Abstract: Objective To study the action spectrum of ultraviolet light in chronic actinic dermatitis (CAD).Methods The minimal erythema doses (MEDs) of ultraviolet A (LVA) and ultraviolet B (UVB) were measured,by application of SUV1000 type ultraviolet simulator,in 131 presumptive cases of CAD and 13 cases previously diagnosed CAD.Results Eighty-five of 131 cases were diagnosed as CAD by a lower UVA-MED and/or UVB-MED.A lower UVA-MED was found in 89.41% of the 85 cases,ranging from 1 J/cm² to 20 J/cm²,while a lower UVB-MED in 69.41%,ranging from 2.8 mJ/cm² to 20 mJ/cm².Out of 85 cases,30.58% were found to be sensitive to UVA alone,and 10.59% to UVB alone.Out of the 13 cases previously diagnosed as CAD,3 showed normal MED for both UVA and LVB at the onset,and 9 showed that the UVA/ UVB action spectrums were changed.Conclusions UVA plays an important role in the action spectrum in CAD.The action spectrum of CAD can be changed from UVA to UVB and vice versa.

Key words: Ultraviolet rays, Photosensitivity disorders, Skin tests