The interaction between narrow-band radiation of UVA and that of UVB on erythemal reaction in Japanese subjects.

The interaction between ultraviolet A (UVA) and ultraviolet B (UVB) on erythemal reaction was investigated on the skin of the backs of 24 healthy Japanese volunteers, using narrow-band radiation. The minimal erythema doses (MED) for UVB (MEDB) and UVA (MEDA) were determined and UVA and UVB were both applied to the same site in immediate combination with 2 orders of exposure (A + B, B + A). In experiment 1 the interaction between suberythemogenic doses of UVA and UVB to produce a visible erythema was examined; the sum of the fraction of MEDA and the fraction of MEDB was at most equal to 1 for 3 different doses of UVA (1, 2.4, 4.9 J/cm2) with either order of exposure. This interaction is considered to be photoaddition. In experiment 2 a fixed dose of UVB (1.7 MEDB of each subject) was applied in combination with 4 different doses of UVA (1, 2.4, 4.9, 7.4 J/cm2); the erythemal reaction was compared with the grading scale erythema produced by UVB alone and allotted a score of 1 to 7. The total number of MED irradiated was calculated for each combination exposure and the score obtained was compared with the score expected on the basis of that number of MED. The scores for A + B practically coincided with the expected values, whereas those for B + A were decidedly smaller than the expected values. This suggests photoaddition for A + B and photorecovery for B + A in the bright erythema dose range. The relationship between the Japanese skin type (JST) and the waveband interaction was also examined. In experiment 2, when UVA irradiation was followed by UVB, the subjects in JST group III showed photorecovery for high-dose UVA.