Rare Earth Elements and Technology-Related Trace Metals in Paediatric Scalp Hair: A 2001 Urban Baseline from Spain

Rare earth elements (REEs) and technology-related trace elements are increasingly used in modern products and processes, but biomonitoring data in healthy children and adolescents remain scarce; scalp hair provides a practical, integrative matrix for assessing multi-element patterns over time. Scalp hair collected in April-May 2001 from children (6-9 years; n = 120) and adolescents (13-16 years; n = 97) living in Alcalá de Henares (Spain) was retrieved from archival storage and analysed in 2025 using a single QA/QC-controlled ICP-MS workflow. Seven REEs (Ce, La, Pr, Nd, Gd, Er, and Y) and nine technology-related trace elements (Bi, Sb, Th, U, Pd, Pt, Rh, Ir, and Rb) were quantified after rigorous decontamination; left-censored data were treated using Kaplan-Meier, regression on order statistics, and maximum-likelihood approaches, and population reference values were derived as percentile-based upper limits (P95, 95% CI). In children, REEs were frequently detected and showed strong within-suite covariation, with medians in the low ng g^-1 range (e.g., Ce ≈ 0.011 µg g^-1; La ≈ 0.007 µg g^-1), whereas in adolescents, most REEs were near reporting limits. Sb and U were ubiquitous in both age groups, while platinum-group elements were largely undetected. Shale-normalised REE patterns were subparallel across normalisers, La/Ce anomalies were centred below unity, and weak soil-hair correlations suggested multiple microenvironmental exposure pathways. These data provide a robust pre-diffusion baseline for REE metals in European youth, offering a benchmark for future urban exposome assessments.