Crude coal tar plus near ultraviolet light suppresses DNA synthesis in epidermis.

Crude coal tar and a refined product, when they are combined with near ultraviolet (UV) light (UVA, 320-400 nm), depress DNA synthesis in vivo in normal and proliferating skin of the hairless mouse. Near ultraviolet light alone does not interfere with DNA synthesis, but coal tar without UVA does have an inhibitory effect on DNA synthesis of normal skin. This effect of coal tar without UVA is not as great as the effect of the combination of UVA and coal tar. Between 0.9 and 1.6 joules/sq cm of UVA are required for its biologic effect on DNA synthesis in coal-tar-treated skin.