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.