Investigating Wearable Sensor Placement for Heat Risk Monitoring of Firefighters: A Skin-Ear Temperature Study

In firefighting, heat stress is a significant physiological hazard that leads to an increased risk of heat-related illness. Heat-risk monitoring of core body temperature (CBT) can help identify early signs of heat strain and can be accomplished with wearable sensing technologies (WST). This study employs non-invasive temperature sensing to examine the relationship between tympanic temperature and skin temperature (ST) and across multiple on-body locations under different experimental conditions. This study was conducted with 10 healthy adults, male and female, under calm and non-calm conditions, at different times of the day, and across multiple sessions, representing moderate physiological strain similar to firefighting training activities. Tympanic temperature was used as a practical proxy for CBT and compared with ST measured at the forehead, wrist, and elbow. Bland-Altman analysis, together with the variance of differences between CBT and ST, was used to identify the most reliable on-body sensor location. Elbow sites showed lower variability for CBT than wrist sites, suggesting greater stability and suitability for wearable deployment in the context of firefighters' gear in situations where forehead placement is considered impractical. Results show that variability is affected by activity state, time of day, sex, and anatomical location. Findings consist of preliminary evidence informing the most suitable placement of a wearable sensor for heat risk monitoring.