Human responses and safe work time limits for personswearing the US Army’s Self-Contained Toxic EnvironmentProtective Outfit (STEPO) with personal CO2 scrubber, O2supply, and upper body cooling system were determined fromhuman tests and human simulation modeling. STEPO is theArmy’s highest-level protective system. A rational thermalphysiological model was assembled to simulate the responsesand extend the human test results to other conditions. Comparisonsbetween human and simulated responses include coreand skin temperature, heart rate, thermal sensation, and waterloss. Safe work time limits were until core temperature andheart rate exceeded 38.3°C (101°F) and 150 bpm.