Assessment of multi-factor influences on elderly usage patterns and heat-stress exposure in neighbourhood public outdoor spaces

•Effective usage rate (effUR) is defined to characterize the utilization of neighborhood POS.•effUR and heat-stress exposure of elderly users affected by both thermal and non-thermal factors.•The elderly users exhibit a higher thermal acceptability in POS with an exercise function.•The elderly users become less sensitive to heat stress when they are accompanied.•A novel method is proposed to evaluate the hourly health-warning-heat-stress exposure intensity in POS. Given the hotter weather in summer, quantifying the impacts of thermal and non-thermal factors on the usage of public outdoor spaces (POS) and exposure to heat stress among older people are essential to evaluate heat-related risks in POS and guide POS designs to guarantee the safety of outdoor activities. In this study, field tests were conducted from June to August in four POS of two public housing estates in Hong Kong, to investigate the elderly's POS usage and thermal response in relation to microclimate parameters, POS exercise function, and companionship. A method for evaluating the intensity of health-warning-heat-stress exposure in POS is proposed, aiming to guide risk reducing measures in POS. The results indicate that, at air temperatures within the 29.0 °C and 35.0 °C range, one-degree increase in mean radiant temperature is associated with around 0.05 decrease in POS effective usage rate (effUR). The effUR is found less sensitive to wind conditions, while 67% of the interviewees prefer stronger wind speed. An average thermal sensation vote of 1 (slightly warm) and a fairly high effUR are found under a wide mPET range spanning from 30.0 °C to 39.0 °C, while moderate-to-strong heat stress may occur when mPET exceeds 35.0 °C. Elderly users’ thermal sensitivities reduce in POS providing exercise functions, and when in the company of and chatting with others. The intensity of health-warning-heat-stress exposure varies over time and differs across POS, which will be exacerbated due to global warming unless climate-adapted cooling strategies are implemented.