Psoralens are materials that make the skin more sensitive to UV light. They are photosensitizing agents found in plants naturally and manufactured synthetically. Psoralens are taken as pills (systemically) or can be applied directly to the skin, by soaking the skin in a solution that contains the psoralens. They allow UVA energy to be effective at lower doses. When combined with exposure to the UVA in PUVA, psoralens are highly effective at clearing psoriasis and vitiligo. In the case of vitiligo, they work by increasing the sensitivity of melanocytes, the cells that manufacture skin color, to UVA light. Melanocytes have sensors that detect UV light and trigger the manufacture of brown skin color. This color protects the body from the harmful effects of UV light. It can also be connected to the skin's immune response.
LED PUVA lamps give much more intense light compared to fluorescent type lamps. This reduces the treatment time, makes the treatment more effective, and enables the use of a weaker psoralen.
The physician and physiotherapists can choose a starting dose of UV based on the patient's skin type. The UV dose will be increased in every treatment until the skin starts to respond, normally when it becomes a little bit pink.
Normally the UVA dose is increased slowly, starting from 10 seconds and increased by 10 seconds a day, until the skin becomes a little bit pink. When the skin is little bit pink the time should be steady.
To reduce the number of treatments, some clinics test the skin before the treatments, by exposing a small area of the patient's skin to UVA, after ingestion of psoralen. The dose of UVA that produces redness 12 hours later, called the minimum phototoxic dose (MPD), or minimal erythema dose (MED) becomes the starting dose for treatment.