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 (r2: 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.