choosing reasonable hypotheses on the use of materials. The results of the two approaches, i.e. index I and a room model, were compared. 1. Introduction In the last 30 years, both researchers and regulators have recognised building materials to be an important issue, for they are the most significant source of indoor gamma ray exposure for the population. In fact, in the proposal for a Council directive laying down basic safety standards for protection against the dangers arising from exposure to ionising radiation (EUBSS) [1], Article 75 and Annex VII are devoted to building materials. This interest was also stimulated by research activities focusing on developing computational methodologies room models and in situ techniques [2] to evaluate and predict the indoor gamma dose rate on the basis of /12/ $33.00 c 2012 IOP Publishing Ltd Printed in the UK

2 350 C Nuccetelli et al the radioactivity and other characteristics of building materials. In the past, the comparison of several room models published in the international literature had shown their substantial equivalence [3]. The sensitivity of the Markkanen room model [4] the model proposed in the European Union to define the concentration index I, as defined in Radiation Protection 112 (RP112) [5] was also studied in relation to some parameters such as room dimensions, thickness and density of walls, floor and ceiling. The index I provides indications for limiting the use of building materials that are too radioactive, and has been recently adopted in a EUBSS proposal [1]. It is worth noting that in the last decades many countries have developed methods to evaluate and classify building materials on the basis of their natural radionuclide content. This paper describes how the Markkanen model has been improved by one of the authors (namely, the ISS room model; see section 4), and applied to a large database of building material activity concentrations, a 10-year collection of data on Italian [6] and European building materials [7]. On this database the authors calculated index I for several materials and products (cement, concrete, bricks, etc), and evaluated the impact of this gamma ray source on population exposure [8]. The results stimulated a discussion about the possible consequences of different choices for the parameters involved in the calculation of index I, e.g. outdoor background to be subtracted, dose criteria, etc. Considerations on radiation protection follow the comparison of the results obtained by the two methods, index I and the ISS room model. 2. Index I in the European technical guidelines The index of activity concentration, I, was introduced in a publication of the European Commission [5] as a tool to identify materials that need supplementary investigations. It suggests basing radioactivity controls of building materials on a defined dose for controls and an exemption level. Therefore, threshold s for index I are calculated on the basis of a dose criterion and account for ways and amounts in which the material is used in a building. In this approach the dose criterion is defined as the dose exceeding the mean environmental outdoor background in Europe of 50 ngy h 1 ( 0.25 msv year 1 with indoor occupancy factor of 0.8) in 1999, as probably taken from the UNSCEAR 1993 Report [9]. In order to assess whether the dose criteria are met, RP112 defines index I in the following equation: I = C226 Ra + C232 Th + C40 K A226 Ra A232 Th A40 K (1) where 226 Ra is used as reference for the parent 238 U. In RP112 equation (1) A Ra 226, A Th 232 and A K 40 are 300 Bq kg 1, 200 Bq kg 1 and 3000 Bq kg 1, respectively. In particular, when materials are used in bulk amounts, indoor dose rates of 0.3 msv year 1 and 1 msv year 1 correspond to an index I of 0.5 and 1, respectively. When materials are superficial or with restricted use (tiles, boards, etc) the s of index I corresponding to 0.3 msv year 1 and 1 msv year 1 are 2 and 6, respectively. The new EUBSS proposal requires [1] that building materials be identified and listed by the relevant competent authority as being of concern from the radiation protection point of view,.... The EUBSS proposal also reads: Identified types of building materials which are not liable to give doses exceeding the reference level of 1 msv per year for indoor external exposure from building materials, in excess of prevailing outdoor external exposure, shall be exempt from requirements at national level,....

