Optimal indoor cooling in hot climates: Experimental and computational insights on human thermal comfort

Rapid transitions between hot outdoor environments and cold air-conditioned spaces are common in tropical cities, yet their physiological effects remain insufficiently quantified. As a result, indoor temperature setpoints are often selected based on steady-state comfort indices, which may overlook transient cold stress, cardiovascular strain during rewarming, and the risk of prolonged discomfort during everyday outdoor–indoor transitions. This study combines controlled human trials with finite-element bioheat simulations to investigate skin and internal temperature responses under realistic thermal shifts. Thirty participants (20 male, 10 female; ages 15–60; varied BMI and activity levels) were exposed to five indoor cooling scenarios (18.7–25.9 °C) after 30 min of outdoor heat exposure (36.5–39.0 °C) in Sylhet, Bangladesh. Skin temperature and subjective discomfort were recorded during 30 min of cooling and up to 120 min of rewarming. A 3D anatomical thermoregulatory model based on Pennes' bioheat equation, developed in COMSOL (1.88 M elements), reproduced transient skin, muscle, and core temperatures. Across all cases, 26.8 °C emerged as the consistent skin-temperature threshold for cold discomfort. Environments below 21 °C induced rapid cooling discomfort within 7–14 min in the coldest case, triggering vasoconstriction, increased metabolic load, and mild hypothermia risk. Moderate settings (21–26 °C) maintained skin and muscle temperatures above critical thresholds, reducing thermoregulatory strain and enabling prolonged comfort. Age, BMI, and habitual activity influenced cooling rates, with younger and more active individuals tolerating longer exposures. Re -exposure to outdoor heat caused rapid skin warming (0.45–0.684 °C/min in the first 15 min), potentially stressing the cardiovascular system. These findings support 21–26 °C as the optimal range for air-conditioned environments in hot climates, balancing comfort, health, and energy efficiency. The integrated experimental–computational approach offers a validated framework for HVAC design, occupational health policy, and climate-adaptive building strategies.