A 3-D virtual human thermoregulatory model to predict whole-body and organ-specific heat-stress responses

Objective This study aimed at assessing the risks associated with human exposure to heat-stress conditions by predicting organ- and tissue-level heat-stress responses under different exertional activities, environmental conditions, and clothing. Methods In this study, we developed an anatomically detailed three-dimensional thermoregulatory finite element model of a 50th percentile U.S. male, to predict the spatiotemporal temperature distribution throughout the body. The model accounts for the major heat transfer and thermoregulatory mechanisms, and circadian-rhythm effects. We validated our model by comparing its temperature predictions of various organs (brain, liver, stomach, bladder, and esophagus), and muscles (vastus medialis and triceps brachii) under normal resting conditions (errors between 0.0 and 0.5 °C), and of rectum under different heat-stress conditions (errors between 0.1 and 0.3 °C), with experimental measurements from multiple studies. Results Our simulations showed that the rise in the rectal temperature was primarily driven by the activity level (~ 94%) and, to a much lesser extent, environmental conditions or clothing considered in our study. The peak temperature in the heart, liver, and kidney were consistently higher than in the rectum (by ~ 0.6 °C), and the entire heart and liver recorded higher temperatures than in the rectum, indicating that these organs may be more susceptible to heat injury. Conclusion Our model can help assess the impact of exertional and environmental heat stressors at the organ level and, in the future, evaluate the efficacy of different whole-body or localized cooling strategies in preserving organ integrity.

A digital tool for prevention and management of cold weather injuries—Cold Weather Ensemble Decision Aid (CoWEDA)

This paper describes a Cold Weather Ensemble Decision Aid (CoWEDA) that provides guidance for cold weather injury prevention, mission planning, and clothing selection. CoWEDA incorporates current science from the disciplines of physiology, meteorology, clothing, and computer modeling. The thermal performance of a cold weather ensemble is defined by endurance times, which are the time intervals from initial exposure until the safety limits are reached. These safety limits correspond to conservative temperature thresholds that provide a warning of the approaching onset of frostbite and/or hypothermia. A validated six-cylinder thermoregulatory model is used to predict human thermal responses to cold while wearing different ensembles. The performance metrics, model, and a database of clothing properties were integrated into a user-friendly software application. CoWEDA is the first tool that allows users to build their own ensembles from the clothing menu (i.e., jackets, footwear, and accessories) for each body region (i.e., head, torso, lower body, hands, feet) and view their selections in the context of physiological strain and the operational consequences. Comparison of predicted values to skin and core temperatures, measured during 17 cold exposures ranging from 0 to −40°C, indicated that the accuracy of CoWEDA prediction is acceptable, and most predictions are within measured mean ± SD. CoWEDA predicts the risk of frostbite and hypothermia and ensures that a selected clothing ensemble is appropriate for expected weather conditions and activities. CoWEDA represents a significant enhancement of required clothing insulation (IREQ, ISO 11079) and wind chill index-based guidance for cold weather safety and survival.

Comparison Between Experimental and Heart Rate-Derived Core Body Temperatures Using a Three-Dimensional Whole Body Model

Determination of core body temperature (Tc), a measure of metabolic rate, in firefighters is needed to avoid heat-stress related injury in real time. The measurement of Tc is neither routine nor trivial. This research is significant as thermal model to determine Tc is still fraught with uncertainties and reliable experimental data for validation are rare. The objective of this study is to develop a human thermoregulatory model that uses the heart rate measurements to obtain Tc for firefighters using a 3D whole body model. The hypothesis is that the heart rate-derived computed Tc correlates with the measured Tc during firefighting activities. The transient thermal response of the human body was calculated by simultaneously solving the Pennes' bioheat and energy balance equations. The difference between experimental and numerical values of Tc was less than 2.6%. More importantly, a ± 10% alteration in heart rate was observed to have appreciable influence on Tc, resulting in a ± 1.2 °C change. A 10% increase in the heart rate causes a significant relative % increase (52%) in Tc, considering its allowable/safe limit of 39.5 °C. Routine acquisition of the heart rate data during firefighting scenario can be used to derive Tc of firefighters in real time using the proposed 3D whole body model.

Investigation on performance of three personal cooling systems in mitigating heat strain by means of thermal manikin

The objective of current study was to examine the effectiveness of three cooling strategies (i. e., electric air fan, evaporative cooling vest, and liquid cooling vest) in mitigating body heat strain in heat wave conditions by means of a Newton-type thermal manikin. A human thermoregulatory model was used to simulate human physiological responses while using the three cooling strategies. Two environmental conditions were selected to simulate heatwave conditions, i. e., 36 ?C, 33% relative humidity (hot-dry), 40 ?C, 27% relative humidity (extremely hotdry). A metabolic rate of 1.2 MET was selected to simulate resting person or person doing light housework. It was found that the electric air fan had cooling benefit in both environments. In addition, the evaporative cooling vest and liquid cooling vest showed similar effectiveness in mitigating body heat strain in both hot-dry or extremely hot-dry environments. Thus the evaporative cooling vest and liquid cooling vest were recommended under heatwave conditions..

Evaluation of a hybrid personal cooling system using a manikin operated in constant temperature mode and thermoregulatory model control mode in warm conditions

In this study, the cooling effect of a portable hybrid personal cooling system (PCS) was investigated on a sweating manikin operated in the constant temperature (CT) mode and the thermoregulatory model control (TMC) mode. Both dry (i.e., no sweating) and wet manikin tests (i.e., sweating) were performed in the CT mode in a warm condition (30℃, 47% relative humidity (RH), air velocity va = 0.4 m/s). For the TMC mode, two case studies were simulated: light work condition (30℃, 47% RH, air velocity va = 0.15 m/s, duration: 60 min, metabolic rate: 1.5 METs) and construction work condition (30℃, 47% RH, va = 1.0 m/s, 40 min exercise [5.5 METs] and 20 min rest [1.2 METs]). Four test scenarios were selected: fans off with no phase change materials (PCMs) (i.e., Fan-off, the Control), fans on with no PCMs (i.e., Fan-on), fans off with fully solidified PCMs (i.e., PCM+Fan-off) and fans on with fully solidified PCMs (i.e., PCM+Fan-on). Under the dry condition, the cooling rate in PCM+Fan-off during the initial stage (e.g., 55 and 50 W for the first 15 min and 20 min, respectively) was higher than that in Fan-on (i.e., 45 ± 1 W); under the wet condition, the cooling rate in PCM+Fan-off (e.g., 45 W for 10 min) was much lower than that in Fan-on (i.e., 282 ± 1 W). The hybrid PCS (i.e., PCM+Fan-on) provided a continuous strong cooling effect. Simulation results indicated that ventilation fans or PCMs alone could provide sufficient cooling while doing light work. For the intensive work condition, the PCS in all three scenarios (i.e., PCM+Fan-off, Fan-on and PCM+Fan-on) exhibited beneficial cooling, and the hybrid PCS showed an optimized performance in alleviating heat strain during both exercise and recovery periods. It was thus concluded that the PCS could effectively remove body heat in warm conditions for moderate intensive activities.