The effect of on-site application density on the UV protection efficacy of sunscreens

Sun protection factor (SPF) and UVA protection factor (UVAPF) are performance indicators consumers recognize for UV protective cosmetics such as sunscreens. However, on-site application density affects actual UV protection, despite these indicators. To understand actual UV protection better, a more reliable manner is needed to verify application density for further discussion of photoprotection efficacy regarding public health.

To estimate the UV protective efficacy of sunscreen in actual use based on the application density of UV protective cosmetics and the analysis of UV protective effect modulated by application density.

The subjects applied the SPF-labeled sunscreens as usual. We measured the application amount and area including any amount on their hands to calculate the average application density on the face. Also, sunscreens were applied at densities of 0.5, 1.0, and 2.0 mg/cm² . The SPF values were measured at each application site to evaluate the effect of application density on photoprotection efficacy.

We established a method of measuring application density utilizing three-dimensional photograph analysis. The median application density of the sunscreen applied in actual use was 1.33 mg/cm² . The measured SPF values decreased in association with the decreased application density of sunscreens. Based on the estimate assuming the first-order correlation, the SPF value required to get the protective effect equivalent to a sunscreen with SPF 15, 30, or 50 at 2 mg/cm² was calculated to be 23.8, 47.5, and 79.2, respectively, with the application density of 1.33 mg/cm² .

We demonstrated a reasonable procedure for estimating the photoprotection efficacy of sunscreens on the face. A suggestion was made to consider the application density for further discussion of photoprotection among consumers, especially for the long term with respect to public health.