Food safety aspects relating to the application of X-ray surveillance equipment: Memorandum from a WHO meeting*

Memoranda/Memorandums Food safety aspects relating to the application of X-ray surveillance equipment: Memorandum from a WHO meeting* Inspection of food-containing cargoes using X-rays is safe since no detectable radioactivity will be induced in the foodstuffs provided that an energy level of 10 MeV and a dose of 0.5 Gy are not exceeded. Introduction Many countries have regulations permitting the irradiation of foodstuffs. In most cases, these regula- tions conform to the Codex General Standard for Irradiated Foods (1), and in particular specify that X- rays used for this purpose should be generated from machine sources operating at or below an energy level of 5 mega-electron-volts (MeV). This limit has been chosen in order to stay well below the energy level where significant induction of radioactivity in the irradiated food may be expected. Ionizing radiation is used not only to accom- plish an effect on food, but also in connection with process and quality control (e.g., detection of the level of filling in cans and of foreign-bodies in containers) and in connection with the use of X-ray surveillance * This Memorandum is based on the report of a WHO Consulta- tion, convened in cooperation with the International Atomic Energy Agency (IAEA), which met in Neuherberg/Munich, Federal Republic of Germany on 13-17 November 1989. The participants were A.M.I. Alsayyed, Doha, Qatar; K.J. Dale, London, England; J.F. Diehl, Karlsruhe, Federal Republic of Germany; J. Farkas, Budapest, Hungary (Rapporteur); M. Frissel, Bilthoven, Nether- lands; H. Frohlich, Frankfurt, Federal Republic of Germany; J.H. Hubbel, Gaithersburg, MD, USA (Chairman); J.R. Lujan, Mexico DF, Mexico; and G. Pauli, Washington, DC, USA. Secretariat K.W. Bogl, Berlin (West); A. Brynjolfsson, Wageningen, Netherlands; F.K. Kaferstein, WHO (Secretary); A.-M. Schmitt-Hannig, IAEA; R.B. Singh, London, England; and H. Stiff, WHO. Joint FAO/WHO Food Standards Programme: E. Casadei, FAO, Rome, Italy. In addition, companies interested in X-ray surveillance equipment were represented by G. Geus and C. Koch, Wiesbaden, Federal Republic of Germany; C.T. Blunden and G. Bennet, Bristol, England; and C.S. Nunan, Palo Alto, CA, USA. Requests for reprints should be sent to Dr F.K. Kaferstein, Food Safety Unit, Division of Environmental Health, World Health Organization, 1211 Geneva 27, Switzerland. A French translation of this Memorandum will appear in a later issue of the Bulletin. Reprint No. 5075 equipment. WHO has recently been informed of new technological developments that have made it pos- sible to use higher energy X-ray systems for the examination of large cargo containers and cargo vehicles to detect the presence of contraband such as illegal drugs, explosives and guns. Some Member States of WHO have already expressed interest in the use of such surveillance equipment. However, for penetrating large cargo containers, these systems operate with X-ray energies of over 5 MeV. Although there may be considerable advantages in using this new technology in combating terrorism, etc., countries may be hesitant in allowing the use of such equipment on cargoes containing food because the energy level is in excess of that specified for food irradiation by the Codex Alimentarius Commission. It was for this reason that WHO, in cooperation with IAEA, convened a meeting to seek international consensus on the food safety aspects arising from the use of high-energy X-ray surveillance systems. All companies known to WHO as developers or manu- facturers of X-ray surveillance equipment were invited to participate. Their representatives presented technical information on such equipment and sur- veillance systems at the meeting. The objectives of the meeting were: - to investigate the usefulness of inspecting food- containing cargoes with the help of ionizing radiation; - in the event of an affirmative answer, to define the parameters (energy-level and dose) necessary for large cargo surveillance with X-rays; and - to consider possible health consequences from exposing food to X-rays with energies greater than 5 MeV and an absorbed dose in the range of 0.5 gray (Gy), in relation to induction of radio- activity; toxicological, nutritional and sensory considerations; and microbiological considera- tions. Bulletin of the World Health Organization, 6 (3): 297-301 (1990) © World Health Organization 1990 297 Memorandum Cargo Inspection is inspction with X-ray survellance equipment useful? A major commitment of the customs authorities the world over is the fight against illegal trafficking in contraband such as drugs and arms. There are at