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.

J. Soc. Cosmet. Chem. 29. 725-740 (1971) ¸ 1971 Society of Cosmetic Chemists of Great Britain Measurement of skin colour in vivo I. M. GIBSON* Presented in September 1970, at the 6th IFSCC Congress, Barcelona Spain. Synopsis--Surface colours can be measured by a number of METHODS based on REFLECTOMETRY, COLORIMETRY and VISUAL STANDARDS. In both humans and animals, difficulties arise because the SKIN COLOUR may change owing to changes in blood flow, caused by the pressure exerted by the measuring instrument, because the skin surface is easily deformed, and because it may be impossible to take the subject to the laboratory. Reflectance spectrophotometry provides most information about the spectral composition of light reflected from a skin sample and instrumental difficulties have been largely solved by the use of FLEXIBLE FIBRE OPTICS. Visual colorimetry is also a practicable technique when fibre optics are incorporated in standard instruments. The simplest method is to view a set of grey standards through a monochromatic yellow filter. This eliminates the colour difference between the standard and skin, and a value can be given to the skin relative to the grey scale. INTRODUCTION The appearance of skin is governed both by the nature of the skin surface and by underlying structures in the epidermis, dermis and sub- cutaneous tissues. The surface properties of skin contribute to its apparent dryness, greasiness and surface texture, whereas the deeper structures are responsible for such properties as wrinkles and colour. The colour of the skin is largely dependent on the presence of melanin in the epidermis, on the natural yellowish colour of the epidermis and dermis, on the number of blood vessels present in the upper dermis and the extent to which they are *Unilever Research Laboratory, Isleworth, Middlesex. 725