Dose-response and time-course characteristics of UV-A1 erythema

To determine the time course and dose-response characteristics of UV-A1 erythema in the Tayside region of Scotland.

Adult volunteers (skin types I and II [n = 13] and III and IV [n = 11]) were exposed to geometric dose series of UV-A1 irradiation from a high-output source on photoprotected lower back and inner forearm skin.

Photobiology unit in a university hospital.

The minimal erythema dose (MED) was recorded visually and erythema was assessed objectively by erythema meter at 4, 8, 24, and 48 hours after exposure.

Peak erythema (lowest visual MED) was seen at 8 hours on the back and arm in 11 subjects with skin types I and II and on the back at 8 hours in 9 subjects and on the arm at 4 hours in 10 subjects with skin types III and IV. The lowest median (range) MED was 20 J/cm(2) (14-56 J/cm(2)) on the back and 42 J/cm(2) (20 to >80 J/cm(2)) on the arm at 8 hours for subjects with skin types I and II and 28 J/cm(2) (20-112 J/cm(2)) at 8 hours on the back and 56 J/cm(2) (28-80 J/cm(2)) at 4 hours on the arm for subjects with skin types III and IV. The D(0.025), an objective measure that corresponds approximately to the visual MED, demonstrated a broad peak from 8 to 24 hours.

Our local population is more erythemally sensitive to UV-A1 radiation than reports suggest. Daily dose regimens may risk cumulative erythema. Lower starting doses should be used in this population. The wide range of MEDs highlights the need for MED testing.