Who Can Avoid the Heat? Inter-Individual Differences in 24-hr Heat Exposure Reveal Disparities in Heat Adaptation and Stress.

Differences in adaptive capacity and sensitivity shape behavioral and physiological responses to overheated environments, which are associated with unequal personal heat exposure (PHE). Although growing evidence documents heterogeneity in PHE across regions and populations, studies rarely examine how adaptive factors influence sub-daily exposure patterns and resulting heat stress relative to regional heat. This study assesses intra- and inter-individual variation in 24-hr PHE and heat loads using regional weather as a non-behavioral baseline. We studied 34 adults aged 45-74 from diverse socioeconomic, occupational, and housing backgrounds in the Phoenix Metropolitan Area. Using the HeatSuite™ system and Kestrel Drop D2 sensors, we monitored 10-min indoor and on-body ambient conditions over 21-day periods during summer 2024, reconstructing PHE based on participants' locations. We used Generalized Additive Models to explain inter-individual differences and Linear Mixed Models to examine associations with vulnerability features. Heat stress was assessed using a physiology-based human heat balance model. Results reveal significant inter-individual differences in individually experienced temperatures (IET) and humidities (IEH), with three sub-daily patterns emerging: highest overall exposures, strong diurnal amplitude, and day-long stable exposures. Median IET and IEH were 25.9°C and 1.4 kPa, with maxima reaching 47.5°C and 5.1 kPa. On average, participants experienced temperatures 10.4°C below regional conditions, with low evaporative cooling requirements to attain heat balance for sedentary activities. However, participants with income <$40k/year experienced higher peak daily temperatures. These findings demonstrate how high-resolution personal monitoring reveals when and why heat interventions may be most effective, informing policy with locally-grounded evidence.