Transepidermal water loss , across

Relative TEWL indicates the value of transepidermal water loss across the compromised skin barriers relative to that of normal, intact skin. 

Transepidermal water loss (TEWL) is a natural occurrence that takes place in the skin layers. TEWL is the result of movement of water from the deep skin layers across the epidermis into the outside atmosphere. It is a tightly regulated process that is controlled by the stratum comeum [9], Occlusive topical bases and devices, such as transdermal patches, block TEWL and cause increased hydration of the skin. Hydration of the skin increases the permeation rates of compounds transdermally. The occluding effect of transdermal patches is an important mechanism that promotes increased diffusion of the compound across the skin into the systemic circulation [10]. 

It has been known for some time that the intercellular lipids of the stratum corneum play a very important role in the skin barrier function. This knowledge has been accumulated from systematic studies on skin permeability of compounds of varying lipophilicity (Scheuplein 1965 Roberts et al. 1977 Durrheim et al. 1980 Surber et al. 1993) and investigations of alterations in transepidermal water loss (Elias and Feingold 1992 Aszterbaum et al. 1992). Because the major route of permeation across the stratum corneum is via the intercellular lipid, the rate at which permeation occurs is largely dependent on the physicochemical characteristics of the penetrant, the most important of which is the relative ability to partition into the intercellular lamellae. 

Beyond the pure physicochemical interactions described, there is increasing evidence that jet fuels themselves can alter skin structure, and it is probably by this additional mechanism that may be associated with increased dermal absorption of permethrin. These chemical-induced modifications in skin structure have been demonstrated by increased transepidermal water loss and significant dermatotoxicity at the macroscopic and molecular levels in skin (Monteiro-Riviere et al., 2001, 2004 McDougal and Rogers, 2004). It is therefore no surprise that chronic exposure to these mixtures of solvents can enhance jet fuel hydrocarbon absorption (Muhammad etal., 2004). This is characteristic for chronic dermal exposures to solvents and strongly suggests that military persormel are more likely to absorb hazardous chemicals across their skin if they are chronically exposed to jet fuels or solvrait-related chemicals. 

Mechanical damage provides one model for a diseased skin state [112, 119, 120]. Mechanical damage and stratum comeum removal by tape stripping can iuCTease transepidermal water loss (TEWL) to a level likely attained in various dermatoses and is the most conunonly utilized method of barrier perturbation due to its simplicity. Abrasion by means of a brash bristle or a needle drawn across the skin s surface has been shown, by transmission electron microscopy, to loosen the top layers of the skin [117]. Abrasion may also enhance and control the delivery of vitamin C, 5-aminolevulinic add, vaccines, and biopharmaceuticals [121-124]. However, mechanical damage has infrequently been anployed either in vitro or in vivo human skin as a means to study absorption enhaneement. 

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