Warable sensor-based thermal comfort assessment for pedestrianconsidering the ever-changing thermal environment and physiological response

Outdoor thermal comfort is important to city dwellers' well-being and health. Pedestrians are especially sensitive to thermal environments, and their thermal comfort is expected to be at risk due to the urban heat island effect combined with climate change. Pedestrian thermal comfort assessment is crucial to comprise sustainable climate change adaptation strategy. However, pedestrian thermal comfort has been simply evaluated using the survey asking pedestrians about their comfort levels via oral or paper interviews. The survey's shortcoming is that it does not reflect the dynamics of the ever-changing environment and the resultant responder's physiology. The development of wearable sensors overcame the survey's limitation and allowed to detect human physiological responses reflecting the changes in the surrounding environment more objectively. Among several physiological parameters, heart rate(HR) is a representative proxy for physiological thermal stress reflecting environmental heat load. It can be easily monitored by a smartwatch wearing an optical blood flow sensor. Therefore, we aim to investigate the applicability of physiological thermal comfort evaluation based on pedestrians' HRs monitored using a smartwatch in real-walking settings. The experiment was conducted on four streets with an east-west orientation in Suwon, Gyeonggi-do, Korea. The four streets were selected with high or low effects of grey and green infrastructure on the streets' thermal environment based on a building-height-to-street-width(H/W) ratio of 2 and the percentage of tree canopy cover(%TCC) of 50, respectively. The 32 voluntary pedestrians walked one street a day for an hour(14:00-15:00) with a smartwatch(Mi-band4) to record HR of each pedestrian. During walking, microclimates (air and globe temperature, relative humidity, wind velocity) were monitored using a portable meteorological station. After walking, the survey was conducted by asking about their feelings while walking as thermal comfort level. We defined the thermal environment created by grey and green infrastructures as the difference between the street's mean radiant temperature(T_mrt) calculated by the street's microclimates and the official air temperature from the automatic weather station. We also suggested the physiological thermal comfort index(PTCI) to quantify physiological thermal comfort including the cardiovascular risk based on HRs. Consequently, we found the tree's effect was contradictory according to the H/W ratio. The increment of 10%TCC reduced T_mrt by 1.1℃ on the low H/W ratio street but rose T_mrt up to 0.1℃ on the high building street. The TCC's heat dissipation hindrance might cause this result because TCC could block the wind path and interfere with air circulation rather than having the cooling effect of the tree-formed shades on streets where high buildings already form sufficient shade. The PTCI results reflected the thermal environment of each street well because a 10%TCC rise decreased the cardiovascular risk by 8% on the low building street but increased the risk up to 7% on the high building street. However, pedestrians could not perceive the thermal environments' distinctions among streets due to interruption of aesthetic quality other than microclimates. Therefore, we identified that physiological thermal comfort based on HR is more appropriate to be used as a basis for establishing adaptation strategies for pedestrians.