Contents lists available at ScienceDirect Journal of Thermal Biology journal homepage: www.elsevier.com/locate/jtherbio Measuring core body temperature with a non-invasive sensor Savyon Mazgaoker a,b , Itay Ketko a,b , Ran Yanovich a,b , Yuval Heled b,c , Yoram Epstein b,c, ⁎ a The Warrior Health Research Institute, Israel Defense Forces, Medical Corps, Tel Hashomer, Israel b Heller Institute of Medical Research, Sheba Medical Center, Tel Hashomer, Israel c Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel ARTICLE INFO Keywords: Double sensor Non-invasive monitoring Core temperature Heat stress Occupational health ABSTRACT In various occupations, workers may be exposed to extreme environmental conditions and physical activities. Under these conditions the ability to follow the workers' body temperature may protect them from overheating that may lead to heat related injuries. The "Dräger" Double Sensor (DS) is a novel device for assessing body-core temperature (T c ). The purpose of this study was to evaluate the accuracy of the DS in measuring T c under heat stress. Seventeen male participants performed a three stage protocol: 30 min rest in a thermal comfort environment (20–22 °C, 50% relative humidity), followed by an exposure to a hot environment of 40 °C, 40% relative humidity -30 min at rest and 60 min of exercise (walking on a treadmill at 5 km/h and 2% elevation). Simultaneously temperatures measured by the DS (T DS ) and by rectal temperature (T re ) (YSI-401 thermistor) were recorded and then compared. During the three stages of the study the average temperature obtained by the DS was within ± 0.3 °C of rectal measurement. The correlation between T DS and T re was significantly better during the heat exposures phases than during resting under comfort conditions. These preliminary results are promising for potential use of the DS by workers under field conditions and especially under environmental heat stress or when dressed in protective garments. For this goal, further investigations are required to validate the accuracy of the DS under various levels of heat stress, clothing and working levels. 1. Introduction In order to increase operational capabilities and to reduce workers' health risks, a perception has been evolved focusing on continuous, non-invasive physiological monitoring systems based on advanced technologies, which could be integrated into the working gear. In various occupations (firefighters, mine-workers etc.) the workers are required to perform under uncompensated heat stress, sometimes while dressed in protective garments, which impair adequate heat dissipation (Havenith et al., 1999; Taylor, 2006). As a result, the operational capabilities of such workers may deteriorate and they might be at the risk to incur various degrees of heat injuries (Epstein and Roberts, 2011; Epstein et al., 2012; Friedl, 2012). It follows that an important parameter that should continuously be monitored is body core tem- perature (T c ). Various devices to measure T c are in use (Mackowiak, 1997), but in most cases they are not practical for routine field application due to wiring, invasiveness, hygiene, difficult to reuse, or uncomfortable for the user. The available systems to measure directly T c are invasive (rectal probe, esophageal probe). They are difficult to apply in various working scenarios for labors who are exposed to extreme environmen- tal conditions and extensive work loads, and may hamper compliance. Among the simplest non-invasive method in assessing T c is measuring skin temperature using a surface thermistor or an infrared light absorption technique (Richmond et al., 2013). Noteworthy, skin temperature underestimates T c (Gagge and Gonzalez, 1996; Mendt et al., 2016). The minimal invasive method to measure T c is the temperature pill, which is the only method available today for remote core body temperature monitoring (O’Brien et al., 1998; Baillot and Hue, 2015). This method, however, exhibits many limitations, such as high cost, the possibility to be influenced by water and food intake or the difficulty to standardizing the location of the sensor along the gastrointestinal tract. Non-invasive estimated of T c are based on computational models, which are composed mainly on time-series analysis of heart rate and or in a combination with skin temperature (Buller et al., 2013; Niedermann et al., 2014; Richmond et al., 2015). In the past several years researches have put effort in the develop- ment of new methods for the measurement of T c by using non-invasive sensors. The common principle of most existing approaches is extract- ing the temperature from measuring heat flux gradients, using sensors attached to the skin surface (Yamakage and Namiki, 2003; Teunissen et al., 2011; Kitamura et al., 2010; Steck et al., 2011). One of those http://dx.doi.org/10.1016/j.jtherbio.2017.03.007 Received 1 October 2016; Accepted 17 March 2017 ⁎ Correspondence to: Heller Institute of Medical Research Sheba Medical Center, Tel-Hashomer, Israel. E-mail address: yoram.epstein@sheba.health.gov.il (Y. Epstein). Journal of Thermal Biology 66 (2017) 17–20 Available online 18 March 2017 0306-4565/ © 2017 Published by Elsevier Ltd. MARK