An optimal amount of water, especially in the stratum corneum, is essential for maintaining normal functioning, healthy skin. 1 Therefore, the amount of water from the skin is carefully regulated, in part through trans-epidermal water loss. 1 Trans-epidermal water loss (TEWL) is an objective measurement of the amount of water passively evaporating from a designated area of stratum corneum in a fixed amount of time due to the water vapour pressure gradient on either side of the skin. It has historically been used as a measure of barrier water function and skin integrity in healthy individuals. 2 Advantages of using a non-invasive marker of skin integrity such as TEWL include a lack of skin barrier destruction, and the potential for early detection of subclinical effects. 3 Elevated TEWL values may reflect barrier dysfunction in diseases such as atopic dermatitis and psoriasis. 4,5 Oppositely, decreased TEWL values reflect an intact or recovered dermal barrier. 5 Thus, TEWL values are typically measured for in vitro permeation testing (IVPT) to reflect a healthy skin sample, and regulatory agencies such as the Food and Drug Administration (FDA) and European Group for Efficacy