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-

GAMMA RADIATION FOR PRODUCT STERILIZATION 723 activity. It has always been my understanding when dealing with the packaging materials and devices which are now being routinely radiation-sterilized by ? radi- ation from cobalt 60, that such a possibility does not exist. Can you confirm this? THE LECTURER: I can confirm that there is no danger of induced radioactivity no matter how high the dose applied. Misapprehension on this point arises through con- fusion with neutron irradiation which does induce radioactivity. The T rays from cobalt 60 have an energy level which is well below the "threshold value" required to cause a photoneutron reaction in elements. This point has of course been carefully examined both in theory and practice. Obviously the process would not have been recommended or adopted if there was any doubt about this. MR. D. J. BUSH: I think those of us who are cosmetic formulators will be most interested in the effects of T radiation on our products. I have observed gassing in some cosmetic creams irradiated in metal tubes though no signs of corrosion were observed. I would be glad of your comments. THE LECTURER: Irradiation can cause gassing, probably due to hydrogen and carbon dioxide production from sensitive compounds. This can be inhibited by the addition of hydrogen scavengers. The small amount of gas production is, of course, readily observed in tightly packed metal tubes. MR. N. F. E. BLACKMORE: You mention that radiation causes glass to discolour under what conditions, if any, does the colour disappear? THE LECTURER: The browning or blackening effect, which depends in extent on the total dose given and in stability on the glass formulation, can always be removed by heating at high temperature in an oven. The colour fades gradually on exposure to sunlight. MR. P. MoxEY: Does ? radiation induce cross linking in plastic packaging materials or plastic devices being sterilized? THE LECTURER: Radiation can cause cross linking, for example, in polyethylene. This effect has been used to advantage with this material to produce polyethylene with elevated melting point. However, the dose required is very much higher than those recommended for microbiological control. A wide range of plastics have been found perfectly suitable for radiation sterilization provided repeated treatment is not envisaged. A M•MBER OF THE AUDTENCF•: Can micro-organisms build up resistance to radi- ation? THE LECTURER: Yes--by repeated sub-lethal irradiation treatments with the deliberate growth of organisms between the treatments this can be achieved, and has been demonstrated in the laboratory. A practical problem is very unlikely to arise since it would entail the survival of organisms within products following treatment, and then the growth of these survivors and their appearance in fresh product for more irradiation and further survival. This would have to occur on several occasions before a significant effect would be observed.