# Applicability of Human Thermophysiological Model for Prediction of Thermal Strain in PPE
## 4 Discussion and Conclusions  
When carrying out an activity in PPE, it is necessary to be aware of the risk of a person overheating associated with risk of heat stress development. It has been demonstrated that the essential parameter of the protective ensembles that influences the occurrence and increase of heat strain is the static clothing permeability index \(i_{m}\), according to which protective clothing can be divided into \"air-permeable\" (\(i_{m}\) 0.3-0.4) and \"impermeable\" (\(i_{m}\) \(\leq\) 0.1). The difference between the two groups of protective clothing in terms of the risk of inducing heat strain can be dozens of minutes in the maximum acceptable working time under the same conditions. As a precaution against excessive heat load, an estimation of the permissible working time in PPE can be made based on simulation calculations using thermophysiological models. For \"air-permeable\" ensembles, both tested thermophysiological models--the simpler PHS and the computationally demanding FMTK--are applicable. Based on the results, the Tiger Plus garment representing air-permeable protective clothing showed a very low MAE value of less than 0.4 \({}^{\circ}\)C for both models tested (PHS, FMTK). However, for \"impermeable\" protective ensembles represented by the Tychem F, only the FMTK model is applicable, as the PHS model overestimated the results. The MAE value of the PHS prediction for the Tychem F impermeable ensemble was approximately 1.6 \({}^{\circ}\)C. For this reason, the FMTK model was selected to build the PTS tool. However, the direct use of the FMTK model is not suitable for the practical use of the usual PPE users for rapid estimation of the situation, as this model working in Matlab or its Matlab Compiler Runtime (MCR) application requires the input of the detailed insulation properties of the ensembles and other input data. The main difficulty of using the FMTK model directly lies in prescribing the local thermal and evaporative resistances to each part and sector, and subsequent calculations would take dozens of minutes. To provide an interactive platform without any time delay for operating the thermophysiological model, the Predictor of Thermal Stress (PTS) tool was developed based on data from precalculated scenarios using the FMTK model for four selected types of protective clothing, especially under the conditions of higher metabolic heat production (high load) and higher temperatures. These scenarios were defined to match conditions during real interventions. The output values of the PTS tool include an estimate of the time when the specified user wearing the selected protective ensemble reaches the internal temperatures of 38 \({}^{\circ}\)C, 38.5 \({}^{\circ}\)C, and 39 \({}^{\circ}\)C under the given conditions. Depending on the work scenario selected (short variant 83 min, respectively long variant 180 min in total) the estimation of: \"Missioncompletion\" (in % of planned time) is displayed, as well as the predicted body core temperature \(T_{re}\) value in case the whole selected work scenario would be finished. When using the PTS tool, it is important to be aware that, for many people, the 38.5 \({}^{\circ}\)C threshold may be already beyond the point of intolerable subjective thermal discomfort and many PPE users would have to quit the activity earlier. Thus, for a more sensitive risk assessment, it is appropriate to set the limit when 38.1-38.2 \({}^{\circ}\)C is reached, as many people may experience problems after exceeding this limit. This \(T_{re}\) limit is within the range of general recommendation of measures against excessive heat strain by ACGIH regarding the body core temperature of 38.0 \({}^{\circ}\)C for unselected unacclimatised workers and 38.5 for medically selected and acclimatized personnel [32].