An experimental study of irritant effects of urea in different vehicles.

The properties of urea as an irritant were investigated. Seventeen healthy volunteers were patch tested with 20% urea using petrolatum and water, respectively, as vehicles. Irritant effects of urea were assessed by clinical evaluation of patch test reactions as well as by various non-invasive methods. The inflammatory response was quantified by laser Doppler flowmetry measuring the superficial blood flow, and by ultrasound A-scan reflecting the edema formation. Impairment of the barrier function was indicated by measurement of transepidermal water loss (TEWL). It is concluded that 20% urea in petrolatum applied under occlusion for 24 h elicits significant inflammation (i.e. increase in blood flow and skin thickness) and causes impairment of the skin barrier (i.e. increased TEWL). The irritant impact of urea on the skin depends upon the vehicle used, the irritant effect being intensified when urea is dispensed in petrolatum compared with water.