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

## TL;DR

This study analyzed rare earth elements and trace metals in the hair of children and adolescents in Spain in 2001 to establish a baseline for future environmental exposure assessments.

## Contribution

The study provides the first baseline data on rare earth elements in European youth hair, offering a pre-diffusion reference for future urban exposome studies.

## Key findings

- REEs were frequently detected in children's hair with low concentrations, while most were near detection limits in adolescents.
- Sb and U were commonly found in both age groups, but platinum-group elements were largely undetected.
- Shale-normalized REE patterns showed subparallel trends and weak correlations with soil, indicating multiple exposure pathways.

## Abstract

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.