272 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Since the term barrier has within its present connotation a concept of obstruction that can be taken in several ways, it seems important at the outset to attach certain descriptive terms to it, which should help to clarify the original use of the word and to define the degrees of effective- ness being dealt with here. A universal barrier, if thought of as a per- fectly exclusive obstruction, does not exist (certainly the skin is not one). Rather, all barriers presently available are imperfectly exclusive that is, no known agent will repel every attack made on it by every element of the environment each one is susceptible of penetration to some degree by some substance or other. Most of these agents are nonselective in their exclusiveness, permitting a wide variety of materials to penetrate their defense mechanisms. Others of them, however, are very selective in their ability to repel some things and not others for example, sun- screens will permit the transmission of light rays within only a limited spectral band. In the pharmaceutical and cosmetic industries the diver- sities of the nonselective agents are used to advantage in the formulation of specific products, designed to accomplish a specific kind of protection against specific attacking substances. In the composition of such a product, the protection may arise from several sources. To begin with, barrier agents are usually thought of as additives certainly the search for new agents is focused on chemical entities that can be added to or carried in vehicular materials. At the same time, a great amount of protection can be derived from the vehicles themselves. (Pharmacists have for generations spoken of the lack of penetration of "epidermatically acting" ointment formulations, e.g., medicihals incorporated into hydrocarbon bases.) The great body of vehicular necessities including emulsified cream bases, might be considered to comprise a group of "pharmaceutical barriers." The special agents dispersed in the vehicles then, might constitute a group of "chemical barriers." Both groups, it can be reasonably argued, effect their protec- tion by setting up "mechanical barriers," mere refinements of the clothing layers of the past. The most obvious mechanical barriers would be relatively heavy, stable layers of unctuous masses, virtually impenetrable by reason of their ability to fill the pores of the skin, their thickness, their density, and the physical- chemical nature of their components, to practically the entire environ- ment. Equally obvious are the cosmetic disadvantages of such protection. The same mechanism of action, in a highly reduced, much less visible form, is effected by the film formers, which group includes perhaps the majority of the nonspecific, protective barrier materials. In the case of ' these agents, the factors of ultimate importance are the physical-chemical properties, chiefly the ability to maintain high interfacial tensions between the film and the attacking elements. True chemical action resides in those

PROTECTIVE BARRIERS FOR THE SKIN 273 compounds that can actually enter into chemical combination with the hostile substances, rendering them harmless or nonirritant by neutraliza- tion or by complexation. When protection is sought against assault by a nonmaterial environmental factor, light in particular, other physical and physical-chemical properties of the barrier components must be utilized. In these cases, protective activity derives from reflection of light rays as by some of the common white pigments, or from selective absorption of harmful rays, as by the sunscreens. While the mechanism of action of barriers is in most cases the formation of mechanical, pharmaceutical, or chemical interference between the usual skin/environment relationship, there seems to be some correlation between barrier activity and skin moisture loss. The effect of a wide range of proved barrier materials on moisture loss of the skin has been measured (2) by strapping desiccators on the forearms of human subjects. Controls were run first on the bare skin then the equipment was placed over areas that had been covered with cosmetic raw materials. The variance between like samples was negligible, and the results were reported as averages of at least three determinations (as many as eight were obtained). The following table summarizes the findings: Material .4verage effect on moisture loss Petrolatum 48% reduction I.anolin, anhydrous 32% " Light mineral oil 28% " Lanolin alcohols (25% in mineral oil) 28% " Isopropyl palmirate 28% " Silicone oil 26% " Squalene 23% " Glyceryl trioleate 23% " Polyoxyethylene glyc!)ls and esters all showed no effect or caused an increase in moisture loss. This work supports a proposal (3) to attach priority of importance to the moisture content, particularly of the horny layer of the skin. Further, it confirms the opinion that fatty emollients soften the skin by restricting moisture evaporation. When such studies are considered from the point of view of mechanism of action, it would appear that effective barrier components may serve in a bilateral capacity, functioning in two direc- tions (both skin/barrier and barrier/environment) rather than in the single (barrier/environment) direction earlier expressed. There are to be found in the literature several systems of classification of barrier creams and protective agents. One of these (4) approaches the problem from a mechanistic aspect the categories include: 1. Pore filling products, such as vanishing creams 2. Bases containing a high proportion of inert powders 3. Water-repellent fatty substances 4. "Invisible glove" film-formingprotectives and 5. Rather special preparations which protect against specific irritants.