The photosensitizing agent is injected into the bloodstream. The agent is absorbed by cells all over the body, but stays in cancer cells longer than it does in normal cells. One to three days after injection, when most of the agent has left normal cells but remains in cancer cells, the tumor is exposed to light. The photosensitizer in the tumor absorbs the light and produces an active form of oxygen (singlet oxygen) that destroys nearby cancer cells. PDT has been used for the past 30 years and is a treatment that works. The limitation of this form of cancer treatment is that the light needed to activate most photosensitizers cannot pass through more than one centimeter of tissue: Most of the organic photosensitizers used in conventional PDT treatment are restricted to be activated only by ultraviolet or visible light which do not have deep tissue penetration.