least two factors which have an important bearing on the efficiency with which this task is performed: (1) the need to unpack and repack cargo items; and (2) the huge volume of cargoes at the present time and the increases anticipated in the future. At Hamburg port, for instance, container traffic increased by 11.7% in 1988 to a total of 1.6 million containers. Dover and Southampton ports together handle approximately 20 tonnes of food per minute every day, which amounts to some 9.5 million tonnes/year. Throughput in the State of Qatar is some 20-40 trailers of foods each day. Similar con- siderations apply to air cargo. At Frankfurt Inter- national Airport, for example, 2.2 million individual consignments are handled annually by the customs authorities; an expansion by about 33% is expected by the year 2000. All the above figures are likely to increase with the anticipated rise in world food trade. Control procedures for detecting and preventing contraband fall into a number of categories, such as the use of (1) conventional manual control; (2) dogs for detecting drugs and explosives; (3) chromatogra- phic, spectroscopic and related methods; and (4) X- ray surveillance. The advantage of the first three of these methods is the immediate provision of incriminating evidence, thus permitting direct assessments to be made. A disadvantage of the second and third methods is that these are highly specialized techniques and therefore of limited general applicability. Also, for biological reasons, dogs cannot repeatedly provide satisfactory results over extended periods. The most important drawback of all three methods is that they are time- consuming and labour intensive and, consequently, do not permit a high throughput of goods generally and large cargo containers in particular. The fourth method, X-ray surveillance, is a rapid and efficient tool for the systematic and serial inspec- tion of cargoes. However, X-ray surveillance systems currently in use operate at 140 kilovolts (i.e., energy levels' up to 0.14 MeV); because of this technical limitation, present systems allow for the inspection of small cargoes only. It is understood that recent ' For the purpose of this report, the term "energy level" is defined as the maximum photon energy producible by the X-ray source. developments, using surveillance equipment with X- ray energy levels of up to 10 MeV, will enable large cargo containers to be screened without the need for opening the container and unpacking the goods. This new technique will therefore facilitate the checking of large volumes of bulk consignments such as perishable goods (e.g., fresh food, flowers, etc.), textiles and leather goods without the need for un- packing. This is a particularly important considera- tion in view of the extraordinary inventiveness of smugglers in thinking up places and means of con- cealment. Perishable goods are an example in point; because of the known difficulties in handling such cargoes (time constraints, financial penalties), these goods are being used, increasingly, to conceal con- traband, mostly drugs. It should be noted that the use of high-energy X-ray equipment requires experienced personnel trained in image interpretation and in its safe operation (for details, see Annex page 301). Any development which facilitates rapid screen- ing of large cargo containers will be advantageous to the customs and other control authorities. However, the technical feasibility and health consequences of such high-energy surveillance systems are issues that are discussed below. Parameters necessary for X-ray surveillance of large cargo containers Energy levels. X-ray surveillance of large cargo items with thicknesses of the order of 2.5 m of water equivalent, or 30 cm of steel, is not possible without increasing the penetrating power of the X-ray beam. The penetrating power can be increased only by increasing the energy levels from those at present used for luggage inspection, typically up to 0.14 MeV, to energy levels of the order of 5 to 10 MeV.b For successful imaging, including use of various kinds of image enhancement techniques, the maxi- mum tolerable attenuation of the primary X-ray beam in traversing the cargo unit appears to be between 10-4 and 10-i. The 10-4 figure comes from presentations at this Consultation by representatives of companies producing fan-beam, moving-cargo high-energy X-ray surveillance equipment. The 10' figure was inferred from published information on rocket-motor flaw detection in 50 cm of steel using 16 MeV X-rays (2). One company described X-ray surveillance equipment using maximum energy levels of 8 MeV; another company demon- strated images derived from equipment operating at energy levels between 6 MeV and 