Exposure of the Belgian Population to Ionizing Radiation

Exposure of the Belgian Population to Ionizing Radiation H. Vanmarcke 1, H. Mol 2, J. Paridaens 1, G. Eggermont 1 1 Belgian Nuclear Research Centre, SCK CEN, Boeretang 200, 2400 Mol, Belgium. E-mail: hvanmarc@sckcen.be 2 European Institute of Higher Education Brussels, EHSAL, Nieuwland 198, 1000 Brussels, Belgium Abstract. The radiation exposure of the Belgian population in 2001 is calculated using the methodologies given in the UNSCEAR 2000 report. The annual average effective dose in Belgium is 4.5 msv, of which 2.5 msv is due to natural radiation sources and 2 msv due to man-made sources, essentially medical radiation exposures. In Belgium, like in most countries with an advanced health care system, medical exposures are now the most important single source of ionizing radiation. Recent data, collected for the yearly report on the state of the environment in Flanders, will be presented and compared to UNSCEAR data. The annual average dose from diagnostic medical examinations is estimated on the basis of National Health Service data to be 2.0 msv, divided into 1.8 msv for radiology and 0.2 msv for nuclear medicine. With 120 CT-scans per year per 1000 population and an average effective dose of 7.7 msv per examination, the contribution from CT amounts to 0.9 msv/year. CT examinations thus provide half of the total radiation exposure in radiology. The average radon concentration in Belgium is estimated indoors at 48 Bq/m³ and outdoors at 10 Bq/m³. With the UNSCEAR dose conversion factor of 9 (nsv/h)/(bq/m³) (in terms of radon decay products) and a small contribution from radon gas dissolved in blood, a radon dose of 1.35 msv is calculated. Note that the UNSCEAR dose conversion factor for radon is 50% higher than the ICRP 65 conversion convention that was adopted in the European directive and implemented in national legislation. The annual exposure to cosmic radiation is estimated at 0.35 msv, including a small contribution from air travel and holidays (for instance winter sports). Finally, the external and internal exposures from K-40 and the natural decay series are assessed at 0.4 and 0.3 msv respectively and the thoron exposure at 0.1 msv. The average exposure in Belgium has almost doubled over the last 100 years, from approximately 2.3 msv in 1900 to 4.5 msv in 2001. Of this increase 0.2 msv comes from natural sources and 2 msv from medical applications. During the same period the average life expectancy in Belgium for man increased from 48 to 75 years and for women from 51 to 81 years. These two effects together resulted in a threefold increase of the lifetime population exposure: for man from 110 msv in 1900 to 340 msv in 2001 and; for women from 120 msv in 1900 to 360 msv in 2001. 1. Introduction Man has always been exposed to sources of ionizing radiation. These sources are the result of natural processes on earth and in space, and are, since the end of the 19 th century, also due to an increasing number of human activities. Although natural sources still provide the major contribution to the radiation exposure of the Belgian population, the increasing medical exposure is approaching. If this trend continues, the collective dose from medical applications will exceed the collective dose from natural sources within a decade or so. The effective doses to the Belgian population are assessed using national data with the methods given in the UNSCEAR 2000 report [1]. The world average values of the UNSCEAR report are given by way of comparison (printed between brackets and in italics). 2. Medical radiation exposures During the last century, ionizing radiation has been increasingly applied in medicine for diagnosis and therapy, with the result that medical radiation exposures have become an important component of the total radiation exposure of populations. The benefits of this widespread practice to patients are selfevident. Recent Flemish data collected for the yearly report on the environment and nature in Flanders [2] will be given and compared to the world average values of the UNSCEAR 2000 report [1]. The medical practice in Brussels and in the Walloon provinces is quite similar so that the Flemish results can be extrapolated to the whole of Belgium. 1

