Dynamic Toxic Exposure Assessment for Vulnerable Populations: Incorporating Evacuation Delay into Hazard Analysis

Conventional exposure assessments for toxic release events often rely on spatial hazard representations that provide limited information on how exposure evolves while evacuation is underway. This study proposes a dynamic exposure assessment framework that integrates time-dependent concentration profiles with scenario-specific evacuation completion times, including vulnerability-related evacuation delay. Exposure is quantified as accumulated exposure from release onset until evacuation completion, while the duration above a selected Acute Exposure Guideline Level (AEGL) reference threshold is used as a complementary temporal exposure metric. The framework is demonstrated using an ammonia-release scenario simulated with ALOHA and three illustrative evacuation-delay scenarios evaluated under identical hazard conditions. The results show that progressively delayed evacuation increases both accumulated exposure and the duration above the selected AEGL-2 reference threshold. However, the exposure contribution of additional evacuation time depends strongly on its temporal position within the evolving concentration profile. Additional evacuation time coinciding with the peak concentration plateau produced the largest marginal increase in accumulated exposure, whereas additional time during the later decay phase contributed comparatively less. A structured sensitivity analysis showed that the principal qualitative findings remained stable under ±15% perturbations of the reconstructed concentration profile, although the AEGL-2 exceedance duration was more sensitive to concentration variations. These findings demonstrate that exposure outcomes depend not only on evacuation duration but also on the temporal alignment between evacuation timing and hazard evolution. The proposed framework provides a structured approach for linking time-dependent dispersion outputs with evacuation assumptions and can be adapted to other toxic release scenarios when appropriate concentration–time data and substance-specific reference criteria are available. The approach may provide additional information for temporally informed emergency planning, particularly for populations requiring additional evacuation time or assistance.