Validity and Reliability of Wearable Heat Stress Monitors in Firefighters During Exercise-Induced Hyperthermia
Rich D. Stevenson, Tom Wilton, Sophie Evans, Xanthe Dyer, Harry Yuen, Oliver Peacock, James L. J. BilzonPurpose:
To evaluate the validity and reliability of commercially available wearable heat stress monitors (HSM) for estimating core body temperature (T c ) in firefighters during exercise-induced hyperthermia and recovery while wearing firefighting personal protective equipment (PPE).
Methods:
Twenty-one firefighters completed two identical laboratory trials consisting of treadmill exercise in PPE followed by seated recovery. T
c
was measured continuously using rectal thermometry (T
rec
) (criterion), a gastrointestinal telemetric pill alongside four wearable HSM (Equivital, Kenzen, Zephyr, CORE (arm and chest placement)). Validity was assessed using mean bias, 95 % limits of agreement (LoA), mean absolute error (MAE), root mean square error (RMSE), Pearson’s
Results:
All HSM demonstrated poor agreement (CCC = 0.26–0.85) along with wide LoA (1.25–2.22 °C) across all phases. During exercise, Equivital showed the strongest overall performance. During recovery, accuracy deteriorated markedly for all devices, with MAE and RMSE approximately doubling and fewer than one-third of readings fell within ± 0.3 °C of T rec . Temperature-dependent analyses revealed progressive increases in RMSE above ~38.5 °C, peaking during early recovery. When exercise and recovery were combined, Equivital and the CORE devices demonstrated the most favourable performance. Overall reliability was moderate to good (ICC = 0.74–0.81), with lower reproducibility for the arm-mounted devices (Kenzen and CORE-arm).
Conclusions:
Wearable HSM can track general trends in T c but lack the accuracy and precision required for critical safety or return-to-work decision-making in firefighters, particularly at higher T c and during recovery.