Examinations with x-rays are the most common medical application. Almost everyone passes from time to time an examination of the chest, teeth and extremities (joints and limbs). The dose from these traditional x-ray examinations is low and the direct advantage of a more precise diagnosis outweighs the radiation risk to the patient. The demand for high-quality images causes a trend away from these simple procedures towards more complex and higher dose imaging procedures. An outstanding example of this trend is the increasing use of Computed Tomography (CT). This is a scanning method that uses computerized x-ray images to provide cross-sectional images of an internal part of the body. In radiotherapy, very high doses are delivered precisely to tumour volumes to eradicate disease, mostly cancer, while minimizing the irradiation of the surrounding healthy tissue. The quantity effective dose is inappropriate for characterizing therapeutic exposures, in which levels of irradiation are by intent high enough to cause deterministic effects. Radiotherapeutic exposures are therefore not accounted for in this overview. 2.1. Trends in frequency of examinations 2.1.1. Diagnostic radiology According to National Health Service data, the average inhabitant of Flanders was subject to 1.2 x-ray examinations in 2001 (excluding dental x-rays) [2]. The temporal trend in the annual frequency is shown in fig. 1. The decrease in the number of examinations in 1993 is due to a modification in the reimbursement system. The number of examinations increases since 1997 with about 2 % per year in spite of efforts to keep the costs for the National Health Service under control. Number of examinations per 1000 population 1250 1200 1150 1100 1050 1000 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 Flanders Belgium FIG. 1. Trends in the number of medical x-ray examinations in Belgium (1990-2001) and Flanders (1996-2001). More detailed National Health Service data shows a shift between the types of examinations (fig. 2). The number of CT and mammography examinations increased at the expense of spine and GI-tract examinations, while the number of chest examinations remained constant at a high level. Developments in imaging technology involving non-ionizing radiation have an influence on the practice of radiology. Ultrasound and magnetic resonance imaging (MRI) are becoming the imaging modality of choice for certain parts of the body. In 2001, they were used in 26 %, respectively 2 % of the total number of imaging procedures [2]. 2.1.2. Nuclear medicine In nuclear medicine, radionuclide preparations are administrated to patients for diagnosis or to a much lesser extend for therapy. The number of diagnostic administrations of radiopharmaceuticals to patients in Flanders was 45 per 1000 population per year in 2001 (fig. 3). The number of examinations 2

in Flanders is 24 % lower than the average for Belgium [2]. The large academic hospitals in Brussels, which attract a lot of Flemish patients, can explain the difference. Belgium is leader in Europe when it comes to the number of examinations per 1000 population (The UNSCEAR estimate for countries with an advanced health care system is 19 per 1000 population per year, a third of the number in Belgium). 30% 25% 20% 15% 10% 5% chest CT spine mammography GI-tract angiography 0% 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 FIG. 2. Trends in diagnostic radiology practice in Flanders (1990-2001). Number of examinations per 1000 population 70 60 50 40 30 20 10 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 Flanders Belgium FIG. 3. Trends in diagnostic nuclear medicine practice in Belgium (1990-2001) and Flanders (1996-2001). Table I gives the relative frequency of the five most important diagnostic nuclear medicine procedures in Flanders. The data come from a survey in 19 Flemish hospitals [3]. Bone scintigraphy accounts for almost half of the examinations. Table I. Percentage contributions by types of procedure to total numbers of diagnostic nuclear medicine procedures in Flanders. Procedure Bone scintigraphy 45 % Thyroid scintigraphy 18 % Myocard perfusion (heart) 12 % Myocard injection (heart) 5 % Lung perfusion 5 % 3

