Tursen Dermatology Clinic, Mersin, Turkey
Water covers most of the earth’s surface and composes most of the human body; this can illustrate the critical linkages between water, health, and ecosystems. From outer space, the earth looks like a “blue” planet because most of its surface is covered by water. Only 2.5% of that water is fresh, and most of that lies frozen and inaccessible in the polar ice caps and Greenland, leaving less than 1% of fresh water accessible in lakes, river channels, and underground. Only about one‐third of the world’s potential fresh water can be used for human needs. As pollution increases, the amount of usable water decreases [1, 2].
Water contributes significantly to health, and a daily intake of 1.5 to 2 L of water should be guaranteed, because good hydration is essential to maintain the body–water equilibrium, although needs may vary among people. However, the worldwide population is far from reaching the recommended allowance for water intake [2–4].
The skin is the largest organ of the body, providing a protective barrier against bacteria, chemicals, and physical insults while maintaining homeostasis in the internal environment. Such a barrier function in the skin ensures protection against excessive water loss .
Dry Skin and Other Disorders
Atopic Dermatitis (AD) and Dry Skin
A pivotal sign of atopic dermatitis (AD) is dry skin, xerosis. The importance of this symptom has been underlined by the finding that approximately one‐third of all patients with AD carry a mutation in the gene coding for the important skin barrier protein filaggrin. Lack of filaggrin results in a skin‐barrier defect, with dry and cracked skin. The mutation in filaggrin does not, however, explain why almost all patients with AD have a skin‐barrier defect. Inflammatory cytokines of the TH2 subtype can downregulate the expression of filaggrin in keratinocytes. The severity of AD dictates the level of treatment. In all cases, the basic therapy is moisturizers to alleviate the dry skin and the impaired skin barrier function. The use of moisturizers has also been shown to reduce the need for potent topical corticosteroids. In mild to moderate cases, topical calcineurin inhibitors or corticosteroids may be used both in a proactive manner to prevent flare‐ups, and as treatment of acute eczema [6–8].
Psoriasis and Dry Skin
In psoriasis, the epidermis is abnormal: the skin appears thickened and scalloped (hyperacanthosis), the stratum granulosum is absent, and the stratum corneum (SC) is much thicker. Patients are well aware that a minor mechanical trauma to the skin, such as light rubbing, can cause psoriasis lesions. This is known as the Koebner phenomenon, named after the Viennese dermatologist who described it around the end of the nineteenth century. Patients also know that ultraviolet rays acting on the skin’s surface are an excellent treatment for psoriasis. In reality, there is a constant exchange between keratinocytes, lymphocytes, and Langerhans cells, and the three essential cells in the skin’s immune system. Molecules that play an important role in psoriatic inflammation, such as IL‐23 and IL‐17, are the target of treatments known as “biological therapy” (monoclonal antibodies). These molecules act on the epidermis and its ceramide content, and reduce its ability to produce antimicrobial peptides, important elements in epidermal anti‐infectious defenses, and other functions. The use of emollient treatments ensures optimum hydration in the epidermis, and may play a role in preventing flare‐ups [8, 9].
Contact Dermatitis and Dry Skin
It is clear that the epidermal barrier plays the most important role in all forms of contact dermatitis (CD). There are several different types of CD: allergic contact eczema, caused by haptens (most commonly nickel); orthoergic CD, caused by irritants; and AD, in which contact with proteic allergens from the environment plays an important role. Among the factors aggravating the skin, there are strong and weak aggressions, and depending on the molecules and the dose, we may either be tolerant or sensitive to these aggressions. This is an important element in the concept of fragile skin – it can also be described as “dose‐dependent” or “aggression dependent,” independently of other individual genetic or pathological factors [10, 11].
