Background Infrared radiation (IR) exposure generates reactive oxygen species and induces matrix metalloproteinase‐1 expression in human skin. Moreover, while not as acute as ultraviolet radiation, repeated infrared irradiation can result in the photoaging of skin. Broad‐spectrum sunscreens can protect skin from IR, but no human in vivo test methods for the evaluation of sunscreens' IR protection effect have been developed. We aimed to develop such a method. Materials and Methods We included 155 Korean subjects in our three‐part clinical study. The IR reflectance of subjects' skin was measured using a benchtop model of an IR light source and a reflectance measuring probe. We measured the IR reflectance in relation to skin color and hydration level to set up our experimental conditions. We then calculated the infrared protection factors (IPFs) of cosmetic emulsions as the IR reflectance ratio between cosmetic sunscreen‐applied skin and non‐sunscreen‐applied skin and assessed the relationship between IPFs and the amount of sunscreen ingredients. Finally, this method was validated using several commercial sunscreen cosmetics. Results Skin color and hydration level did not influence the IR reflectance of subjects' skin. The IPFs of cosmetic sunscreens showed a positive correlation with the amount of inorganic sunscreen ingredients. Conclusion In this study, we developed a simple, fast, and ethically acceptable human in vivo test method for evaluating the IPFs of cosmetic sunscreens.

Ultraviolet (UV) radiation has beneficial effects on humans as the major source of vitamin D synthesis and in the context of heliotherapy. 1 However, excess UV radiation induces acute and long-term damage, such as sunburn, photocarcinogenesis, and photoaging. 2 The human immune system is also altered by UV radiation. 3 To avoid these harmful effects of UV radiation, especially cutaneous damage, people apply sunscreen to their skin. The level of protection provided by sunscreen against UV radiation is affected by multiple conditions, including UV filter distribution [4][5][6] and the thickness or quantity 7-10 of sunscreen applied to the skin surface. To ensure sufficient protection from sunscreen, it must be applied properly. The World Health Organization (WHO) recommends the proper use of sunscreen, which it defines as liberal application 20 minutes before sun exposure and reapplication every