2.2. Doses from medical examinations 2.2.1. Diagnostic radiology The average effective dose per type of examination is compared in table II from three different sources. The values of UNSCEAR 1993 [4] have been adapted in the UNSCEAR 2000 report [1] to the continuing developments in medical imaging. The results of a recent study in 20 Flemish hospitals for 5 important types of examinations, including CT, are in line with the values of the UNSCEAR 2000 report [5]. CT, GI tract, angiography and spine are higher dose imaging procedures, while the doses from chest examinations and extremities are low. Table II. Comparison of effective doses to patients from diagnostic x-ray examinations (msv). Type of examination UNSCEAR 1993 [4] UNSCEAR 2000 [1] Mol 2001 [5] Chest 0.14 0.14 0.15 Extremities 0.06 0.06 - Spine 1.7 1.8 1.7 Pelvis and hips 1.2 0.83 - Head 0.16 0.07 - Abdomen 1.1 0.53 0.92 Gastrointestinal tract 5.7 5.0 - Cholesystography 1.5 2.3 - Urography 3.1 3.7 7.9 Angiography 6.8 12 - PTCA - 22 - Mammography 1.0 0.51 - CT 4.1 8.8 7.7 Extremities 1% Chest 2% Other 6% Spine 15% Abdomen 3% CT-scan 52% GI tract 8% Angiography 10% Mammography 3% FIG. 4. Dose distribution from diagnostic x-ray examinations in Flanders in 2001. Multiplying the National Health Service data on the number of examinations with the effective dose per examination gives the dose distribution shown in fig. 4. The dosimetric data from UNSCEAR 2000 were used when no local data were available [5]. Trends in the annual average effective dose of diagnostic radiology are summarized in fig. 5. The average dose in Flanders is estimated at 1.8 msv in 2001. The exposure is dominated by CT, which provides 52 % of the annual effective dose. With 120 CT-scans per year per 1000 population and an average dose of 7.7 msv per examination, the contribution from CT amounts to 0.9 msv/year. The dose from CT doubled between 1990 and 2001. 4

The increase of the CT dose is partly compensated by a decrease in conventional examinations, in particular examinations of the spine and the GI tract (fig. 2). 2 1.8 Average annual dose (msv) 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 Flanders: all examinations Belgium: all examinations Belgium: CT 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 FIG. 5. Trends in annual effective dose in Flanders (1996-2001) and Belgium (1990-2001) from diagnostic radiological examinations. The large and increasing share from CT is given separately. 2.2.2. Nuclear medicine A working group of the Belgian Society for Nuclear Medicine has issued guidelines for the administered activities for different examinations [6]. Table III shows the characteristics of the most important diagnostic nuclear medicine procedures. 99m Tc forms the basis of most of the radiopharmaceuticals. The typical dose for an examination is between 1 and 10 msv and the administered 99m Tc activity between 100 and 900 MBq. A survey at 19 nuclear medicine departments in Flanders estimates the average dose per diagnostic procedure at 4.2 msv [3]. This value multiplied by the number of examinations (45 per 1000 population) results in an average dose of 0.2 msv/year (The corresponding UNSCEAR estimate for countries with an advanced health care system is 4.3 msv per procedure and 19 procedures per 1000 population, which comes down to an average dose of 0.08 msv/year). Table III. Characteristics of the most important nuclear medicine procedures for a typical healthy adult male (70 kg). Procedure Radiopharmaceutical Reference activity MBq Effective dose for reference activity msv Bone scintigraphy Tc-99m-HDM/MDP 740 4.2 Thyroid scintigraphy Tc-99m-pertechnetate 110 1.4 I-123 for 55 % uptake 40 14.4 for 35 % uptake 40 8.8 for 15 % uptake 40 4.3 Myocard perfusion (heart) Tl-201 150 33.0 Tc-99m-tetrofosmin rest 900 6.8 strain 900 6.3 Tc-99m-MIBI rest 900 8.1 strain 900 7.1 Lung perfusion Tc-99m-MAA 110 1.2 5

