Hair curl type and scalp health, part 2: FTIR spectroscopy studies and analysis of inflammatory cytokines

Background Part 1 of this study demonstrated that Walker Type 4 hair is associated with lower wash frequencies, leading to poor scalp conditions and that increasing wash frequency improves scalp health. This second part investigates how hair type and wash frequency affect scalp hydration, sebum levels and composition, scaling, and inflammatory cytokines using instrumental measurements. Methods Healthy female panellists with chemically undamaged hair were recruited in the US and South Africa based on natural curl type: wavy (Type 2), curly (Type 3) or kinky (Type 4). Assessments were performed at baseline (Visit 1) and 1 week later (Visit 2). Between visits, panellists washed their hair four times with a standard shampoo and conditioner. In‐vivo ATR‐FTIR spectroscopy was used to measure scalp sebum levels and composition. Scalp pH was recorded, and ELISA assays quantified IL‐1α and IL‐1RA from sebum collected via Sebutape™. Results Spectroscopic measurements at Visit 1 showed that lower wash frequencies were associated with higher sebum build‐up on Type 4 scalps. However, when washing habits were normalized across all groups, sebum levels did not vary by hair type. Controlled washing (four times between visits) significantly reduced scalp sebum levels and the quantity of sebum fatty acids. Scalp pH was initially higher in Type 4 hair than Type 2, particularly in the South African cohort. Following the controlled washing protocol, scalp pH decreased for all hair types by Visit 2, with no significant differences remaining between groups. Cytokine analysis revealed an association between sebum levels and cytokine concentrations, providing a tentative link to the inflammation and scaling observed in Part 1. Conclusions Part 2 confirms that specific scalp properties are related to washing habits rather than hair curl type. Lower wash frequencies, primarily observed with Type 4 hair, resulted in higher sebum build‐up. Frequent washing reduced sebum, fatty acids, scalp surface pH and cytokine levels. No differences in these markers were observed between curl types after 1 week of normalized washing. Therefore, increasing wash frequency may help reduce the incidence of scalp irritation in individuals with Type 4 hair.