Paediatric Scalp-Hair Biomonitoring of Nutritionally Relevant and Mixed-Source Elements in Urban-Industrial Spain

Scalp hair offers a non-invasive matrix for retrospective paediatric biomonitoring, although interpretation is complicated by endogenous incorporation, exogenous deposition and element-specific analytical detectability. We characterised B, Ca, Co, Fe, Li, Mg, Mo, Ni and Se in archived scalp hair collected in 2001 from 120 children aged 6-9 years and 97 adolescents aged 13-16 years residing in urban-industrial Alcalá de Henares, Spain. Samples were analysed in 2025 by ICP-MS using a common quality-controlled analytical workflow, with left-censored observations evaluated using censoring-aware methods. B and Li were highly censored (89.2-94.8% and 86.7-99.0%, respectively), whereas Fe, Mg, Mo and Se were consistently quantified. Adolescents showed higher median Ca (325 vs. 168 µg g^-1; q = 1.43 × 10^-9), Mg (24.0 vs. 9.05 µg g^-1; q = 8.91 × 10^-19) and Ni (0.169 vs. 0.0597 µg g^-1; q = 2.10 × 10^-5), but lower Fe (5.16 vs. 7.54 µg g^-1; q = 2.28 × 10^-11), Co and Mo than children. Sex-associated differences were also observed for several elements, although their magnitude and detectability varied between age strata. Residential heterogeneity was strongest among adolescents: median Ca in the mixed residential-industrial Zone IV was 967 µg g^-1 compared with 232-385 µg g^-1 in Zones I-III (q = 2.75 × 10^-12), while median Mg was 113 versus 18.8-24.3 µg g^-1 (q = 2.26 × 10^-6). These specimens, collected in 2001, stored dry in paper envelopes for approximately 24 years, and analysed together in the 2025 ICP-MS campaign, provide a distinctive historical paediatric biomonitoring baseline from a well-defined Spanish urban-industrial population. The findings support scalp hair as a screening-level matrix for investigating population patterns across developmental stage, sex and residential setting, while not supporting its use as a direct measure of nutritional status, internal dose or health risk.