2.2.3. Summary The average effective dose from diagnostic medical imaging in Flanders in 2001 is estimated at 2.0 msv/year; 1.8 msv/year from x-ray examinations and 0.2 msv/year from nuclear medicine procedures. (The corresponding UNSCEAR estimate for countries with an advanced health care system is 1.3 msv/year (radiology 1.2 msv/year and nuclear medicine 0.08 msv/year)). 3. Exposures from natural radiation sources With the exception of cosmic radiation, the natural radioactivity results from the decay of radioactive nuclides with half-lives of more than 500 million years. Important nuclides are uranium-238, uranium- 235, thorium-232 and potassium-40. Both external and internal exposures arise from these sources (The world average values of the UNSCEAR 2000 report are printed between brackets and in italics). 3.1. Radon and thoron exposure The above-mentioned long-lived radionuclides, except for potassium-40, form the beginning of natural decay series: the uranium series ( 238 U), the thorium series ( 232 Th) and the actinium series ( 235 U). Each of the three decay series has an isotope of the noble gas radon. Traditionally these isotopes are called radon ( 222 Rn), thoron ( 220 Rn) and actinon ( 219 Rn). As they are chemically inert, they can move in the earth's crust and in building materials and they may eventually reach the atmosphere. Actinon has the shortest half-life and the lowest abundance of the three natural decay series. That is why the isotope's concentration in the air is negligibly low. Radon and thoron are present in indoor air. The much longer half-life of radon (3.82 days) by comparison with thoron (55.6 seconds) causes the contribution of radon to the radiation exposure of the population to be much more important than that of thoron. Radon measurements in thermometer shelters in Belgium gave an average value of 10 Bq/m³ in outdoor air (10) [7]. The radon concentrations indoors are higher. The average concentration in Belgium is estimated at 48 Bq/m³ (40) with a geometric mean of 38 Bq/m³ (30) and a geometric standard deviation of 2.0 (2.3) [8]. The highest values, up to several thousands of Bq/m³, are found in the Ardennes. Direct measurements of the concentrations of the short-lived decay products of radon are difficult and limited. They are estimated from considerations of equilibrium (or disequilibrium) with radon. The equilibrium factor is the ratio of the Equilibrium Equivalent radon Concentration (C EEC ) to the radon concentration (C Rn ). F = C EEC /C Rn with C EEC = 0.105 C 218Po + 0.515 C 214Pb + 0.380 C 214Bi where C 218Po, C 214Pb and C 214Bi are the concentrations of the short-lived radon decay products in air. UNSCEAR suggests a rounded value of 0.6 for the equilibrium factor for the outdoor environment and 0.4 indoors. There is no consensus in the scientific community on the value of the dose conversion factor for radon. The epidemiologically based conversion factor of ICRP 65 [9] is derived from the risk estimate of the superseded BEIR IV report of 1988 [10]. The more recent BEIR VI report of 1998 [11] suggests an increased risk per unit radon exposure. As the dosimetric evaluation, using the ICRP 66 lung model [12], also shows higher values, the UNSCEAR Committee decided to keep its previous value of 3.6 (nsv/h)/(bq/m³) ( 3.6 = 9 in terms of EEC x 0.4 equilibrium factor). For the representative radon concentrations, equilibrium and occupancy factors, and the dose coefficient in terms of EEC, the following annual effective doses are derived: Indoors: 48 x 0.4 x 9 x 7000 x 10-6 = 1.2 msv/year (1.0) Outdoors: 10 x 0.6 x 9 x 1760 x 10-6 = 0.1 msv/year (0.1) Total = 1.3 msv/year (1.1) 6

