How Different Is Human Hair? A Critical Appraisal of Global Hair Fibre Characteristics and Properties Towards Defining a More Relevant Framework for Hair Type Classification

This review evaluates the differences in human hair fibre characteristics and classification methods, highlighting the limitations of the common geo-racial classification into African, Asian, and Caucasian hair. It suggests that this approach does not adequately capture global hair diversity and proposes a more complex eight curl type classification, which may be difficult to reproduce. Key findings include the inverse relationship between hair curl and cross-sectional shape/area, with straighter hair being circular and curlier hair being elliptical. There is no significant difference in hair amino composition, but some proteins are associated with curly hair. Eumelanin is prevalent in all hair colors except red, which has high pheomelanin. Straight hair has higher tensile strength, while curly hair is more fragile due to structural issues. African hair has the highest lipid levels, Caucasian hair has the highest water sorption, and Asian hair the lowest. The review recommends reporting specific hair characteristics and donor data to improve study robustness and comparability.