Heavy metal concentrations and inter-metal relationships in commercially available hair dyes of different colours: a chemometric evaluation.

Cosmetic hair colouring products may contain trace inorganic contaminants introduced through pigments, raw materials, manufacturing processes, or packaging. Repeated use may contribute to low-level human and environmental exposure. This study determined the concentrations of seven heavy metals (Cr, Co, Cu, Pb, Cd, Ni, and Fe) in 21 commercially available hair dyes representing seven colour categories obtained from retail outlets in Ibadan, Nigeria. Samples were digested using a nitric acid-hydrogen peroxide mixture and analysed by flame atomic absorption spectrometry. Method reliability was verified using procedural blanks, replicate analysis, and spike recovery (95-104%). Metal concentrations (mg/kg) exhibited colour-dependent variability. Chromium ranged from 0.230 ± 0.245 mg/kg in green dyes to 2.53 ± 0.12 mg/kg in blue dyes, while copper showed the highest variability, reaching 7.03 ± 4.67 mg/kg in red dyes. Lead and nickel also varied across colour categories, with relatively higher nickel concentrations observed in blue dyes. Multivariate analyses (principal component analysis and hierarchical clustering) revealed partial grouping patterns and inter-metal associations, suggesting potential common sources linked to formulation components. Although concentrations were generally within available guidance ranges, the co-occurrence of multiple metals suggests the need for continued monitoring of cosmetic products as a screening-level indication of potential contamination concerns. Given the limited sample size and market scope, the findings should be interpreted as exploratory baseline data for cosmetic surveillance and environmental monitoring rather than definitive health risk characterization.