Fig. 6 illustrates the lack of consensus between the different authorities. The only difference in the calculation of the indoor radon exposure is the value of the dose conversion factor for radon [13]. 4 3.2 msv/year 3 2 1 1 1.2 0.8 1.5 1.2 0 BEIR IV UNSCEAR 93 ICRP 65 ICRP 66 BEIR VI UNSCEAR 00 FIG. 6. The residential radon exposure in Belgium according to BEIR, ICRP and UNSCEAR (radon concentration: 48 Bq/m³, equilibrium factor: 40 % and occupancy factor: 80 %). Note that the UNSCEAR dose conversion factor for radon at home is 50 % higher than the value given in the European Basic Safety Standards that is based on ICRP 65 [14]: Radon at home: 1.1 Sv per J h/m³, which is equivalent to 2.4 (nsv/h)/(bq/m³) Radon at work: 1.4 Sv per J h/m³, which is equivalent to 3.1 (nsv/h)/(bq/m³) For completeness, the contribution from a minor pathway of exposure to radon has to be added, namely dissolution of radon gas in blood with distribution throughout the body. The dose estimate for the representative radon concentrations in Belgium with the method given in the UNSCEAR report is 0.06 msv/year (0.05). The short half-life of thoron (55.6 seconds) limits the thoron exhalation of soil and building materials and thus the contribution of thoron to the radiation exposure of the population. UNSCEAR estimates the average concentration of thoron outdoors at 10 Bq/m³ and approximately the same indoors. It is not possible to use the concentration of the thoron gas in dose evaluation, since the concentration is strongly dependent on the distance from the source. With the estimated equilibrium equivalent concentrations of thoron indoors of 0.3 Bq/m³ and outdoors of 0.1 Bq/m³, and a dose conversion factor of 40 (nsv/h)/(bq/m³), the annual effective doses are: Indoors: Outdoors: 0.3 x 40 x 7000 x 10-6 = 0.084 msv/year 0.1 x 40 x 1760 x 10-6 = 0.007 msv/year Total (rounded off) = 0.1 msv/year (including a minor contribution from thoron gas dissolved in blood) Note that the UNSCEAR dose conversion factor of 40 (nsv/h)/(bq/m³) is close to the value given in the European Basic Safety Standards for thoron at work [14]: 0.5 Sv per J h/m³, which is equivalent to 37.5 (nsv/h)/(bq/m³). The average exposure to radon, thoron and their short-lived decay products in Belgium is (rounded value): 1.3 (radon in air) + 0.06 (radon in blood) + 0.1 (thoron) = 1.45 msv/year (1.2). 3.2. Internal exposures other than radon and thoron Ingestion is the main exposure pathway of the population with significant contributions from potassium-40 and from the uranium and thorium decay series. Potassium is more or less uniformly distributed in the body following intake in foods, and its concentration is under homeostatic control: Adults: 55 Bq/kg 0.165 msv/year Children: 61 Bq/kg 0.185 msv/year There are no control mechanisms to keep the concentration of the radionuclides from the uranium- and thorium-series in the body at a fixed level, so that the doses are dependent on the intake. The main contributor to this dose is polonium-210. UNSCEAR estimates the effective doses from the ingestion of uranium- and thorium-series radionuclides at: 7

Adults: Children: Infants: 0.11 msv/year ( 210 Po contribution = 0.07 msv/year) 0.20 msv/year ( 210 Po contribution = 0.10 msv/year) 0.26 msv/year ( 210 Po contribution = 0.18 msv/year) The total effective dose from internal exposures other than radon and thoron is assessed at 0.3 msv/year. 3.3. External terrestrial exposure External exposures arise from terrestrial radionuclides present at trace levels in soil and building materials. Irradiation is mainly by gamma radiation from radionuclides in the uranium and thorium series and from potassium-40. Hundreds of soil samples from all over Belgium were measured in the eighties by SCK and WIV [15]. The average values of the spectrometric analyses of the soil samples, the dose conversion coefficients from the UNSCEAR 2000 report and the calculated absorbed dose rates in air are given in table IV. Table IV. External exposure rates derived from the average radionuclide concentrations in soil in Belgium (and UNSCEAR worldwide average). Concentration in soil Bq/kg Dose conversion coefficient (ngy/h) / (Bq/kg) Absorbed dose rate ngy/h 40 K 380 (420) 0.0417 16 (18) 226 Ra (uranium series) 26 (33) 0.462 12 (15) 232 Th 27 (45) 0.604 16 (27) Total absorbed dose rate outdoors from soil measurements: 44 (60) The three components of