3 Natural radioactivity in building material in the European Union 351 Table 1. Specific dose rate for the natural radionuclides accounted for in the definition of I [5]. Radionuclide 226 Ra Th K 0.08 Specific dose rate (ngy h 1 per Bq kg 1 ) The activity concentration index shall be used as a screening tool for identifying materials that may be exempted or subject to restrictions. For this purpose the activity concentration index I may be used for the classification of the materials into four classes, leading to two categories of building materials (A and B). Category (corresponding default dose) Use A ( 1 msv) B (>1 msv) (1) Materials used in bulk amounts A1 I 1 B1 I > 1 (2) Superficial and other materials A2 I 6 B2 I > 6 with restricted use The division of materials into (1) or (2) according to their use shall be based on national building codes. The choice of 1 msv as the dose reference level accounted for the analysis carried out by the authors on the collected data [8] which led to an assessment of the impact of the choice of a given dose criterion on the EU market. 3. Analysis of the robustness of index I The A x s of index I in the RP112 guide were calculated using a dose criterion of 1 msv year 1 exceeding the gamma dose received outdoors, i.e. 50 ngy h 1, with the following equation: 10 3 Sv year 1 = (SDR x A x Gy h 1 ) 0.7 Sv Gy h year 1 (2) where SDR x is the specific dose rate (ngy h 1 per Bq kg 1 ) for the radionuclide x (see table 1) and A x is the activity concentration (Bq kg 1 ) of radionuclide x when floor, ceiling and walls are made of concrete, the material considered for the RP112 hypothesis Index I versus outdoor background We analysed the variability of index I versus the outdoor background. In order to evaluate how the average European gamma background outdoors had changed after the publication of RP112, a new calculation was made by merging information from the 2000 and 2008 UNSCEAR reports [10, 11]. This was necessary in order to account for the increased number of member states (MS) and the availability of updated national data. When introduced in equation (2), the new estimation of the average absorbed dose rate outdoors, i.e. 53 ngy h 1, did not significantly change the results. However, the absorbed dose rate in air outdoors is highly variable among MS. For this reason an analysis was made of the differences in A x s obtained when the minimum or maximum national outdoor background s are used. In table 2, the A x s calculated in these cases are compared with the RP112 ones. The minimum average absorbed dose rate in

4 352 C Nuccetelli et al Table 2. Analysis of A x variability versus the outdoor background considered. A x (Bq kg 1 ) 50 (RP112) Background subtracted (ngy h 1 ) 18 (min. European ) A 226 Ra 276 (300 a ) A 232 Th 231 (200 a ) A 40 K 3176 (3000 a ) a Rounded used in RP (max. European ) Dose criterion (msv year 1 ) Table 3. A x s of I for dose criteria of 1, 0.5 and 0.3 msv year 1. Computed A 226 Ra (Bq kg 1 ) A 232 Th (Bq kg 1 ) A 40 K (Bq kg 1 ) Rounded Computed Rounded Computed Rounded 1 (RP112) a a It is worth noting that RP112 guide for the dose criterion of 0.3 msv year 1 does not recalculate the A x s in the definition of index I but maintains that this is equivalent to writing I = 0.5. air outdoors is for Cyprus (18 ngy h 1 ) and the maximum for Portugal (84 ngy h 1 ), as found in the 2000 UNSCEAR report. The results of the analysis reported in table 2 can be discussed considering that annex VII of the EUBSS proposal reads: where appropriate, actual doses for comparison with the reference level shall be assessed using more elaborate models which may also take into account the background outdoor external exposure from local prevailing activity concentrations in the undisturbed earth s crust. This means that a MS can adapt I to its radiological situation, producing a significantly different limitation on materials. Indeed, the three denominators of I for RP112 would be significantly different for Cyprus and Portugal, as can be seen in table 2. Therefore, the materials exempted from, or subject to, restrictions as for movement and/or use would be quite different from country to country in the EU The index I applied to different types of building material with three dose criteria Index I was calculated for some types of building materials present in the database with the aim of evaluating the percentage of materials exceeding a given dose criterion. Three dose criteria have been considered, namely those identified by the RP112 guidance (0.3 and 1 msv year 1 ), plus an intermediate level of 0.5 msv year 1. The A x s were computed using a formula analogous to equation (2) with the appropriate dose criterion written on the left. Results are shown in table 3. For each dose criterion, with the rounded s of A x reported in table 3, we assessed the percentage of bricks, concrete and phosphogypsum samples of the database possibly subject to control or restriction (see table 4). The calculation was made only for those samples for which 226 Ra, 232 Th and 40 K activity concentrations were all available. The results of table 4 indicate that 95% and 82% of bricks and concrete samples, respectively, exceed the dose criterion of 0.3 msv year 1. By applying the RP112 method for this dose criterion (I > 0.5), a percentage of 91% would be obtained for bricks and 62% for concrete. Therefore, for this dose criterion the RP112 method is less conservative than the use

