Normal skin urea effects

Moisturizers often contain humectants of low molecular weight and lipids. Humectants, such as urea, glycerin, lactic acid, pyrrolidone carboxylic acid (PCA), and salts, are absorbed into the stratum corneum and there, by attracting water, increase hydration [54]. Lipids, for instance petrolatum, beeswax, lanolin, and various oils in moisturizers, have traditionally been considered to exert their effects on the skin solely by forming an inert, epicutaneous, occlusive membrane. They are incorporated into formulations on the basis of their technical and sensory properties rather than on their possible epidermal impact [52, 54]. However, topically applied lipids may also penetrate to the living cells of normal epidermis, be metabolized and significantly modify endogenous epidermal lipids [92]. In normal skin, a... 

Loden M (1995) Biophysical properties of dry atopic and normal skin with special reference to effects of sldn care products. Acta Derm Venereol Suppl (Stockh) 192 1-48 Loden M (1996) Urea-containing moisturizers influence barrier properties of normal sldn. Arch Dermatol Res 288 103-107... 

Evidence from at least one randomized controlled trial (level I) show that 10% urea is effective for the treatment of psorisasis, ichthyosis, and dry feet, and 4 to 10% for the treatment of dry atopic skin and senescent skin. Evidence from another well-designed clinical study (level II) supports the treatment of hand dermatitis with urea. Evidence at level I also exist showing barrier improving effects of urea in both normal and in dry skin disorders (atopic skin, ichthyosis). Furthermore, strong evidence exists (level I) for reduced susceptibility to SLS, but not to other external agents. No evidence has been found for successful treatment of seborrhoic dermatitis, perioral dermatitis, and keratosis pilaris with urea. 

The urea- and melamine-formaldehyde (UF and MF) resins present similar hazards. Free formaldehyde, which is present in trace amounts and may be liberated when their resins are processed or slowly afterwards, can irritate the mucous membranes (and can cause skin sensitisation). Formaldehyde is a metabolite occurring normally in the human body and is converted to formic acid by enzymic oxidation. Trace amounts of free formaldehyde can have an irritating effect on mucous membranes (and can cause skin sensitisation). Formaldehyde in the cured resin is believed to be due to left unreacted free formaldehyde, in addition, it is thought that it may be also be due to a demethylolation reaction and/or cleavage of methylene-ether bridges. UF resins and foams are banned in a number of countries. 

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