# Applicability of Human Thermophysiological Model for Prediction of Thermal Strain in PPE
## 4 Discussion and Conclusions
In similar studies on work in PPE in hot environments, comparable maximum body core temperatures above 38 \({}^{\circ}\)C [9], 38.1 \({}^{\circ}\)C [33], 38.2 \({}^{\circ}\)C [34], 38.4 \({}^{\circ}\)C [35], or even up to 39.9 \({}^{\circ}\)C [36] were observed in the real work environment. For extreme situations, the PTS tool presents a limit of \(T_{re}\) = 39 \({}^{\circ}\)C also, which is completely unbearable for most PPE users and can even be considered as a highly health-threatening condition. Acclimation to a hot environment may be induced by regular training under hot conditions to increase the endurance against thermal exhaustion [37]. However, it should be remembered that the main purpose of the PPE covered in this study was to provide protection against hazardous environment to experts in the chemical field and/or biological field, medical staff, investigators, decontamination team members, and many others. Therefore, for many of them, one can hardly expect excellent fitness comparable to sportsmen with regular training, and the estimation should regard the \"normal\" fitness level of the occasional users of this type of PPE. The estimated values of reaching \(T_{re}\) 38.0 \({}^{\circ}\)C and 38.5 \({}^{\circ}\)C are of great interest to give an idea of the risks of heat stress, especially when planning strenuous activity in impermeable clothing under relatively undemanding climatic conditions or, conversely, planning long-term physically less-demanding activity in air-permeable protective ensembles under climatically demanding conditions. It is important to keep in mind not only the external climatic conditions and the thermal insulating properties of the garments, but also, especially, the physical demands of the planned activity, as these play an essential role in heat stress development. The risk of heat stress can vary many times between physically less-demanding activities (slow walking, patrolling, etc.) and more-demanding ones (walking uphill or stairs, running, walking on difficult terrain, working with heavy tools, carrying a load or a victim, etc.) even for the same person under the same conditions (external temperature, type of PPE). The PTS tool was validated by real tests in a climatic chamber with a group of probands under various defined conditions. The results of the comparison between the real average time and the predicted time using the PTS tool can be expressed in the form of a MAPE value of 12.6%. The source of inaccuracies in the validation calculations comparing the real tests and PTS predictions is undoubtedly the input of inaccurate (calculated) metabolic activity. In real tests of thermal strain in the climatic chamber, metabolic output was calculated for each test indirectly from the VO2 max, \(HR\) increase, and other individual proband parameters, because it was not possible to directly measure the energy expenditure using an O2/CO2 analyzer during the tests and apply the appropriate respiratory protection to the given protective ensemble at the same time. Moreover, the PTS tool applies interpolation between a few strict input values of the metabolic activity. Besides, the cause of deviations in the validation calculations was also due to some differences in the initial body core temperature \(T_{re}\) of the individual probands; see Table 5. However, the simulation calculations using the FMTK model, which are the basis for the look-up tables, were always performed for a default initial \(T_{re}\) of 36.9 \({}^{\circ}\)C, although in real tests, the initial \(T_{re}\) temperatures of the different probands ranged from 36.0 to 37.5 \({}^{\circ}\)C. In this context, it should be taken into account that the PTS tool may slightly underestimate the risk of overheating if the initial internal body temperature at the start of the activity is in reality higher. The recommended range use of the PTS tool is 5-45 \({}^{\circ}\)C, although 45 \({}^{\circ}\)C may seem excessively high in many cases.