Abstract: Objective To accurately evaluate the individual heat stress risk of outdoor workers exposed to high temperature. Methods The thermal environment and human physiological parameters were investigated at a typical outdoor construction site for 220 kV power system self-renovation in Guangzhou City from July to August of 2019,to analyze the relationship between high temperature exposure and changes in working hours and heart rate. Results During the survey,the average wet bulb globe temperature（WBGT） values changed in the range of 28.4-40.2 ℃,and working durations of workers decreased significantly with increasing WBGT. Based on the heart rate calculation,the average metabolic rates of six working types were obtained. The metabolic rate of breaking operation was the highest[（300±62）W]. There was statistically significant difference in the metabolic rate among workers in different working type（P<0.05）. With the recommended safety limits of WBGT and metabolic rates in the standard,the metabolic rates of most working types were in the mild level of grade Ⅱ and moderate level of grade Ⅲ,and the majority of average metabolic rates were above the recommended limits,indicating that the metabolic rates of most workers in hot environments exceeded the limit. Conclusion The study proposes a real-time evaluation method for heat stress of workers exposed to outdoor hot temperature based on heart rate monitoring,which can provide reference for population protection and risk identification in hot environments.

Key words: Hot environment exposure, Workers, Real-time heart rate, Metabolic rates, Thermal safety evaluation