Acne and Dry Skin
The facial skin of acne patients differs from normal skin of people without acne in many ways. Higher sebum production, the size of the sebaceous glands, higher TEWL (Transepidermal water loss), and lower SC hydration (decreased conductance) all support the SC permeability barrier impairment associated with acne. In addition, significantly reduced free sphingosine and total ceramides in patients’ SC are reported, indicating a deficient intercellular lipid membrane correlated with impairment of the SC permeability barrier. The increase in TEWL and decrease in SC hydration (conductance) were greater in patients with acne of moderate severity as compared to those with mild acne severity. Filaggrin is a key protein in epidermal differentiation and contributes to the structural and functional integrity of the SC. Alteration of filaggrin expression is associated with modification in the skin barrier, and leads to some skin diseases, such as AD. Within acne lesions, there is an increase in filaggrin expression in keratinocytes lining the follicle wall. In addition, P. Acnes has been shown to increase filaggrin expression in cultured keratinocytes and also in explants of human skin. Importantly, it is not known if the changes in filaggrin expression noted in acne are primary or secondary events. Some topical medications, systemic medications, and physical procedures used to treat acne and/or acne scarring can lead to alterations in the SC permeability barrier function based on documentation of increased TEWL, and in some cases visible signs of xerosis. Increases in TEWL have been reported with benzoyl peroxide, tretinoin, tazarotene, and isotretinoin .
Transepidermal Water Loss (TEWL)
TEWL is the outward diffusion of water through skin. TEWL measurements are used to gauge skin water barrier function. An increase in TEWL reflects an impairment of the water barrier. TEWL measurements allow parametric evaluation of the effect of barrier creams against irritants, and characterization of skin functionality in clinical dermatitis and in irritant and allergic patch test reactions. An evaporimeter determines TEWL by measuring the pressure gradient of the boundary layer resulting from the water gradient between the skin surface and ambient air. TEWL measurements can be affected by the anatomical site, sweating, skin surface temperature, inter‐ and intraindividual variation, air convection, ambient air temperature, ambient air humidity, and instrument‐related variables to name a few. Although TEWL is influenced by many variables, experiments show that evaporimeter measurements are reproducible in vitro and in vivo [4, 8].
TEWL is regarded as one of the most important parameters for characterizing skin barrier function, but an agreed‐upon definition of what “normal” TEWL is does not exist. In order to determine generalizable TEWL values for healthy adults, a systematic review and meta‐analysis was conducted. The databases MEDLINE and EMBASE and publication lists were screened. After full‐text appraisal of 398 studies, 231 studies were excluded due to unclear or insufficient reporting. One hundred and sixty‐seven studies providing data for 50 skin areas were included in the final data synthesis. Pooled sample sizes ranged from n = 5 for the left cheek and the left lower back, to a maximum of n = 2838 for the right midvolar forearm area. The lowest TEWL of 2.3 (95% CI 1.9–2.7) g/m2/h was calculated for the breast skin, the highest TEWL of 44.0 (39.8–48.2) g/m2/h was calculated for the axilla. TEWL in individuals 65 years and older was consistently lower compared to the group of 18‐ to 64‐year‐old individuals. The quality of reporting TEWL in humans should be increased in future studies.
Maintaining Skin Integrity in the Aged
Dry skin is characterized by decreased lipid content and a delayed reconstitution of the epidermal barrier after skin irritation. These are problems of high relevance in the aged population, especially in the development of irritant CD. Asteatotic and perineal irritant dermatitis are the most important subtypes of irritant CD in the elderly. This contribution presents a compressed survey on these subtypes and elucidates their relation to an impaired barrier function. Typical irritants affecting aged individuals are explained and compared with irritants that seem to be more significant in younger people. Results of biophysical investigations, such as measurement of TEWL, are discussed regarding their age‐dependence. TEWL decreases with age, which was formerly interpreted as an indication of a decreased sensitivity. Today, we know that reconstitution of the epidermal barrier after irritation is delayed once it has been impaired. Reasons are decreased activities of enzymes involved in lipid synthesis and processing, a changed cytokine profile, a reduced acidification of aged skin, and alterations in the function of epidermal stem cells. Owing to these new insights, a re‐evaluation of the sensitivity of aged skin has to be initiated, especially with regard to occupational dermatology .