5 Natural radioactivity in building material in the European Union 353 Table 4. Percentage of EU bulk materials exceeding the three dose criteria. Bulk material Number of samples with a complete data set Dose criterion (msv year 1 ) Brick % 68% 5% Concrete % 59% 5% Phosphogypsum % 89% 84% of the parameters reported in table 3, but this would not be a problem for the EU, owing to the choice of a different dose criterion. For the dose criterion of 1 msv year 1, only 5% of samples would not be exempted. Thus, the goal of not exceeding 1 msv year 1 seems easy to achieve, but this dose level received from building materials might not be negligible, considering also the effective dose from other possible radiation sources indoors. On the other hand, it is unquestionable that the adoption of 0.3 msv year 1 as an exemption level would prefigure a situation that would be difficult to manage for the EU market. Finally, the adoption of an intermediate dose criterion of 0.5 msv year 1 would imply that about 60% of the main bulk materials could not be exempted, which means that this scenario would also produce important consequences requiring further economic evaluations. As for phosphogypsum, the results of table 4 show that a high percentage of samples exceed the upper dose criterion as well: the use of this building material may undergo strong restrictions when the EUBSS is implemented in MS. However, in many countries it is used as plasterboard. In this case, as a superficial building product, the limits for I of RP112 are 2 for the dose criterion of 0.3 msv year 1, and 6 for 1 msv year 1. The percentage of phosphogypsum samples exceeding the two limits are 58% and 0%, respectively. 4. Gamma dose from building material In order to get an independent estimate of the potential exposure of the EU population to gamma radiation from natural radioactivity in building materials, the activity concentration s collected in the database [7] were used as input for the ISS room model. The ISS room model, which is based on the Markkanen model, accounts for the gamma energy lines in a very detailed way; in fact, the natural spectrum from to 2.6 MeV is represented by 34 effective lines to be compared with the five lines of the Markkanen model [4]. As a result of this, build-up factors and mass attenuation coefficients in building materials and air are calculated more accurately. Both the ISS room model in the present application and the RP112 guide consider a m standard room with 20 cm thick walls, floor and ceiling. In these models only gamma rays are considered, not the radon dose rate. Finally, a general comment is necessary: the ISS room model like the Markkanen model and others developed to get a deeper assessment of indoor gamma dose rate due to building materials allows the user to change parameters like room size and building product through its density, thickness and composition. Consequently, these codes are the most appropriate tools to evaluate the robustness of screening methods like the index I, that of necessity is inaccurate to be a simple and fast tool Application of the ISS room model to bulk building materials in EU Table 5 reports the results of the use of the ISS room model with average s of activity concentration of some building materials. Table 6 shows the calculations made with minimum

6 354 C Nuccetelli et al Table 5. Country average gamma dose rate in the four standard rooms, assuming a reasonable use of brick, concrete, cement and phosphogypsum, as compared with the average absorbed dose rate in air outdoors in the same country (from [10, 11]). Absorbed dose rate in air indoors (ngy h 1 ) Country Population a ( 10 6 ) Outdoor abs.dose rate in air (ngy h 1 ) Concrete floor + ceiling = (f + c) Phosphog. wall = (ph.w.) Concrete Bricks 3 concrete walls Austria Belgium Bulgaria Cyprus b Czech Rep Denmark Finland France Germany Greece Hungary Ireland Italy Lithuania Luxembourg Netherlands Poland Portugal Romania Slovakia Slovenia Spain Sweden UK Arithmetical average Population weighted average 55 3 walls 4 walls 4 walls + (f+ c) 3 walls 4 walls 4 walls + (f+ c) + (ph.w) + (f+ c) 3 brick walls + (ph.w.) + ( f+ c) a EUROSTAT 2009 s dated 2007 [12] total UE population = b As data for Cyprus were lacking in the literature, 30% of cement activity concentrations were used as s for concrete.

