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Keywords:

  • erythema;
  • phototherapy;
  • pigmentation;
  • UVA;
  • UVB

Background/purpose: 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.

Conclusion: 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.