Many factors can influence the responsiveness of a consumer's skin to a potential irritant. Some factors are intrinsic, inherent to the subjects themselves (e.g., sensitive skin, atopic skin), the body site, and previous traumas to the considered skin area. Other factors are external, such as composition of product, conditions of exposure, occupation of the subject, and climatic factors [4,5,7]. The reason why these factors are covered in this chapter are evident. Some cosmetics with anti-irritant ingredients are designed for some specific skin sites, such as the face, or considered as seasonal products, such as cosmetics against winter dryness of the skin.
Factors inherent to the constitution of the skin of the subjects that may influence skin responsiveness are numerous. A marked interindividual variability in response to irritants have been reported and ascribed to host-related factors. Considering the interindividual variability of subjects to skin irritants, one must mention here the concept of ''sensitive skin.'' The term sensitive skin clearly has a different meaning for consumers than for cosmetic scientists and dermatologists [4,6]. Consumers use the term sensitive skin to indicate that their skin readily experiences adverse reactions or unwanted changes to
external factors, such as the use of personal care products. Subjects with sensitive skin tend to more readily develop skin reactions to cosmetics and other topical drugs than do normal persons. Many attempts have been made by cosmetic scientists and dermatologists to describe and demonstrate in a scientific way what sensitive skin is. Visible effects, such as erythema and skin dryness, are noticed. However, half of adverse reactions are purely sensory perceptions, subjective symptoms of stinging, itching, burning, and feelings of dryness with or without visible effects.
Regional Differences in the Sensitivity of Normal Skin
It has been clearly demonstrated that when measuring the potential irritancy of cosmetic ingredients, great regional differences in the sensitivity of normal skin are observed [27,28]. Several factors must be considered in order to explain the observed regional differences in skin sensitivity, such as differences in total skin thickness, skin permeability, the amount and composition of epidermal and sebaceous lipids, blood microcirculation, hydration level of the horny layer, thickness of the horny layer, and desquamation rate and local daily exposure to irritant products. Most skin-irritation phenomena are noticed in the face.
Influence of Gender, Age, and Ethnic Group
Contradictory data are presented in the scientific literature about the influence of ethnic group on skin sensitivity . It has been demonstrated that the irritant response may be higher in babies and children and decrease with age . Concerning skin sensibility to irritants related to gender, many studies show that women are more reactive than men [31,32]. However, this difference could be attributable to the fact that women are more exposed to household chemicals and more frequently use face care cosmetics, rather than related to real physiological differences. Other factors are external to the subject, such as composition of their usual products, conditions of exposure, occupation of the subject, and climatic factors.
Acute skin exposures of a very irritant chemical cosmetic ingredient are very rare and attributable to accidents, inadequate use, or problems in the manufacturing of the cosmetic product. The list of very irritant products are known and must be totally avoided or used at very low concentrations; we will be dealing mostly with subacute and chronic exposure of the skin. Subacute exposure will provoke an immediate impairment of the skin barrier. Repeated exposures to certain cosmetic products with very limited impairment of the skin barrier can induce, after a certain time, significant cutaneous reactions.
There is clearly a seasonal or climatic effect on the amplitude of the skin irritation reaction. Generally, much higher irritation reactions are observed in winter than in summer. This difference is related to a dehydration factor: a situation of dryness of the horny layer provoked by ambient air with very low relative humidity. This situation is particulary present on the lower legs and more frequent in older subjects; typical symptoms include winter xerosis, extreme dryness, scaling, and rough skin surface. Furthermore, in the win
ter the epidermis is more aggressed by extreme temperature changes between the inside and outside world. In the summer period, the upper layers of the epidermis are well hydrated, and the skin is smooth unless excessively exposed to sun damage. Actinic aging of the skin is characterized by various clinical symptoms, including dryness of the skin.
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