In the cutaneous tissue, part of the moisture is present within the cells (low molecular weight hydrophilic substances) as in a sponge lattice, while another part, moisture in transit, comes from perspiratio insensibilis, a simple and constant evaporation process by which the water in the dermis reaches the outside after having crossed the whole epidermis. Environmental conditions (prolonged and excessive exposure to sunlight and overheated locations), incorrect hygienic treatments (improper cleansing), some pathological conditions (psoriasis, ichthyosis, etc.), and the use of drugs may affect these phenomena. Therefore, the hydration and well-being of the skin depend on the amount of water in the stratum corneum (the skin layer exposed to the environment), and is related both to the levels of NMF (Natural Moisturing Factor), and therefore of PCA (Pyrrolidon Carboxylic Acid, the natural "sponge" of the skin), and to the correct amount of perspiratio insensibilis, which is itself also conditioned by the presence of the right amount of surface and intercellular lipids.The acid pH film is another major skin protective factor because, in addition to preventing the entry of pollutants and microorganisms, it regulates the moisturizing process preserves the epidermal ecosystem in its proper balance and determines the differences between the various skin biotypes. However, the stratum corneum (SC) does not simply protect the human body from water loss, but it also contains water for plasticity and barrier function. Changes in SC water permeability and water content are therefore closely linked to skin health and disease.