Effect of the stratum corneum barrier function on cutaneous photosensitivity (UVB):
Relationship between trans-epidermal water loss and the minimum erythema dose

This research result was published in the journal Photodermatology, Photoimmunology, and Photomedicine in May 2019^*1 .

The cutaneous barrier function prevents the invasion of various external stimuli into the body while protecting the skin against moisture loss.
While people with atopic dermatitis and skin sensitive to stimuli are generally aware that their cutaneous barrier function is low, people with healthy skin tend to know less about this function. Recent research has demonstrated that the transdermal invasion of foreign substances is a greater cause of allergic diseases than oral intake. Findings have further shown that skincare (barrier care) during infancy helps prevent the development of allergies later in life. The role of the cutaneous barrier function has thus been receiving growing attention over the past years.
Kao has been among the first to investigate the cutaneous barrier function of the stratum corneum. Collaborative research by Kao and the Tokyo Women's Medical University has identified decreased levels of intercellular lipids, especially ceramides, in the stratum corneum of patients with atopic dermatitis^*2 .

In November 2010 Kao researchers analyzed the skin color and cutaneous barrier function of 285 healthy Japanese female volunteers (age 30 to 49 years old, average: 39.6±5.2 years old). The researchers measured the amount of water lost via the skin (trans-epidermal water loss, TEWL) and assessed skin sensitivity against UVB by determining the lowest UVB irradiating dose to induce erythema (minimum erythema dose, MED).
Darker skin is generally thought to be less prone to sunburn. The analysis, however, indicated that skin sensitivity against UV was negatively correlated not with skin color, but with the cutaneous barrier function (Figure 1). The negative correlation between UV sensitivity and the cutaneous barrier function was significant, and was reproduced twice in more than 250 female volunteers.

This research showed that skin with a reduced stratum corneum barrier function is prone to UVB-induced erythema. A person with a low cutaneous barrier function can thus be assumed to be more susceptible to UVB damage. Conversely, a person who is easily sunburned by UVB may have a low cutaneous barrier function. The findings suggest that persons with low barrier function or high susceptibility to UVB sunburn should carefully administer skin care to reinforce their cutaneous barrier function and protect their skin against UVB irradiation.
Kao is going to use these findings in its ongoing search for novel sun protection technologies for skin with low barrier function and high sensitivity, as well as technologies to improve the barrier function in all types of skin. The company continues to research new ways to keep the skin healthy and beautiful under all environmental conditions, including strong sunshine.

【MED:Minimal Erythema Dose】

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