Minimal erythema dose after multiple UV exposures depends on pre-exposure skin pigmentation.
OBJECTIVE
Phototherapy consists of multiple ultraviolet (UV) exposures. Most previous studies have focused on erythema following a single UV exposure in fair-skinned persons. Although it is well known that phototherapy lowers the daily UV-threshold dose for erythema in clinical practice, this is insufficiently documented under controlled experimental conditions. The purpose of this study was to quantify the change in the daily threshold for a dose specific erythema grade after 1-4 consecutive daily UV exposures.


METHODS
Forty-nine healthy volunteers (skin type II-V) with varying pigmentation quantified by skin reflectance. Two UV sources were used: a narrowband UVB (Philips TL01) and a Solar Simulator (Solar Light Co.). Just perceptible erythema after 24 h was chosen as the minimal erythema dose (+); besides + and ++ were assessed.


RESULTS
We found a positive and significant exponential relationship between skin pigmentation and UV dose to elicit a specific erythema grade on the back after 1-4 UV exposures. After repetitive UV exposures the UV dose had to be lowered more in dark-skinned persons compared with fair-skinned persons to elicit a certain erythema grade. This applied to both UV sources and all erythema grades.


CONCLUSIONS
In the dark-skinned persons the daily UV dose after the 4 days UV exposure should be lowered by 40-50% to avoid burns compared with the single UV exposure. For the most fair-skinned persons essentially no reduction in the daily UV dose was needed. Our results indicate that the pre-exposure pigmentation level can guide the UV dosage in phototherapy.
Henriksen
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Department of Dermatology, Bispebjerg Hospital, University of Copenhagen, Bispebjerg Bakke 23, DK-2400 Copenhagen NV, Denmark. mette.henriksen@yahoo.com
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D000328:Adult; D000704:Analysis of Variance; D004307:Dose-Response Relationship, Radiation; D004890:Erythema / Q000209:etiology*; D005260:Female; D006801:Humans; D008297:Male; D012044:Regression Analysis; D012867:Skin / Q000528:radiation effects*; D012880:Skin Pigmentation; D014466:Ultraviolet Rays / Q000009:adverse effects*