Mercury stable isotopes in human biomonitoring: source attribution, exposure pathways and monitoring readiness

Mercury (Hg) pollution remains a global environmental challenge, and human biomonitoring is essential for linking environmental contamination to exposure pathways. However, total Hg and species concentrations in hair, urine and blood often cannot separate overlapping sources or distinguish environmental inputs from biological processing. Hg stable isotopes provide an additional line of evidence because mass-dependent fractionation (δ 202 Hg) and mass-independent fractionation (Δ 199 Hg, Δ 201 Hg and Δ 200 Hg) retain information on source, environmental transformation and biomarker chemistry. This Review synthesizes peer-reviewed evidence for Hg isotope applications in human biomarkers and related exposure media, with emphasis on source attribution, matrix selection and monitoring readiness. Hair isotope measurements currently provide the strongest evidence for screening dietary methylmercury exposure from fish, marine mammals and rice, especially when paired with local source samples and Hg speciation. Urine is more relevant for inorganic Hg and elemental Hg vapor exposure, but the evidence base is smaller and more affected by analytical and biological uncertainty. The exploratory quantitative synthesis comprised 360 extracted observations aggregated into 26 study/location/biomarker/source records. Between-study heterogeneity was substantial (I 2 generally >95%). Leave-one-out linear discriminant analysis achieved 25.0% accuracy and 15.0% balanced accuracy, both below the 41.7% majority-class baseline; leave-one-reference-out accuracy was 16.7%. Isotope contrasts should therefore not be treated as universal diagnostic cutoffs. We propose an explicit, ordinal decision framework for deploying Hg isotope biomarkers in environmental monitoring programs and identify reporting standards needed for reproducible source attribution. These priorities can help move Hg isotope applications from geochemical case studies toward carefully validated tools for environmental pollution monitoring.