High-intensity outdoor labor in hot climates poses significant risks to worker health, safety, and productivity, making reliable evaluation of thermal stress a priority in occupational environments. This chapter presents findings from a field study conducted at construction sites in Southern China during the summer of 2019, where both microclimatic conditions and physiological indicators were monitored to assess thermal safety. Relationships between environmental parameters (air temperature, mean radiant temperature, air velocity, and relative humidity), physiological responses (heart rate and ear canal temperature), and cognitive performance were examined alongside workers’ subjective feedback. Results indicate that once air temperature exceeds 34 °C, reductions in work intensity become necessary to prevent discomfort and physiological overload. Elevated heart rate and ear canal temperature were closely linked to heat exposure, while cognitive performance declined as thermal stress intensified, especially among older workers. Overall, nearly 40% of participants reported discomfort, and operational efficiency fell by approximately 8% for every marked increase in heat load. These findings underscore the importance of integrating physiological and psychological monitoring into occupational safety frameworks, offering valuable insights for the development of adaptive strategies to protect outdoor laborers under extreme thermal conditions.