10 MeV; and a third company sugges- ted the possibility of using energy levels greater than 10 MeV. 298 WHO Bulletin OMS. Vol 68 1990. Food safety and use of X-ray survelllance equipment The penetrating power of the X-ray beam does not increase indefinitely with increasing photon energy. There is a minimum in the attenuation cross- section vs. photon energy, above which the X-ray beam becomes less penetrating (3). For carbon, this minimal attenuation energy is 55 MeV, but drops to 8 MeV for copper and to 3.5 MeV for lead. Another factor to be considered is the contribu- tion to the attenuation from photonuclear interac- tions (4) in the 6-30 MeV region which accounts for only 2-6% of the total attenuation, but which can be a major mechanism for inducing radioactivity in the cargo material. This consideration is discussed in more detail below. Dose levels. Information on the dose requirements for imaging with multi-MeV photons appears to be currently available only from commercial developers of such equipment. For imaging with a cone-beam and a two- dimensional imaging screen (stationary cargo), the presentation by a representative of a developer of this type of equipment highlighted the need for a dose of 0.05 Gy at the surface of the cargo nearer to the X- ray source. This would imply a dose at the detector side of the cargo of 0.05 x 1O'- Gy (i.e., 5 x 10-6 Gy) required by the detector system to produce an acceptable image. For imaging with a fan-beam (moving cargo) facility using two beams at right angles, a much lower dose may be possible (in this context, a dose as low as 0.00025 Gy at the source side was quoted by one producer). To allow for flexibility, for overlap of the exposures in some systems, for sufficient resolution, and for the need to re-examine cargoes in some instances, the Consultation considered a maximum dose of 0.5 Gy absorbed by the food. Possible health consequences Exposure of food to X-rays wiff energy levels >5 MeV and a maximum dose of 0.5 Gy Induction of radioactivit. Several possibilities exist to induce radioactivity in food. The induction depends on an interaction between X-ray photons or neutrons with atoms in the food. Most interactions of this kind do not lead to the induction of radioactivity. One type of interaction produces radioactive isomers. Energy from a photon is absorbed by an atom and afterwards emitted as radiation. Neutrons may be emitted following interactions of photons with atoms in the food (e.g., deuterium), or from outside sources (for example, as used in a thermal neutron detection scanning device). The absorption of a neutron by an atom may also induce activity. Electrons induce radioactivity primarily by indirect means; photons are created (Bremsstrahlung) as the electrons strike the target material. These photons, in turn, interact with the nucleus of the atom in photo- nuclear reactions. Many of these reactions have threshold energies below which reactions do not occur. Thresholds are always dependent on the iso- tope and the type of reaction. All these physical processes are well known and documented and amenable to calculation. Results of such calculations are reported by Becker (5,6) and by Leboutet & Aucoutuviev (7). Based on such calculations, the Consultation recognized that high-energy radiation can induce radioactivity in any absorbing medium, such as food. For example, one can calculate that even natural background radiations (e.g., cosmic rays) induce radioactivity in food. The factors affecting the radioactivity include the type of radiation (electro- magnetic, probability of induced electron or neutron), the energy of the radiation, and the par- ticular elements found in the food. These factors can also interact; for example, high-energy X-rays can induce reactions that produce neutrons, leading to further reactions caused by the neutrons. Experimental studies that are relevant to deter- mine the effects of low-dose/high-energy X-rays on food are usually not designed to determine induced radioactivity at the combinations of energy level, dose, and time after exposure that would be used in surveillance systems. However relevant experimental data are available from studies designed to evaluate the use of activation analysis and the application of X-rays and electrons in food irradiation and medical uses at energy levels up to 24 MeV and at doses up to 50 kGy. Such studies, both theoretical and experi- mental, can be used to extrapolate downwards to a lower dose such as that of 0.5 Gy considered by the Consultation for surveillance systems. These studies show no evidence that detectable levelsc of radio- activity would be induced at these lower doses. In light of the large variations of background radioactivity in food that are of no concern, the Consultation concluded that radioactivity below the detection limit is also of no concern. A criterion of no detectable, induced radioactivity may be more strict All foods contain radioactivity, usually at levels in the range of 30-300 becquerel/kg. The amount of radioactivity in any specific food varies, depending on its elemental composition. The amount of increased radioactivity that can be measured is typically about 1% of the natural background In the food. For the purpose of this report, the Consultation considered this level to be the detection limit. WHO Bulletin OMS. Vol 68 1990. 