To the Editor.—
Taylor and Stern have shown1 that a course of phototherapy for psoriasis resulted in the development of an average tolerance equivalent to about 17 times the pretreatment minimal erythema dose. The radiation source used in their study was a fluorescent UV-B sunlamp, and the authors acknowledge that their results may not be applicable to groups of patients treated with different lamps. We agree with this reservation and would suggest that very different results would have been achieved had other lamps been used.The significant quantity of highly erythemogenic UV-C(<290 nm) and short-wave UV-B radiation (290 to 300 nm) emitted by fluorescent "sunlamps" of the type used by Taylor and Stern would be absorbed strongly by proteins in the stratum corneum, and a small increase in thickness would result in a large reduction in radiation transmitted to the viable layer of the epidermis and, hence, an increase in minimal erythema dose.
Diffey BL, Farr PM. Magnitude of UV-B—Induced Tolerance. Arch Dermatol. 1992;128(3):410-411. doi:10.1001/archderm.1992.01680130132024