Validation and adjustment of the mathematical prediction model for human rectal temperature responses to outdoor environmental conditions

Models to predict rectal temperature (T_re) have been based on indoor laboratory studies. The present study was conducted to validate and adjust a previously suggested model for outdoor environmental conditions. Four groups of young male volunteers were exposed to three different climatic conditions (30°C, 65% rh; 31°C, 41% rh; 40°C, 20% rh). They were tested both in shaded and open field areas (radiation: 80 and 900 W·m⁻², respectively) at different work loads (100, 300 and 450 watt). Exercise consisted of two bouts of 10 minutes rest and 50 minutes walking on a treadmill, at a constant speed (1.4m · s⁻¹) and different grades. The subjects were tested wearing cotton fatigues and protective garments. Their T_re and heart rate were monitored every 5 min and skin temperature every 15 min, oxygen uptake was measured towards the end of each bout of exercise; concomitantly, ambient temperature, relative humidity and solar load were monitored. We concluded that: (a) the corrected model to predict rectal temperature overestimates the actual measurements when applied outdoors; (b) radiative and convective heat exchanges should be considered separately when using the model outdoors; (c) radiative heat exchange should also be considered separately for short-wave radiation (solar radiation) and long-wave emission from the body to the atmosphere. Finally, an adjusted model to be used outdoors was suggested.