As the global greenhouse effect intensifies, the urban heat island effect becomes increasingly severe; consequently, people are exposed to high-temperature and metabolically intensive thermal environments. The issue of thermal safety is becoming increasingly serious. This study investigated the human thermal comfort requirements at different temperatures and metabolic rates. Twenty participants were made to perform four tasks at four ambient temperatures (Ta; 28, 30, 32, and 34 degrees C) and metabolic rate levels (2.4, 3.2, 4.0, and 4.4 met), and their physiological reactions, symptoms, and responses to subjective thermal comfort were continuously measured. The results revealed that 15.2% of the participants exhibited adverse symptoms when Ta = 28 degrees C, whereas 53.2% of the participants exhibited adverse symptoms when Ta = 34 degrees C. The symptoms included intense sweating, facial heat, rapid heartbeat, dyspnea, chest tightness, feebleness, general weakness, blurry vision, skin sensitivity, and nausea. Prolonged exercise in high-temperature environments may affect the body's heat resistance, resulting in discomfort such as fatigue, headaches, and nausea. Furthermore, participants with high metabolic rates preferred higher wind speeds and lower humidity to improve their thermal comfort in warmer thermal environments. The results offer valuable insights towards mitigating heat exposure-related occupational health risks during work or exercise in high-temperature environments.