Limitations of employing a unified physiological equivalent temperature scale for assessing thermal comfort in Taiwan’s Cwa urban climate

Taiwan's national reference for outdoor thermal comfort assessment originates from a single non-urban site in a Cfa zone, whereas the country's most heat-vulnerable metropolitan areas lie in the Cwa zone. Conventional thermal comfort evaluations rely on single standardized scales, which may not fully reflect variation across different climates and outdoor contexts. This study employed the modified physiologically equivalent temperature (mPET) thermal index as a thermal comfort indicator and established a regionally adaptive and mPET-based thermal perception scale suitable for Taiwan's Cwa climate. Field studies were conducted in two representative Cwa sites-Zhongxing New Village and the Central Taiwan Science Park-and encompassed seven outdoor spaces with distinct spatial functions and microclimatic characteristics. Microclimatic measurements, questionnaire surveys, and user count observations were considered to examine the relationships among thermal conditions, thermal sensation, thermal comfort, and spatial behavior. The results indicate that a neutral temperature for these sites is 23.3 °C mPET and that a 90% acceptable thermal comfort range is between 21.4 and 25.8 °C mPET. Behavioral thermal adaptation, including shade-seeking and time-shifting behaviors, was also observed in this study. In addition, contextual comparison with PET thresholds reported for other climate zones indicated the regional specificity of the neutral temperatures and physiologically equivalent temperature classifications derived for Taiwan's Cwa climate, suggesting that a single physiologically equivalent temperature scale may be insufficient for nationwide evaluations. The study findings highlight a need for climate-sensitive and behaviorally informed thermal comfort assessments. The results offer initial empirical support to aid with climate adaptation strategies and outdoor space planning in humid subtropical urban environments, with further validation across additional Cwa sites warranted.