## 1 Introduction
### Wearable Sensing Devices for Core Temperature Measurement  
Early detection of heat stress is crucial for preventing occupational illnesses and fatalities caused by environmental factors such as heat. Wearable sensors like Slate Safety (SS) and ZephyrTM show potential to address early detection needs by identifying individuals at risk and allowing appropriate interventions. Slate Safety (SS): The SS system is a wireless wearable device that can be attached to a worker's arm and is designed to monitor their heart rate, core temperature, and exertion level with the aim of identifying and preventing potential heat-related illnesses or injuries [20]. The SS device incorporates a heart rate sensor to monitor physiological data. Using Slate Safety algorithms, the device extrapolates additional biometrics including exertion, CT, calories burned per minute, steps per minute based on readings from the sensor [20]. It provides continuous updates at fifteen-second intervals for multiple workers with a battery life of over 24 h. Signals are transmitted via cellular connection and can be stored for later uploading. Safety alert limits can be set independently for each worker [20]. ZephyrTM Bioharness (Zephyr): The United States Food and Drug Administration-approved Zephyr is a brand of wearable wireless sensors that records physiological parameters including heart rate, respiratory rate, core temperature, activity levels, and posture for up to 26 h [21]. It utilizes the Kalman Filter algorithm which was developed using a large dataset of ambulatory heart rate to provide estimated CT [22]. It is composed of a BioModuleTM that weighs 85 g and an adjustable strap with skin conductive electrodes that fit on the chest at the lower sternm for both men and women. The BioModuleTM is also capable of measuring respiratory rate through Smart Fabric sensors [21]. There is limited research evaluating the efficacy of these wearable sensors. Prior studies [19; 23; 24] reported that the sensors are accurate but identified systematic differences between the wearable sensors and the ICTS, yet the source of these differences was not clearly explained. Our study aims to address this gap by examining the effectiveness of these wearable sensors and assessing their output within clinically acceptable limits of bias (\(\leq\)\(\pm\)0.5 \({}^{\circ}\)C) and precision (\(\leq\)\(\pm\)1 \({}^{\circ}\)C), as defined by previous studies [5; 25; 26; 27]. The criterion that a device should not exceed a 0.5 \({}^{\circ}\)C difference is rooted in the fact that 0.5 \({}^{\circ}\)C is the minimum temperature difference that has been linked to the occurrence of complications caused by hypothermia [27]. Therefore, adhering to the limits of bias (\(\leq\)\(\pm\)0.5 \({}^{\circ}\)C) and precision (\(\leq\)\(\pm\)1 \({}^{\circ}\)C) ensures that the core temperature measuring devices can provide measurements that are sufficiently close to the true values, with a low level of systematic bias and reasonable precision [5; 25; 26; 27]. Adopting these limits of bias and precision for core temperature measurements can help protect a significant portion of the workforce [25; 27]. Accurate and reliable measurements are crucial for monitoring occupational heat stress and preventing heat-related illnesses among workers. By using a measuring device that meets these accepted limits, employers can better assess the risk of heat-related illnesses and implement appropriate preventive measures to protect workers' health and well-being. In situations where individuals are exposed to extreme heat or engage in strenuous physical activities, a higher level of accuracy may be required to ensure early detection of heat-related conditions and timely intervention. Nonetheless, as technology advances and new research emerges, it is crucial to continuously evaluate and improve core temperature measuring devices. To achieve the goal of this study, the following hypotheses were tested:  
* H1a: SS wearable sensor CT data are within the clinically acceptable limits.