Tar Phototoxicity and Phototherapy for Psoriasis

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Abstract
The phototoxicity of coal tars was determined by comparing the ultraviolet light (UVL) energy required to produce erythema at tar treated sites (minimal phototoxic dose [MPD]) with the energy required to produce the same degree of erythema at untreated control sites (minimal erythema dose [MED]). The ratio of MED/MPD is the phototoxic index (PI). Tars that were phototoxic had a PI of > 1. Using a UVA (320 to 400 nm) and a UVB (290 to 320 nm) light source, 15 subjects and six tars were tested. All tars were phototoxic to UVA but not to UVB (P < 0.0001). Although tar and UVL is a widely accepted treatment for psoriasis (Goeckerman therapy), the light sources employed at normal exposure times provide insufficient UVA energy to produce a phototoxic reaction to the tars that are used. The therapeutic response seen in psoriatic patients treated with tar and UVL should therefore not be attributed to tar phototoxicity.

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