To achieve percutaneous delivery, the pH of the formulation must be below 3.5 to accomplish protonation of the ascorbic acid. Previously we have determined that maximal concentrations into skin can be achieved with daily applications of 15% L-ascorbic acid for 3 days. We have previously reported that topical L-ascorbic acid is capable of protecting skin against erythema produced by UVB and UVA irradiation by a mechanism unrelated to absorption of the UVR. Moreover, we have demonstrated that topical L-ascorbic acid can provide protection against UVB-induced immunosuppression and systemic tolerance to contact
allergens.