Limited efficacy of low-volume ice slurry ingestion on physiological heat strain in Japanese outdoor workers during summer

BACKGROUND: Occupational heat exposure is a growing public health concern in Japan, where hot, humid summers contribute to heat-related illnesses (HRIs). AIM: To characterise the thermophysiological responses of Japanese outdoor workers to summer heat exposure and explore the effect of a practical pre-cooling approach. SUBJECTS AND METHODS: Two field-based experiments were conducted in August-September. In Experiment 1, heart rate (HR) and core body temperature (CT) were continuously monitored under routine working conditions. In Experiment 2, workers performed their duties after pre-work ingestion of a 100 g carbohydrate-electrolyte ice slurry. HR and CT were assessed using non-invasive wearable sensors, and subjective thermal perception was recorded during the workday. RESULTS: Under high environmental heat stress (maximum Wet Bulb Globe Temperature: 30.2 ± 2.4 °C and 29.4 ± 2.1 °C), both CT and HR increased significantly during midday and afternoon periods. No significant differences were observed between the conditions in terms of HR, CT, or subjective thermal perception. CONCLUSION: Japanese outdoor workers exhibited a sustained elevation in thermo-physiological strain during summer. A low-volume pre-cooling strategy did not measurably modify these responses, indicating a limited physiological impact under real-world conditions.