Human Work Capacity as a Dynamic Boundary Condition of Industrial Sustainability

Industrial sustainability is commonly evaluated through environmental impact, resource consumption, and operational resilience indicators. These metrics describe system performance but do not define whether production remains within human physiological limits. This study develops a dynamic capacity-constrained sustainability model that treats human work capacity as a bounded system state rather than a descriptive social variable. The model formulates capacity as a continuous-time variable governed by aggregated exceedance of thermal and physical tolerance limits and by a recovery parameter representing biological restoration. A stability threshold is derived analytically, defining a critical exceedance level above which steady-state capacity declines below the minimum functional requirement for stable operation. Sensitivity analysis demonstrates that equilibrium capacity decreases nonlinearly with increasing exceedance and depends on the recovery rate. A numerical illustration under summer thermal exposure conditions shows that two production configurations with identical environmental and resource indicators may fall on opposite sides of the stability boundary due to differences in aggregated exceedance. The results indicate that sustainability assessment requires integration of measurable physiological constraints. Human work capacity functions as a dynamic boundary condition that conditions system stability beyond conventional environmental performance metrics.