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 degrees C), simulated-HW (35.4 degrees C). Contact thermistors measured T-sk (T-forehead, T-finger); radio pills measured gastrointestinal temperature (T-gi). Proximal-distal temperature gradients (Delta Tforehead-finger) were also measured. Measurements were grouped into ambient conditions: 22, 25, and 35 degrees C. T-gi and T-forehead only displayed a significant relationship in 22 degrees C (r: 0.591; p < 0.001) and 25 degrees C (r: 0.408; p < 0.001) conditions. A linear regression of all conditions identified T-forehead and Delta Tforehead-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 degrees C. Additional residual analysis identified T-forehead to be responsible for a plateau in T-gi prediction above 37 degrees C. Contact T-forehead was shown to be a statistically suitable indicator of T-gi in non-HW conditions; however, an error of ~1 degrees 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.