299 Memorandum than necessary. However, present-day technology is capable of producing X-ray surveillance equipment which does not induce detectable amounts of radio- activity. Therefore, such a criterion provides a suf- ficient margin of safety to eliminate the need for considering cumulative effects of repeated X-ray sur- veillance inspections or occasional deviation from intended conditions of use due to human error. ToxicologIal, nutritional and senaory considerations. The Consultation considered the question of whether high-energy X-ray surveillance of food-containing cargo might cause chemical changes of toxicological or nutritional concern, or changes in the sensory quality of food. The conclusion was that, at the considered radiation dose of 0.5 Gy for X-ray cargo inspection, radiation-induced chemical changes in foods are so minute that no toxicological risks, losses of nutrients or changes in sensory quality can be foreseen. The dose level that might require considera- tion of such risks or changes is considerably greater than that needed for surveillance; therefore, even repeated inspections of the same cargo would not be of concern. Microbiological considerations. The microbiological safety of irradiated foods has been investigated in many laboratories in relation to food preservation by ionizing radiation, and was a subject of discussion at several international meetings of experts, including the Joint FAO/IAEA/WHO Expert Committee on Wholesomeness of Irradiated Food (8). The con- clusion of these reviews was that the microbiological safety of irradiated food is fully comparable with that of foods preserved by other acceptable preservation methods. Regarding the energy levels of X-ray surveillance equipment which are higher than those at present permitted for food preservation, the Consultation concluded that the events following the primary interactions, including chemical and radiobiological effects, are the same and are independent of the different proportion of various primary energy absor- ption processes during interaction of X-rays with matter as a function of increasing photon energies. Thus, in principle, the same main questions which have been scrutinized in the past in relation to microbiological safety of radiation-preserved food may be considered also for high-energy X-ray sur- veillance of food. However, the much lower dose requirement of the latter technique should be taken into consideration. Regarding dose requirement for selective changes in the composition of the microflora and for changes in the diagnostic characteristics of microorganisms, and considering the fact that nothing of significance has been found regarding radiation-induced mutants even at the dose levels of food preservation by irradiation, the Consultation concluded that no microbiological hazard will arise from the use of the proposed X-ray surveillance systems. Conclusions The Consultation concluded that of all the issues discussed, only the induction of radioactivity may be of concern regarding the potential effects of health. Evaluation of the likelihood of inducing radioactivity in food has mostly been based on theoretical calcula- tions because the X-ray surveillance systems current- ly under consideration are not capable of producing detectable levels of activity. Calculations applied to the different possibilities can be quite complex. It is not essential to make precise calculations, however, if a sufficient safety margin is built in to the deliberation. This condition is met when no detectable radioactivity is induced in foodstuffs. The Consultation concluded, on the basis of available evidence, that no detectable radioactivity will be induced in foodstuffs when an energy level of 10 MeV and a dose of 0.5 Gy are not exceeded. The safety of the food will not be affected as a con- sequence of such exposure. However, this conclusion is not intended to preclude other safe surveillance systems designed to operate at a higher energy level or dose. In such cases, assurance should be provided that, at the point of consumption, food would not contain a measur- ably detectable amount of induced radioactivity. Acknowledgements The Consultation was supported by a grant from the United Kingdom government. References 1. Jolnt FAO/WHO Food Standards Programme. Codex general standard for irradiated foods and recommen- ded international code of practice for the operation of radiation facilities used for the treatment of foods. Codex alimentarius, Vol. XV, 1st edition. Rome, Codex Alimentarius Commission, 1984. WHO Bulletin OMS. Vol 68 1990. Food safety and use of X-ray surveillance equipmont 2. Bakke, T.O. (quoting T.E. Kirchner, American Science and Engineering Inc., Cambridge, MA), Giant scanner inspects rocket motors. Popular science, 224: 95 (1984). 