Psoriasis is a chronic skin disease at which the speed of production of new cells
within the epidermis is about four times higher than normal. A skin that is affected by
psoriasis renews itself in about a week rather than in four weeks, being the normal
rate of renewal. The structure of the fast renewing skin differs significantly from that
of a normal skin and an increased expelling of dead skin cells causes flakes with
often a scale like structure (psore is Greek for scale). Radiation with UV-B had
proven to be effective for the treatment of several types of psoriasis. Furthermore
ultraviolet radiation suppressed the immune system and reduced the inflammatory
responses as often seen by skin diseases like psoriasis and eczema. A positive
result could sometimes be obtained with a so-called PUVA treatment where PUVA
stands for Psoralen- or Phototherapy UV-A. With this method the sensitivity of the
skin for ultraviolet radiation was first increased with help of a photosensitizing agent
called psoralen, a chemical compound that existed in certain plants and that could be
chemically synthesised since 1970. Subsequently the now sensitive skin was
radiated with UV-A and the methods for obtaining the proper doses was comparable
with that for the determination of the doses required for tanning. PUVA therapies were
also successfully applied in the treatment of eczema. Treatment of psoriasis always
required the supervision of a physician.