Technology-Related Trace Elements in Paediatric Scalp-Hair Biomonitoring: Biomarker Validity, Environmental Relevance, and Implications for Risk Assessment.

Technology-related trace elements (TTEs), including rare earth elements (REEs), platinum-group elements (PGEs), silver, antimony, bismuth, barium, strontium, vanadium and radionuclide-associated elements, are increasingly relevant to environmental exposure assessment. Children and adolescents are important sentinel populations, and scalp hair offers a non-invasive matrix for multielement analysis. However, a reported hair concentration may combine follicular incorporation, sweat and sebum deposition and external particles, and may also be influenced by collection, washing, cosmetic treatment and analytical contamination. Separately, left-censoring and the statistical treatment of non-detects can materially affect population summaries and comparisons. This critical narrative review integrates paediatric, occupational, environmental and multimatrix evidence through a structured evidence matrix and an element-by-element audit of biomonitoring assessment values. Hair can identify spatial contrasts and source-related patterns, but current evidence generally does not support direct inference of absorbed dose or clinical risk. In long-term occupational REE exposure, urine shows stronger exposure-response performance than blood; paired hair-urine data for thorium discriminate exposed groups without establishing quantitative equivalence; antimony findings vary by exposure setting; and barium biomonitoring equivalents are available for urine and plasma, but not hair. No health-based guidance values validated specifically for paediatric scalp hair were identified for any of the target TTEs. A seven-stage weight-of-evidence framework is proposed, positioning hair as a complementary screening and hypothesis-generating matrix within multimatrix assessment.