Determination of the Minimal Erythema Dose for Ultraviolet A1 Radiation

Background: There is limited knowledge of the potential of ultraviolet A1 (UVA1) radiation to induce erythema.

Objectives: To investigate the minimal erythema dose (MED) for UVA1 in a German and Austrian population and to determine UVA1 dose-response curves for erythema and pigmentation.

Methods: We determined the UVA1-MED in 59 participants using a geometric increase in the dose. Visual readings of erythema and pigmentation were performed at 10 min, 2 h and 24 h after the UVA1 irradiation. Colorimetric measurements were taken to determine erythema (a* value) and pigmentation (L* value) objectively.

Results: More than 30% (18 of 59) of the participants had a high-dose UVA1-MED (> 70-130 J cm ); 56% (33) had no detectable UVA1-MED after 24 h and eight (14%) had a medium-dose UVA1-MED (20-70 J cm ). We observed a stepwise decrease of the a* value measured at the UVA1-MED skin site from 10 min to 2 h and 24 h (P < 0·001). In contrast, there was a stepwise increase of the L* value measured at the UVA1-MED skin site from 10 min to 2 h and 24 h (P < 0·001).

Conclusions: Most participants had no measurable UVA1-MED even after commencing high doses such as 130 J cm . Only a minority of participants (14%) may develop UVA1-induced erythema in the low- and medium-dose region. Hence, it appears to be safe to start UVA1 phototherapy with 20 J cm of UVA1 and continue with subsequent daily dose increments of 10 J cm , up to the desired maximum dose.