PROTECTIVE BARRIERS FOR THE SKIN 277 summertime, can become more than a little dangerous. It is hoped that adequate labeling instructions will destroy this false image of protection certainly, salespeople ought to be instructed to give warnings of the limi- tation of protection of these products. The first concern of the cosmetic chemist with these protective agents is his assignment to incorporate them into formulas for finished products that will possess a high degree of customer acceptance. Seven criteria by which finished barrier creams may be judged have been proposed (15). 1. Good consistency 2. Ease of application 3. Persistent adherence to skin 4. Ability to form a coherent, impervious and flexible film 5. Freedom from irritating properties 6. Ease of removal when desired 7. Aesthetic acceptability The usual approach to the problem of creating an acceptable and effective preparation is an empirical one. Several formulas may be worked through, until one is found that meets the criteria fairly well, or lacking that con- tingency, until the several formulas can be mutually evaluated and one of them selected from the series. This archaic, trial and error method of approach is time consuming and costly and all too often, the choice is reduced to the best formula from a wholly unacceptable lot. A far better, but less frequently applied method makes use of the physical and chemical properties of the proposed constituents of a formula, from which, by scien- tific reasoning, may be predicted the properties of preparations under study. 7['his approach, of course, will not ensure success on a first at- tempt, but it can eliminate from consideration some formulas which ob- viously will not conform to the preselected standards. Formulation of barrier creams presents a number of problems to the chemist. For ex- ample, among those physical and chemical properties that have to be considered most carefully are pH, surface activity, and rheology. A desirable pH for most barrier preparations is 5.5 to 5.6 (16), if they are to be most effective. On the other hand, agents used to protect the skin from acids and bases will be most efficacious at pH values below 5.5 or above 5.6, depending on the type of barrier desired. UnIbrtunately, it is virtually impossible to measure the pH of a finished cream accurately even with the most superior pH meter. The apparent pH values obtained in the nonaqueous systems that characterize creams are at best merely qualitative and have no quantitative validity. However, a knowledge of the properties of the individual ingredients and the application of established theories to predictive calculations can ensure within reasonable limits a particular pH for a finished product.

278 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Barrier creams frequently are water-in-oil or oil-in-water emulsions. Therefore, a knowledge of surface properties of the barrier agents, emulsi- fiers and other components of the formula is essential to the chemist who hopes to prepare stable emulsions. Some agents, for example the sili- cones, are immiscible with most materials used in cosmetic and pharma- ceutical preparations, so they are almost exclusively found in emulsion forms (17). Silicones are repellent to both water and oils (18), a property that is advantageous from the point of view of protection but certainly disadvantageous to the formulator. It is known that auxiliary emulsi- fiers, such as methyl cellulose, are practically worthless for the emulsifi- cation of these silicone oils, and the more powerful and expensive surfac- rants must be used (19). A British firm claims that polyvinylpyrrolidone (PVP) in concentrations of 0.5 to 1.0 per cent enhances stability in ad-- dition it is supposed to increase protection of the emulsion barrier creams (20). A careful study of the surface chemistry involved in the interaction of the ingredients of a cream can help the chemist select the type and con- centration of the surfactant that will effect the most stable emulsion and that will prevent undesirable phase inversion, a phenomenon that could completely defeat the purpose of the barrier. Another important factor in the formulation of barrier creams--or of any cream--is rheology, since many substances used in them are thixo- tropic in nature. Careful control of mixing or blending conditions can be critical variation in time or efficiency of blending can alter markedly the consistency of the finished product. Many other considerations, of course, must be taken into account before formulation is begun. Problems of standardization of raw materials, coloring and scenting are beyond the scope of this discussion however, they are none the less important. It should be re-emphasized that, for his own maximum benefit, the formulator should resolve as many problems as he can anticipate before, rather than after, formulations are tested in the laboratory. When the magnitude of the problem of cosmetic protection for the skin is contemplated in its entirety, it becomes quite apparent that research in this area has just gotten started. The idea of skin protection has been expanded from the limitations of a generation ago (standard hand lotions, suntan products, and such others that were aimed at protecting the skin against the vagaries of the weather) to embrace products that will provide a cosmetic defense against common household irritants and, even more important economically, against industrial raw materials with a high potential for producing occupational dermatoses. "Some concept of the importance of industrial skin protectives may be obtained from the fact that diseases of the skin due to occupation rep- resent from l0 to 12 per cent of all diseases of the skin, and that they