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.