Solar radiation and the validity of infrared tympanic temperature during exercise in the heat

We investigated the validity of infrared tympanic temperature (IR-T_ty) during exercise in the heat with variations in solar radiation. Eight healthy males completed stationary cycling trials at 70% peak oxygen uptake until exhaustion in an environmental chamber maintained at 30°C with 50% relative humidity. Three solar radiation conditions, 0, 250 and 500 W/m², were tested using a ceiling-mounted solar simulator (metal-halide lamps) over a 3 × 2 m irradiated area. IR-T_ty and rectal temperature (T_re) were similar before and during exercise in each trial (P > 0.05). Spearman's rank correlation coefficient (r_s) demonstrated very strong (250 W/m², r_s = 0.87) and strong (0 W/m², r_s = 0.73; 500 W/m², r_s = 0.78) correlations between IR-T_ty and T_re in all trials (P < 0.001). A Bland-Altman plot showed that mean differences (SD; 95% limits of agreement; root mean square error) between IR-T_ty and T_re were - 0.11°C (0.46; - 1.00 to 0.78°C; 0.43 ± 0.16°C) in 0 W/m², - 0.13°C (0.32; - 0.77 to 0.50°C; 0.32 ± 0.10°C) in 250 W/m² and - 0.03°C (0.60; - 1.21 to 1.14°C; 0.46 ± 0.27°C) in 500 W/m². A positive correlation was found in 500 W/m² (r_s = 0.51; P < 0.001) but not in 250 W/m² (r_s = 0.04; P = 0.762) and 0 W/m² (r_s = 0.04; P = 0.732), indicating a greater elevation in IR-T_ty than T_re in 500 W/m². Percentage of target attainment within ± 0.3°C between IR-T_ty and T_re was higher in 250 W/m² (100 ± 0%) than 0 (93 ± 7%) and 500 (90 ± 10%; P < 0.05) W/m². IR-T_ty is acceptable for core temperature monitoring during exercise in the heat when solar radiation is ≤ 500 W/m², and its accuracy increases when solar radiation is 250 W/m² under our study conditions.