Determination of the minimal erythema dose for ultraviolet-A1 radiation

Summary

Background

The knowledge on ultraviolet A1 (UVA1) radiation's potential to induce erythema is limited.

Objectives

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

Methods

We determined the UVA1-MED in 59 subjects using geometric dose increments. Visual readings of erythema and pigmentation were performed at 10 min, 2 h, and 24 h after UVA1 irradiation. In addition, colorimetric measurements were performed in order to objectively determine erythema (a* value) and pigmentation (L* value).

Results

More than 30.5% (18/59) of subjects had UVA1-MED within the therapeutic UVA1 high-dose region (> 70 – 130 J/cm²). 55.9% (33/59) of subjects had no detectable UVA1-MED after 24 h. 8/59 (13.6%) subjects had a UVA1-MED within the medium-dose region (20 – 70 J/cm²). We observed a stepwise decrease of the a* value measured at the UVA1-MED skin site after 10 min, 2 h, and 24 h (P < 0.0001). By contrast, there was a stepwise increase of the L* value measured at the UVA1-MED skin site after 10 min, 2 h, and 24 h (P < 0.0001).

Conclusions

Most subjects had no measurable UVA1-MED even after commencing high doses such as 130 J/cm². Only a minority of subjects (13.6%) may develop UVA1-induced erythema within the low- and medium-dose region. Hence, it appears to be a safe approach to start UVA1 phototherapy with 20 J/cm² and carry on with subsequent daily dose increments of 10 J/cm² up to the desired maximum dose.

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