Transient thermal comfort and physiological responses following a step change in activity status under summer indoor environments

•The variation in thermal comfort after two activity status steps was investigated.•The delay in sweating occurs while walking.•The heat stored during walking takes >20 min to dissipate at 23–29 °C.•People are more sensitive to the change in core temperature at activity down-steps. People always experience changes in their daily activities. However, previous dynamic thermal comfort studies have focused on the changes in environmental parameters. Transient subjective and physiological responses following a step change in the activity status have rarely been addressed. To address this, 21 healthy male subjects dressed at 0.5 clo underwent three sessions: sitting for 15 min, walking for 20 min, and sitting for 20 min under three typical summer indoor conditions (operative temperatures (Top) and relative humidity: 23 °C and 60 %, 26 °C and 50 %, and 29 °C and 40 %). There were two walking speeds: 2 and 4 km/h. In addition to subjective votes, physiological parameters such as metabolic rate, skin temperature, and skin wettedness were investigated. The variations in the rate of heat loss from the skin, the rate of heat storage (S), and the core temperature were analysed using the JOS-3 model. The results showed that S was above zero while walking because of the delay in sweating. The heat storage took more than 20 min to dissipate at a typical summer indoor Top (23–29 °C) after experiencing moderate activity intensities (2.2–3.4 met). People were more sensitive to changes in core temperature during the downstep of activity than during the upstep of activity. The findings from this study revealed patterns of change in thermal comfort following a step change in activity status, which can be used to expand the theory of dynamic thermal comfort.