3. Hubbell, J.H. et al. Pair, triplet, and total atomic cross- sections (and mass attenuation coefficients) for 1 MeV- 100 GeV photons in elements Z=1 to 100, J. phys. chem. ref. data, 9: 1023-1147 (1980). 4. Fuller, E.G. & Hayward, E., ed. Photonuclear reactions Stroudsburg, PA, Dowden, Hutchinson & Ross, 1976. 5. Becker, R.L. Radioactivity induced in foods by 10 MeV electron irradiation. Natick, MA, US Army Natick Research and Development Command, 1977. 6. Becker, R.L. A determination of the radioactivity induced in foods as a result of irradiation by electrons of energy between 10 and 16 MeV. Natick, MA, US Army Natick Research and Development Command, 1979. 7. Leboutet, H. & Aucouturler, J. Theoretical evaluation of induced radioactivity in food products by electron and X-ray beam sterilization. Radiat. phys. chem., 25: 233 (1987). 8. WHO Technical Report Series No. 659, 1981 (Wholesomeness of irradiated food report of a Joint FAO/IAEA/WHO Expert Committee). Annex Operational radiological safety aspects Electron linear accelerators are being used through- out the world in increasing numbers in a variety of important applications. Foremost among these is their role in the treatment of cancer with both photon and electron radiations in the energy range 4-40 MeV. To a greater extent linear accelerators are replacing Co' sources and betatrons in medical applications. Commercial uses include non-destruc- tive testing by radiography, food preservation, product sterilization and radiation processing of materials such as plastics and adhesives. Scientific applications include investigations in radiation biology, radiation chemistry, nuclear and elementary- particle physics and radiation research. Guidelines and standards on the radiological safety aspects of the operation of such accelerators have been developed on a national"b and inter- nationalc basis. In view of the rapidly growing number of cargo container shipments throughout the world, a new field of application for linear accelerators with photon energies of about 10 MeV has been estab- lished for X-ray surveillance of large containers. In principle, the same registration, licensing and inspection procedures established by the appropriate regulatory authority apply as for all linear acceler- ators operating in the same energy range. In coun- tries where a proper radiation protection infrastruc- ture is not available, the Consultation suggests that the manufacturer should notify the IAEA. However, the responsibility for protection of personnel, facilities, the public and the environment from all types of hazards related to linac (linear accelerator) operations must rest with the management of the organization using these systems. Under its direction, a safety unit should be established and a safety programme appropriate to the special needs of the application should be developed and implemented. A radiation safety programme should be developed in coordination with the facility's overall safety programme, and in compliance with national, regional and local requirements. Recommendations of international organizations such as IAEA, the International Commission on Radiological Protec- tion (ICRP), the International Commission on Radiation Units and Measurements (ICRU), the International Electrotechnical Commission (IEC) and the Commission of the European Communities, as well as national commissions, should be con- sidered in the development of this programme. a United States Atomic Energy Commission. Safety guidelines for high-energy accelerator facilities. Washington DC, National Accelerator Committee, USAEC Division of Operational Safety, 1967 (see the latest version). b United States Atomic Energy Research and Development Administralon. Operational safety standards. Washington OC, AECM Section 0550, USERDA (periodically revised). c IAEA Technical Report Series No. 188 (Radiological safety aspects of the operation of electron linear accelerators). Vienna, International Atomic Energy Agency, 1979. WHO Bulletin OMS. Vol 681990. 301