the external radiation field make approximately equal contributions to the gamma radiation dose. Direct measurements of absorbed dose rates in air were carried out at the same locations where the soil samples were taken. Excluding cosmic ray exposure, an average value of 43 ngy/h (59) was found, which is close to the value calculated from the soil concentration measurements. In the same study absorbed dose rate measurements were performed in a few hundred dwellings [15]. A somewhat higher average value of 60 ngy/h (84) was found, due to the change in source geometry from half-space to a more surrounding configuration indoors. To estimate effective doses, account must be taken of the conversion coefficient from absorbed dose in air to effective dose. The smaller body size of children and infants results in higher dose conversion coefficients (adults: 0.7, children: 0.8 and infants: 0.9). The average annual effective dose to adults assuming an occupancy factor indoors of 0.8 is: Indoors: 60 x 7000 x 0.7 x 10-6 = 0.30 msv (0.41) Outdoors: 43 x 1760 x 0.7 x 10-6 = 0.05 msv (0.07) Total = 0.35 msv (0.48) The doses to children (0.40 msv/year (0.55)) and infants (0.45 msv/year (0.62)) are directly proportional to the increase in the dose conversion coefficient from absorbed dose in air to effective dose. The average effective dose to the whole population, including children and infants, from external terrestrial radiation in Belgium is estimated at 0.4 msv/year (0.5). 3.4. Cosmic radiation Cosmic rays interact with the atmosphere producing a cascade of interactions and secondary reaction products. The resulting ionization is a function of both altitude and latitude. The cosmic ray interactions also produce a range of radioactive nuclides known as cosmogenic radionuclides. 8

The external dose rate outdoors increases with geomagnetic latitude. The values for the two components of the cosmic radiation field at sea level in Belgium (and worldwide) are: photons and the directly ionizing component: 32 nsv/h (31); the neutron component: 9 nsv/h (5.5). As Belgium is a country near sea level the altitude correction is small: photons and the directly ionizing component: 1.02 (1.25); the neutron component: 1.1 (2.5). This results in a total effective dose rate outdoors of 32 x 1.02 + 9 x 1.1 = 42.5 nsv/h (52). Applying an indoor shielding factor of 0.8 and assuming indoor occupancy to be 80 % (of time or 7000 h/year) the average effective dose is: 42.5 (1760 + 7000 x 0.8) 10-6 = 0.31 msv/year (0.38). The dose from cosmogenic radionuclides is dominated by the internal dose from 14 C: 0.012 msv/year. Including a small contribution from air travel and holidays (for instance winter sports) the average exposure to cosmic radiation in Belgium can be estimated at: 0.31 + 0.012 + air travel and holidays = 0.35 msv/year (0.4). 4. Sources and trends of radiation exposure in Belgium The radiation exposure of the Belgian population from natural and man-made sources is compared in table V to the average exposure for countries with an advanced health care system from the UNSCEAR 2000 report [1]. The average annual dose in Belgium is 4.5 msv. Almost half comes from diagnostic medical examinations. The second largest contribution is from radon and thoron exposure. The annual dose, calculated with the UNSCEAR dose conversion factor, is 1.45 msv. Note that the UNSCEAR dose conversion factor for radon is 50 % higher than the ICRP 65 conversion convention [9] that was adopted in the European Basic Safety Standards [14]. Much more significant than the average values is the wide range of both indoor radon concentrations and diagnostic exposures to patients. For instance, the dose limit for occupationally exposed workers of 20 msv/year is equivalent to a few CT-scans. Table V. Average exposure from radiation sources in Belgium and worldwide. The medical exposure is for countries with an advanced health care system. Average annual effective dose Source Belgium Worldwide (UNSCEAR) msv/year msv/year Natural radiation Cosmic radiation 0.35 0.4 External terrestrial radiation 0.4 0.5 