GAMMA RADIATION FOR PRODUCT STERILIZATION 721 Vale (14), which change colour during irradiation in the plant. These labels are affixed to the outside of each package and ensure against confus- ing irradiated and unirradiated material. The label is made by impregnating PVC with an acid sensitive dye which changes colour due to release of a small amount of hydrochloric acid from the plastic by irradiation. The use of microbiological test pieces in a routine way to measure the effectiveness of a radiation plant is irrelevant in the light of the reproduci- bility of treatment from package to package. However, there is a place for such a test piece in making a comparison between the effectiveness of producing bacterial inactivation between different radiation facilities. It is possible that although the same dose is delivered at different plants as measured by dosimetry methods, the manner of dose delivery might affect the bacterial inactivation obtained. Such differences might arise particu- larly in comparing an electron machine facility with a cobalt-60 plant. The use of test pieces for this purpose is proposed at the commissioning of plants in a recommended International Code of Practice for the sterilization of medical products (15), and suitable preparations are being developed and tested. GENERAL APPRAISAL There can be no doubt about the applicability of radiation sterilization in the medical products field. The process is being used in many countries and there are at least 20 commercial or semi-commercial cobalt-60 plants in operation and in addition about seven electron machines. The greatest impact is on the introduction of pre-packaged disposables. like the plastic syringe, where the properties of radiation in terms of penetration and lethal effect on micro-organisms without rise in temperature are used to advantage. The process has considerable potential in the treatment of food although adverse effects on the quality of many products limit its use at high doses for sterilization. Other processes involving the elimination of specific pathogens, e.g. salmonellae, or the preservation of foods for limited periods require doses which do not cause observable change. The radiation steriliza- tion of pelleted feeds for specified-pathogen-free and germ-free laboratory animals has been in commercial operation for several years. Interest within the cosmetics industry is increasing with the demand for higher microbiological standards in products. Its usefulness must be decided for individual products and their packs although the process may have application to the inactivation of contaminants in raw materials.

722 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Considerable throughput is needed before investment in radiation plant is warranted. The cobalt-60 source is subject to unavoidable decay whether in use or not, and this factor adds to the usual ones which require continuous operation of plant involving high capital cost. Plant cost will depend on throughput, dose requirement and density of the material being processed but might be in the order of oe100,000 for a 'continuous' type and half this figure for a 'batch' type. Existing radiation facilities are often available for the commercial irradiation of volumes of material which do not warrant specialized plant construction. (Received: 13th September 1970) REFERENCES (1) Christensen, E. A., Holm, N. W. and Juul, F. A. Radiosterilization of medical devices and supplies. In Radiosterilization of medical products, 265 (1967) (STL/PUB/157, IAEA, Vienna. Available HM Stationery Office, London). (2) Ley, F. J. and Tallentire, A. Sterilization by radiation or heat--some microbiological considerations. lVharm. J. 1911 59 (1964). (3) Krabbenhoft, L. K., Anderson, A. W. and Elliker, P. R. Influence of culture media on the radiation resistance of micrococcus radiodurans. Appl. Microbiol. 15 178 (1967). (4) Bridges, A. E., Olivo, J. P. and Chandler, V. L. Relative resistance of micro organisms to cathode rays. II Yeasts and moulds. Appl. Microbiol. 4 147 (1956). (5) Erdman, E., Thatcher, F. S. and MacQueen, K. F. Studies on the irradiation of micro- organisms in relation to food preservation. 1. The comparative sensitivities of specific bacteria of public health significance. Can. J. ]½Iicrobiol. 7 199 (1961). (6) Bridges, B. A. Microbiological aspects of radiation sterilization. Progress in Industrial Microbiology V 283 (1964). (7) 0liver, R. and Tomlinson, A. H. The sterilisation of surgical rubber gloves and plastic tubing by means of ionizing radiation. J. Hyg. 58 465 (1960). (8) Burr, M. M. and Ley, F. J. Studies on the dose requirement for the radiation sterilisation of medical equipment. Pt. I. Influence of suspending media. J. Appl. Bacteriol. 26 484 (1963). (9) Ley, F. J. and Tailentire, A. Radiation sterilisation--the choice of dose. Pharm. J. 216 (1965). (10) Tallentire, A., Dwyer, J. and Ley, F. J. Microbiological quality control of sterilised products: Evaluation of a model relating frequency of contaminated items with increasing radiation treatment. y. Appl. Bacteriol. 34, pt 3 (to be published). (11) Radiation stability of materials (1965). Radioisotopes Review Sheet G1. Available from Wantage Research Laboratory. (12) Hills, P. R., Petley, P. T. and Roberts, R. Olfactory changes in irradiated essential oils. Perfumery Essent. Oil Record $2 413 (1961). (13) Whittaker, B. Red perspex dosimetry in manual of radiation dostmerry. Editors: Holm, N. W. and Berry, R. J. 363 (1970). (Marcel Dekker, New York). (14) Farrell, J. J. and Vale, R. L. Radiation paint verifies product irradiation. Nucleonics 78 (1963). (15) IAEA Recommended code of practice for radiosterilization of medical products. In Radiosterilization of medical productq, 423 (1967). (STI/PUB/157 IAEA, Vienna. Available HM Stationery Office, London). DiSCUSSiON MR. M. J. BussF•: You devote a section to the effects of 7 radiation on the physical properties of some materials. Yesterday in general discussion on methods of steriliza- tion it was suggested that irradiation with 7 rays might result in induced radio-