Correlation Between Sun Protection Factor and Hair Color Difference Index in a New Method for Evaluating Sunscreens'...

This study aimed to establish a novel method for evaluating ultraviolet (UV) protection efficacy on hair, focusing on the relationship between sun protection factor (SPF) and UV-induced hair color changes. We propose a hair SPF (HSPF) that measures the extent of hair color change following UV irradiation.

In in vitro tests, we used UV standard samples with known SPF values (P2, P5, and P8) to construct a standard curve according to hair color changes before and after UV irradiation. Hair samples were analyzed using the CIELAB color space system, calculating the ΔE value to represent the color difference. To assess the UV protection properties of various hair care formulations (oils, essences, and balms), we compared their HSPF with the standard curve. Meanwhile, in vivo tests followed the ISO 24444 guidelines, involving ten participants to validate the correlation between the in vitro and in vivo methods.

Higher SPF products resulted in smaller ΔE values, indicating less color fading and greater protection. The in vitro HSPF values strongly correlated with the in vivo SPF results, particularly for oils and balms (R² = 0.734). However, formulations with high-moisture content exhibited lower correlation.

The HSPF offers a new perspective on the level of hair protection from UV damage and may be promising for the future development and efficacy testing of UV protection products for hair.