Population differences in skin structure and physiology and the susceptibility to irritant and allergic contact dermatitis: implications for skin safety testing and risk assessment.
Skin safety testing and risk assessment utilize a comparative toxicological approach whereby the inherent toxicity (irritation or sensitization) is related to exposure to determine the potential risk to a consumer population. However, consumers cover a broad spectrum of individual characteristics in terms of skin types and functions, as well as their basic habits and practices in use of consumer products. Thus, we try to use very conservative estimates for both inherent susceptibility to the toxic effect as well as the potential worst‐case exposures. While the inherent variation can be considerable even within a country, it is magnified even further when products are marketed globally. Questions have arisen, therefore, as to whether a skin safety testing approach conducted locally or regionally can adequately predict for adverse effects for the global consumer. A good deal of speculation exists that population differences are real and should mandate population‐specific safety testing prior to marketing products in certain regions of the world. In an attempt to address this question, this review has summarized extensive literature covering basic skin biology and function and susceptibility to irritant and allergic skin responses. Throughout this literature, there are individual studies demonstrating population differences in skin properties or in the responsiveness to chemical insult. Some of the research on this topic has pointed fairly convincingly to a demonstrable population difference. The best example of this is the tendency for blacks to have a lesser skin reactivity than Caucasians, likely due to a more impenetrable barrier. In contrast, the comparative data for Caucasians and Asians, for males and females, and for different age clusters are far less compelling. In terms of tendencies, there are little data to support any real difference in skin barrier function or in skin irritation responsiveness between Caucasians and Asians or between males and females. Comparing different age profiles, there does appear to be a slight decline in reactivity to irritants among the elderly (> 65 years). Susceptibility to skin sensitization tends to be more related to exposure than any inherent susceptibility, though some data exist to suggest a slightly increased sensitivity in females versus males. On the basis of these data, standard procedures for skin safety testing and risk assessment can be considered relatively conservative. Most clinical skin safety studies are conducted in test populations that are comprised primarily of female Caucasians between the ages of 18 and 65 years. If anything, the skin reactivity “character” of this population cohort tends to skew toward the more reactive. This is not to say that population‐specific safety testing should not be done. There may be important reasons to pursue a population‐specific testing strategy in certain situations, perhaps due to regional, sex‐specific, or age‐specific marketing or to satisfy a regulatory or external relations need. However, there is yet no scientific justification to mandate such a strategy, based on our current knowledge of skin responsiveness and how it compares across diverse populations. There are clearly gaps in our understanding of population differences, particularly in regard to skin irritation that should guide future clinical research efforts. Side‐by‐side population testing for both acute and chronic skin irritation, more comparative testing of Asian subpopulations, testing for neurosensory irritation, and testing objective skin responses among hyperreactive subpopulations of multiple races are all areas in which additional research is needed. While current testing strategies have provided an excellent track record of success in providing safe and effective products to the world's consumers, results of such research will, in the future, help us to refine and further improve our skin testing and risk assessment capabilities.
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