Predicting Deep Body Temperature (T b ) from Forehead Skin Temperature: T b or Not T b ?

There is a need to rapidly screen individuals for heat strain and fever using skin temperature (T sk ) as an index of deep body temperature (T b ). This study's aim was to assess whether T sk could serve as an accurate and valid index of T b during a simulated heatwave. Seven participants maintained a continuous schedule over 9-days, in 3-day parts; pre-/post-HW (25.4 °C), simulated-HW (35.4 °C). Contact thermistors measured T sk (T forehead , T finger ); radio pills measured gastrointestinal temperature (T gi ). Proximal-distal temperature gradients (ΔT forehead-finger ) were also measured. Measurements were grouped into ambient conditions: 22, 25, and 35 °C. T gi and T forehead only displayed a significant relationship in 22 °C (r: 0.591; p < 0.001) and 25 °C (r: 0.408; p < 0.001) conditions. A linear regression of all conditions identified T forehead and ΔT forehead-finger as significant predictors of T gi (r 2 : 0.588; F: 125.771; p < 0.001), producing a root mean square error of 0.26 °C. Additional residual analysis identified T forehead to be responsible for a plateau in T gi prediction above 37 °C. Contact T forehead was shown to be a statistically suitable indicator of T gi in non-HW conditions; however, an error of ~1 °C makes this physiologically redundant. The measurement of multiple sites may improve T b prediction, though it is still physiologically unsuitable, especially at higher ambient temperatures.