Urine temperature as a core temperature index of exercise-induced hyperthermia

Introduction
Workers working in hot environments are at risk for heat stroke. To prevent this critical disease, it is important to identify high body temperature of workers without delay. However, there has been no reliable and practicable method established for the measurement of core temperatures at workplaces. ISO 9886 (2004) recognizes urine temperature (Tur) as an index of core temperature when it is measured in the environment between 15-25°C. We measured the Tur of 14 subjects in state of exercise-induced hyperthermia to verify the practicality of monitoring core temperature in hot environment and compared with other body temperature indices.

Methods
We measured temperature of voiding urine of the subjects in the toilet after 45minutes exercise at ambient temperature settings of 40°C. Temperatures at rectum (Tre), auditory canal (Tac), oral (Tor), and axilla (Tax) were measured as well. We analyzed the correlation and the differences of each indice with Tre. 

Results
Tur correlated most strongly with Tre (R=0.822). Tac also had a good correlation (R=0.820) whereas Tor (R=0.576) and Tax had poor or no significant correlation (Pearson’s test). The mean difference between Tre was the smallest with Tac followed by Tur, Tor and Tax. Each difference was 0.06±0.22°C, 0.36±0.21°C, 0.71±0.50°C, 0.88±0.79°C, respectively. 

Discussion
Tur was considered to be highly reliable core temperature index and useful in detecting exercise-induced hyperthermia. Tac also showed good correlation with Tre but in some subjects values of Tac exceeded those of Tre, which indicate that they might have been influenced by the high ambient temperature. Tor and Tax were not suitable indices to estimate body temperatures right after exercise, as they were easily affected by ambient air from opened mouth or by sweated skin and difficult to obtain accurate measurements.