Relationship between sun-protection factor and application thickness in high-performance sunscreen: double application of sunscreen is recommended

Background. High-performance sunscreen protects both healthy consumers and photosensitive patients from strong ultraviolet (UV) exposure. The sun-protection factor (SPF), which indicates the efficacy of UV protection, is determined using a prescribed sunscreen application thickness of 2.0 mg/cm2. Therefore, users should apply at least 2.0 mg/cm2 of sunscreen to obtain the level of UV protection expected from a product. In most cases, however, users apply insufficient amounts of sunscreen. Aim. To determine the amount of sunscreen applied under specific conditions, and the relationship between application thickness and SPF value in high-performance sunscreen. Methods. The amount of applied sunscreen was calculated under practical conditions and conditions that directed a double application. The SPF values of high-performance sunscreen applied at three thicknesses (2.0, 1.0 and 0.5 mg/cm2) were determined according to the international SPF testing method. Results. The relationship between SPF value and application thickness correlated in a logarithmic curve. The mean application thickness under practical conditions was approximately 1 mg/cm2, and directing subjects to use a double application increased the application thickness to nearly 2 mg/cm2. Conclusion. Encouraging a double application of sunscreen will help users apply products at a thickness sufficient to achieve expected SPF efficacy. We recommend that guidance on double application of sunscreen should be posted in public locations where sunscreen is likely to be in use.