Radon and thoron 1.45 1.2 Internal exposures other than radon 0.3 0.3 Total 2.5 2.4 Man-made Diagnostic medical examinations 2.0 1.3 Other man-made exposures < 0.05 < 0.05 Total (rounded values) 2.0 1.3 Total 4.5 3.7 The average effective dose in Belgium has almost doubled over the last 100 years from 2.3 msv/year in 1900 to 4.5 msv/year in 2001. Of this increase about 0.2 msv/year comes from natural sources and 2 msv/year from medical applications: A gradual increase of the radon exposure from about 1.3 msv/year in 1900 to 1.45 msv/year in 2001. This is caused by the reduced ventilation of buildings and by the application of building materials with enhanced radium levels, such as phosphogypsum. 9

A small increase of the cosmic radiation of about 0.05 msv/year from air travel and holidays (for instance winter sports). The medical use of ionizing radiation is the largest man-made source of radiation exposure. The contribution is estimated at 2.0 msv/year in 2001 and a further increase is expected. A small contribution from all other man-made sources of less than 0.05 msv/year. During the same period (1900-2001) the average life expectancy in Belgium for man increased from 48 to 75 years and for women from 51 to 81 years. These two effects together resulted in a threefold increase of the life-time population exposure: for man from 110 msv in 1900 to 340 msv in 2001 and; for women from 120 msv in 1900 to 360 msv in 2001. References 1. UNSCEAR, Sources and effects of ionizing radiation, Report to the General Assembly of the United Nations with Scientific Annexes, United Nations publication E.00.IX.3, New York (2000). 2. Vanmarcke, H., Paridaens, J., Eggermont, G., Mol, H., Schoeters, K., Brouwers, J., MIRA-T 2003: hoofdstuk 2.6 Ioniserende straling, Report on the Environment and Nature in Flanders, Vlaamse Milieumaatschappij (VMM), ISBN 90 209 5440 7, pp. 191-201 (2003) (in Dutch). 3. De Geest, E., A multi centre study of the administered activity in nuclear medicine departments in Belgium, presentation EANM conference, Vienna (2002). 4. UNSCEAR, Sources and effects of ionizing radiation, Report to the General Assembly of the United Nations with Scientific Annexes, United Nations publication E.94.IX.2, New York (1993). 5. Mol, H., Dosisinventarisatie radiodiagnostiek in Vlaanderen, VUB study on behalf of the Vlaamse Milieumaatschappij (VMM), Brussel (2001) (in Dutch). 6. Belgian Society for Nuclear Medicine (BSNM), Guidelines for the reference administered activities, on www.belnuc.be (2002). 7. Poffijn, A., Personal communication in the framework of the UNSCEAR survey on exposures to natural radiation sources, (2001). 8. Poffijn, A., Charlet, J.M., Cottens, E., Hallez, S., Vanmarcke, H., Wouters, P., Radon in Belgium: the current situation and plans for the future, in Proceedings 1991 International Symposium on Radon and Radon Reduction Technology, Philadelphia, VI-7, (1991). 9. International Commission on Radiological Protection, Protection against radon-222 at home and at work, Publication 65, Annals of the ICRP, 23 (1993). 10. BEIR IV, Health risks of radon and other internally deposited alpha-emitters, US National Research Council Report, National Academy Press, Washington, DC (1988). 11. BEIR VI, Health effects of exposure to radon, US National Research Council Report, National Academy Press, Washington, DC (1998). 12. International Commission on Radiological Protection, Human respiratory tract models for radiological protection, ICRP Publication 66, Annals of the ICRP, 24 (1994). 13. Vanmarcke, H., Paridaens, J., The significance of ICRP, BEIR and UNSCEAR to the radon exposure in Belgium, in Proceedings 3rd Symp. on Protection against radon, Liège, 41-45, (2001). 14. European Commission, Council Directive 96/29/EURATOM of 13 May 1996 Laying down the Basic Safety Standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation, Official Journal of EC, Series L, No. 159 (1996). 15. Gillard, J., Flémal, J.M., Deworm, J.P., Slegers, W., Measurement of the natural radiation of the Belgian territory, Report of SCK CEN, BLG 607, (1988). 10