Psoralen has been present in sunscreens in order to allow suntanning despite the reduced UV-intensity that acts on the deeper layers of the skin.
In other countries these substances have been present in sunscreens until the first epidemiological results have shown that users have a fourfold risk of developing melanoma.
The increased production of melanin is the reaction of the skin to UVB-induced direct DNA damage.
[6] Several substances are known to increase the amount of direct DNA damage (thymine dimers).
[7][8] Other Web sites state correctly that: "Coumarins produce photosensitivity therefore advise the patient to avoid direct sunlight after treatment.