Background: Topical preparations are sometimes applied before phototherapy without consideration of their potential to block UVB.
Objective: Our purpose was to examine the ability of topical preparations to block UVB.
Methods: Volunteers pretreated with mineral oil, a clear liquid emollient, 5% crude coal tar, 6% salicylic acid ointment, emollient creams, and petrolatum underwent minimal erythema dose testing. Transmission of UVB through a clear film coated with the preparations was measured.
Results: Tars and salicylic acid blocked UVB. Thick application of petrolatum and emollient creams can reduce transmission of UVB. Mineral oil and a clear liquid emollient did not significantly affect transmission or erythemogenicity of UVB.
Conclusion: Clear liquid emollient and mineral oil can be used before phototherapy. If not removed before phototherapy, preparations containing tar or salicylic acid, or thickly applied petrolatum or emollients, can block UVB and presumably reduce its efficacy in the treatment of psoriasis.