7 Natural radioactivity in building material in the European Union 355 Table 6. Minimum and maximum gamma dose rate in the four standard rooms, assuming a reasonable use of brick, concrete, cement and phosphogypsum. (Note: evaluation was not possible for all countries and types of rooms due to missing source data [7].) Country 4 concrete walls + (f+ c) Absorbed dose rate in air indoors (ngy h 1 ) 4 brick walls + (f+ c) 3 concrete walls + (ph.w.) + (f+ c) 3 brick walls + (ph.w.) + (f+ c) min max min max min max min max Austria Belgium Bulgaria Cyprus a Czech Rep. Denmark Finland France Germany Greece Hungary Ireland Italy Lithuania Luxembourg Netherlands Poland Portugal Romania Slovakia Slovenia Spain Sweden UK Arithmetical average a As data for Cyprus were lacking in the literature, 30% of cement activity concentrations were used as s for concrete. and maximum s. It is important to keep in mind that most of the collected data are not statistically representative for MS. That is, they only allow for a rough description of the situation at the European level. Moreover, for radiation protection purposes, measurements were generally made on materials that were presumed to be more active, which means that the published s are probably an overestimation of actual ones. A reasonable use of building materials was assumed to simulate four types of rooms: (i) all four walls made of concrete, (ii) all four walls made of bricks, (iii) three walls are of concrete and one of phosphogypsum, (iv) three walls are of brick and one of phosphogypsum. In all four scenarios of the ISS room model the floor and ceiling are always made of concrete. In table 3, three possible dose criteria, namely 1, 0.5 or 0.3 msv year 1, were assumed for the annual effective dose due to gamma rays from building materials. Taking into account an outdoor background of 50 ngy h 1 (RP112 approach), these effective doses correspond to a derived indoor gamma dose rate in air of about 250, 150 and 110 ngy h 1, respectively. The analysis of the four room configurations in bold in table 5 allowed us to conclude that in only one country might s higher than 250 ngy h 1 (1 msv year 1 ) be found. A different conclusion can be drawn as regards the dose criteria 0.5 and 0.3 msv year 1. In these cases many countries show dose rates exceeding the derived dose rates (150 and 110 ngy h 1 )

8 356 C Nuccetelli et al Table 7. Application of the ISS room model: percentage of samples exceeding the specified absorbed dose rate in air. Standard room 4 concrete walls + concrete floor and ceiling 4 brick walls + concrete floor and ceiling Absorbed dose rate in air indoors >110 ngy h 1 >250 ngy h 1 64% 5% 82% 0% in the different room configurations. From the analysis of table 6, it is clear that in most cases maximum s exceed 150 ngy h 1 and about half of the countries show more than 250 ngy h 1. As regards minimum s, only one country has a gamma dose rate exceeding 150 ngy h 1 and none a higher than 250 ngy h Comparison of the two approaches In order to estimate the differences determined by the use of the two screening methods for building materials, we compared the results obtained by applying index I (see table 4) and the ISS room model. Input data were, in both cases, the activity concentrations of concrete and bricks collected in the database [7]. Two standard rooms were chosen: one with all walls made of concrete, the other with all walls made of bricks; both with ceiling and floor made of concrete. To account for the contribution of floor and ceiling to the brick-walled room, the average activity concentration of concrete for each country was calculated from the database [10]. The number of samples exceeding 1 and 0.3 msv year 1 was assessed for both scenarios. As derived reference s of the indoor gamma dose rate in air we used the s of 250 and 110 ngy h 1 (see section 4.1), corresponding to 1 and 0.3 msv year 1, respectively. The results, in terms of percentage of samples exceeding the above mentioned reference s, are summarised in table 7. As for the concrete room, the analysis showed that 5% and 64% of materials determine dose rates exceeding 250 and 110 ngy h 1, respectively. These s are similar to the results of the RP112 method reported in section 3.2, i.e. 5% (1 msv year 1 ) and 62% (0.3 msv year 1 ). In the case of bricks, the results obtained by the two approaches are not the same: with RP112 the samples exceeding 1 msv year 1 and 0.3 msv year 1 are 5% and 91%, respectively, compared with 0% and 82% obtained with the ISS room model. Therefore, the application of I may be considered conservative for both dose criteria. This result is mainly due to the fact that in the ISS room model used in this work the actual density of bricks ( 1.5 g cm 3 ) was used in the dose rate computations, instead of the density of concrete (2.3 g cm 3 ) used to elaborate I [5]. Indeed, the use of the actual density of bricks results in a decrease of more than 20% for the gamma dose rate per unit of activity concentration coefficients for all radionuclides (uranium and radium chain, thorium chain and potassium), as observed in [3]. This remark confirms the reliability of I as a screening tool. Indeed, it remains on the protective side. 5. Conclusion Being a relatively fast and simple screening tool for building materials, index I of RP112 [5] remains the tool of choice, but the aim of this paper was to analyse in detail the consequences of its possible implementation in MS. The main results are summarised in the following.

