Recrystallization in different sunscreen formulations after cutaneous application.
BACKGROUND: Data on the structure of sunscreens prior to and after application to the skin and the possible impact of these factors on their efficacy are still scant. AIMS: The microscopic structure of several commercial sunscreen products containing benzophenone-3 (BP-3) was analysed under a light microscope, prior to and 2 h after application of the products to the skin, and compared with various control preparations with or without BP-3. METHODS: All tested formulations were mounted on microscope slides, viewed under a light microscope and photographed. Samples were taken directly from original packages or from freshly prepared prescription formulations. Two hours after application to the skin, the samples were taken and processed for microscopy. RESULTS: In some commercial sunscreens numerous crystals were formed upon cutaneous application, whereas others contained crystals both prior to and after epicutaneous application. A single commercial product (a lamellar ambiphyl emulsion, SPF28) retained its regular structure throughout the study. Control preparations with or without BP-3 and/or octyl methoxycinnamate contained crystals after cutaneous application. CONCLUSIONS: Ingredients, most probably BP-3, in many commercial sunscreens are prone to recrystallization while on the skin which might interfere with their UV light-absorbing function.