Validation of Fiala-based thermo-physiological model and predicted heat strain method for predicting rectal temperature and sweat rate in protective clothing

Protective clothing is essential equipment in hazardous environments but often induces severe heat stress due to its high insulation and limited water vapor permeability. This study experimentally validated the Predicted Heat Strain (PHS)

Method: (ISO 7933:2004), and the Fiala-based thermo-physiological model (FMTK) for 140 human-subject tests in protective suits ranging from air permeable to fully impermeable. Experiments were conducted in a climatic chamber at ambient temperatures from -10°C to 35°C with intermittent activity: 1.5 met during sitting and approximately 5 met during walking. Measured rectal temperature values and the sweat rate estimated based on mass balance were used to validate the model. In most permeable suits scenarios, both models achieved acceptable accuracy for rectal temperature (MAE ≤ 0.3°C). However, under warm and hot conditions with impermeable ensembles, PHS significantly overestimated rectal temperatures, with MAE values of 1.1°C and 1.0°C for Tychem F at 25°C and 35°C, and 0.9°C and 1.0°C for OPCH90 at 25°C and 35°C, respectively. In the same scenarios, FMTK remained more stable, with MAE values between 0.1°C and 0.2°C. For sweat rate, both models showed larger deviations, particularly at higher temperatures, with MAE values ranging from 38 to 189 g·h⁻¹·m⁻² for PHS and from 29 to 210 g·h⁻¹·m⁻² for FMTK. Overall, the

Results: confirm the greater robustness of FMTK for rectal temperature prediction and highlight the limited applicability of both models for sweat rate prediction under protective clothing conditions.