Stage-resolved zinc isotope dynamics in firefighters' hair after firefighting

Firefighting exposes firefighters to combustion-derived particulate matter enriched in hazardous metals, yet biomarkers capable of integrating repeated fire events and recovery under real exposure conditions remain limited. This study quantified hair metal concentrations and δ⁶⁶Zn in 53 hair samples collected from 15 Beijing firefighters across five stage-defined periods spanning pre-mission, active firefighting, and recovery. Hair δ⁶⁶Zn ranged from -0.79‰ to -0.13‰ (mean -0.44‰) and displayed a non-linear incident-recovery trajectory, enriched in the heavier Zn isotopes after repeated missions and remaining relatively enriched during early recovery before partially returning toward baseline at the late recovery stage. Linear mixed-effects modeling of δ⁶⁶Zn, accounting for repeated measures, showed that the Zn isotope response was significantly structured by stage-dependent associations with Ni, Pb, and Co, together with substantial between-subject variability (ICC = 0.82). To place these shifts in an environmental context, we compared the firefighter hair data with published Zn isotope signatures of combustion-influenced APM. Combustion-influenced APM commonly shows enrichment in the heavier Zn isotopes, indicating that the post-firefighting enrichment of hair δ⁶⁶Zn is directionally consistent with a combustion-related Zn perturbation recorded in a biologically filtered keratin matrix. Overall, these findings highlight the potential of hair δ⁶⁶Zn as a time-integrated tracer of firefighting-related combustion exposure.