First aminolevulinic acid (ALA) is applied to the skin. It is allowed to remain there for three hours, absorbing into highly metabolic cells (namely, cancer cells, which consume oxygen and nutrients faster than regular cells). Then the ALA solution is illuminated with an LED blue light for about twenty minutes. The chemical reaction that occurs produces by-products called porphyrins that proceed to obliterate the cancer cells. That phototoxic effect makes the skin red, swollen, and irritated, and then causes it to peel. Recovery from PDT takes about six to ten days. Oddly enough, the worse the skin looks after treatment, the better the results because local destruction of precancerous lesions causes the irritation and trauma to the area. For patients who have had skin cancer or a lot of sun damage and actinic keratoses, I recommend getting PDT three times a year.