192 JOURNAL OF COSMETIC SCIENCE may be of major importance to the plasticity of the skin (14). Moreover, lipid-rich formulations improve skin distensibility, while creams and gels with lower lipid content have a more pronounced effect on (short term) skin hydration, as discussed above (14). The vehicle regreasing effect seems to be limited to the application period, and not a long-term effect, while the related increase in stratum corneum moisture content might be longlasting (10). Finally, it should be noted that compounds with distinct dielectric constants have been shown to influence the electrophysical properties of the stratum corneum, as measured by capacitance- or conductance-based instruments (10). Thus, it is plausible that topi­ cally applied moisturizing creams might be a source of false-positive or false-negative results using these instruments (15). Although a good correlation between capacitance values and water content of the tested creams has been demonstrated (15 ), a sufficient time lag following application of compounds should be allotted before any such mea­ surements are registered. Moreover, the regression curve of vehicle effects should be studied as a function of time. BARRIER PROTECTION AND BARRIER RECOVERY A number of factors are involved in the determination of the effectiveness of dermato­ logical and cosmetic vehicles to protect the skin barrier. Commonly used barrier creams, which are either w/o emulsions or cosmetic vehicles with strongly lipophilic character, are claimed to protect against hydrophilic irritants. On the other hand, barrier creams that are o/w emulsion systems, or act like hydrophilic systems, are mistakenly thought to protect against lipophilic irritants. Dermatological skin protection (especially in work conditions) is based on different product groups in situations where barrier or protective creams are employed. For example, pre-exposure skin care includes the use of o/w and w/o emulsions, tannery substances, zinc oxide, talcum, perfluorpolyethers, chelating agents, and UV protectors (16). However, cleansing products and post-exposure skin care are two other important components of skin protection. The post-exposure skin care is based on dermatological and cosmetic vehicles, long-term moisturizers, fast humec­ tants, and lipid-rich formulations. Although protective effects have been shown using specific test conditions, double-blinded, placebo-controlled, randomized trials are still lacking, especially under conditions that approximate real workplace situations (16). In fact, cumulative stress tests with repetitive application of irritants appear to be the best conditions for approximating work conditions (17-23). The distinction between skin protection and skin care is not always clear. For example, in nurses a barrier cream was compared with its vehicle for effects on clinical improve­ ment. Interestingly, both clinical skin status as well as stratum corneum hydration improved significantly in each treatment group, without evidence of a difference be­ tween the vehicle and the barrier cream groups (24). Thus Berndt and colleagues proposed to abolish the distinctions between skin care and skin protection products (24). Correct instructions for consumer use should be stressed with regard to regular and frequent application of a protection product in order to be effective (25). In addition, a recent study discussed whether claims could be made with respect to protective and preventive properties of topically applied body lotions and barrier creams (26). In this particular study, enhanced stratum corneum hydration, improved barrier function, and a faster barrier recovery were reported after SLS barrier disruption (26).

ROLES OF VEHICLES FOR SKIN TREATMENT 193 Exposure to tensides represents a common potential workplace irritant. Protection against tensides seems to be more effective with a lipid-enriched, lipid-external-phase emollient, such as w/o emulsions (27 ,28). In contrast, Held and colleagues showed that a four-week pretreatment of normal skin with some w/o cosmetic vehicles increases susceptibility to detergents (sodium lauryl sulfate) (29). Incorporation of high amounts of emulsifiers into the vehicle and subsequently emulsifying the intercorneocyte lipids could be the reason for the controversial results. Moreover, emulsifiers can act as carriers of aggressive substances, enhancing their penetration into the skin. Alternatively, the deposited lipids can function as traps (solvents) for tensioactive molecules. Thus the long-term application of barrier creams in working conditions where detergents are present should be carefully evaluated. Clinical observations have established the knowledge that skin irritants are more harm­ ful in dry skin conditions. Therefore, vehicles often are used to increase the water content of the stratum corneum as a preventive measure (30,31). Moisturizer-containing cos­ metic vehicles prevent irritant skin reactions induced by detergents, and may also accelerate the regeneration of permeability barrier function in irritated skin (32). Cos­ metic vehicles with moisturizing properties usually contain, either singly or in combi­ nation, humectants such as ammonia, lactic acid, citric acid, and pyrrolidone carboxylic acid and their salts, and urea, glycerol, sorbitol, and amino acids. Most of these agents belong to a group considered "natural moisturizing factors" (NMF), as it is similar to the blend of hydrosoluble ingredients found in the stratum corneum. Their common prop­ erties include the increase of hydration and the enhancement of water-binding capacity in the upper stratum corneum. Reduced NMF content in the stratum corneum can diminish water absorption capacity and may result in perturbation of corneodesmolysis, leading to hyperkeratosis (33). It has been shown that dry environmental conditions increase epidermal DNA synthesis and amplify the hyperproliferative response to barrier disruption (34-3 7). Furthermore, changes in environmental humidity contribute to the seasonal exacerbations/amelioration of cutaneous disorders such as atopic dermatitis and psoriasis, diseases that are characterized by a defective barrier, epidermal hyperplasia, and inflammation (3 5 ). Application of topical glycerol prevented the epidermal hyperplasia and dermal mast cell hypertrophy and degranulation induced by exposure to low hu­ midity (3 5 ). Cosmetic vehicles with barrier properties can also prevent certain types of epidermal damage. For example, Fartasch and colleagues have shown that alterations of the lower part of the stratum corneum and damage to the nucleated layers of the epidermis are induced by sodium lauryl sulfate (38). In this model, formation of lamellar lipid mem­ brane structures was disturbed in the lower stratum corneum. In contrast, the upper stratum corneum showed intact intercellular lipid bilayers. The barrier disruption effect of SLS was prevented by the application of a barrier cream ( discussed in more detail below), with diminished sodium lauryl sulfate penetration as the likely mechanism (38). DERMATOLOGICAL AND COSMETIC VEHICLES IN A TOPIC DERMATITIS The value of well-formulated vehicles in the treatment of atopic dermatitis is widely recognized. In atopic dermatitis, stratum corneum hydration and water binding capac­ ity are reduced (39-41), and impaired permeability barrier function is readily ob-