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.