9 Natural radioactivity in building material in the European Union 357 The 1999 European average estimate of absorbed dose rate in air outdoors in RP112 [5] was updated using the 2000 and 2008 UNSCEAR reports in order to get data about the present 27 EU MS, obtaining a of 53 ngy h 1. The new average absorbed dose rate in air is fairly similar to the previous one (50 ngy h 1 ) and therefore it does not significantly affect the A x s. Conversely, the application of national outdoor gamma background s affects the index remarkably: the three rounded parameters A x (300, 200 and 3000 for 226 Ra, 232 Th and 40 K, respectively) for the minimum and maximum of outdoor background (in Cyprus and Portugal, respectively) would become (241, 202 and 2776) and (313, 262 and 3601), respectively. It can be concluded that if each MS adapted index I to its mean background, the exemption on materials would vary widely within the EU. The consequences of the future use of index I in the EU was also assessed by comparing the results of its application with those obtained from a more precise calculation and a more accurate modelling. The assumption that for a dose criterion of 0.3 msv year 1 the same activity concentration index can be used if its limit is set at 0.5 instead of 1 [5], turned out not to be conservative. In any case, this is not the dose criterion used in the EUBSS proposal. As regards the comparison between the application of the RP112 method and the ISS room model, it can be concluded that, in the case of concrete, the percentages of materials which would be exempted are similar, whereas for bricks they are a little lower with the RP112 method. From a radiation protection point of view, staying on the conservative side is particularly important because these dose criteria only account for the effective dose due to gamma radiation, whereas the exposure indoors is actually much higher than that. Indeed, the population at home is also exposed to radioactivity from 222 Rn from soil and, to a lesser extent, from building materials and water, natural radionuclides in drinking water, beta radiation from certain superficial materials, 137 Cs in some types of wood, etc. Acknowledgment The authors are very grateful to Monica Brocco (ISS) for the linguistic revision of the manuscript. References [1] Proposal for a Council Directive laying down basic safety standards for protection against the dangers arising from exposure to ionising radiation ( protection/radiation protection en.htm) [2] Nuccetelli C and Bolzan C 2001 In situ gamma spectroscopy to characterize building materials as radon and thoron sources Sci. Total Environ [3] Risica S, Bolzan C and Nuccetelli C 2001 Radioactivity in building materials: room model analysis and experimental methods Sci. Total Environ [4] Markkanen M 1995 Radiation Dose Assessments for Materials With Elevated Natural Radioactivity. Report STUK-B-STO 32 (Helsinki: Finnish Centre for Radiation and Nuclear Safety) ( stuk-b/stuk-b-sto32.pdf) [5] European Commission 1999 Radiation Protection 112 Radiological Protection Principles Concerning the Natural Radioactivity of Building Materials (Luxembourg: European Union) ( nuclear/radiation protection/doc/publication/112.pdf) [6] Risica S, Bolzan C and Nuccetelli C 1999 Radioactivity in building materials: experimental methods, calculations and an overview of the Italian situation Proc Work. Radon in the Living Environment (Athens) pp [7] Trevisi R, D Alessandro M, Risica S and Nuccetelli C 2012 Natural radioactivity in building materials in the European Union: a database and an estimate of radiological significance J. Environ. Radioact

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