Thermal safety of indoor factory workers during summer in the subtropics: A dynamic thermal indicator

Extremely hot environments in indoor factories without effective air-conditioning systems are hazardous to the health of workers. This pose a potential threat to production safety on the factory floor, with workers often facing serious thermal safety challenges. Therefore, in this study, a full-scale experiment was conducted in three indoor factories in hot summer and cold winter regions of China during the summer of 2024 to investigate the effects of indoor thermal environments and activity states on worker heat stress by measuring thermal parameters and worker physiological indices in the field. The study resulted in the following findings: (1) Factories with high equipment surface temperatures exhibited 6 degrees C higher average temperatures than others. (2) For air temperatures below 33.7 degrees C, the work status dominated metabolic rate and core body temperature variations by 69.3 % and 85.1 %, respectively. For air temperatures above 37.7 degrees C, the work status remained critical for metabolic rate (83.5 % weighting), with air temperature influence on core body temperature increasing to 44.1 %. (3) In an airconditioned environment, the standard effective temperature (SET) of workers was always within acceptable range at work in contrast to a SET of 38.4 degrees C in non-air-conditioned environments. (4) An improved heat index model incorporating metabolic rate adjustment enhanced the mean thermal sensation vote (MTSV) prediction accuracy in non-air-conditioned settings. (5) Symptoms of worker discomfort were more severe in non-airconditioned than air-conditioned environments. These results quantitatively demonstrate the critical role of activity-adaptive thermal management and provide guidelines for optimizing factory safety and worker comfort.