Quantifying Indoor Heat Stress and Strain across Climate Contexts and Adaptive Capacities

Indoor homes will be an increasing source high heat exposure as temperatures rise and cities grow. Many cases of indoor heat illness and death are associated with poor or no air conditioning (AC), as well as substandard housing (e.g., mobile homes). While AC is often seen as a preventative measure, many at-risk people either lack or cannot always afford the electricity for AC. In this work, we compare indoor personal heat exposure and heat stress experienced by both healthy and heat-sensitive individuals in extreme heat, under both dry and humid conditions, with different levels of AC functioning across U.S. cities. To do so, we apply building energy simulations and human heat balance modeling (HHB) and conduct a three-step analysis using permutations of climate type, housing archetype, cooling-adaptive capacity, and age groups (young and older adults). First, we extract the diurnal variation of the outdoor thermal environments for dry and humid conditions as input data in the BES model. Second, we determine the indoor thermal environment using a BES model (e.g., EnergyPlus) for both extreme heat types and combinations of housing and adaptive capacities to overcome heat exposure. Third, we determine indoor (and outdoor reference) heat exposures and heat stress by applying a human-environment heat exchange model was applied to young and older (>65 years) females resting (1.8 METs). The energy balance impact of cooling resources (e.g., air conditioning, electric fans) are also modeled. Overall, 96 different simulations of indoor thermal exposures were run. Results show that even in appropriately matched climates (based on International Energy Conservation Code), a faulty AC system exposes people to high levels of heat stress. In hot indoor climates, increasing airflow from 0.2 to 3.5 m/s using fans enhances convective and evaporative heat fluxes sufficiently to alleviate heat stress in both young and older females, approaching thermoneutral conditions. This finding aligns with the fact that most air temperatures remain below 35°C (skin temperature assumed in the model). However elevated temperatures and humidity proves to be the most dangerous conditions during extreme heat, attributed to the higher insulation and solar heat gain, as well as reduced air ventilation. This research demonstrates how relying solely on AC as a solution to indoor heat may not effectively protect the most heat-sensitive groups. In the case of a faulty AC, a synergistic approach with fans emerges as a suitable alternative. Future work will investigate other personal cooling options. This study provides a framework to improve the uncertainty around environmental context in heat stress methodologies by including housing infrastructure as an exposure factor when connecting regional weather to heat-health outcomes.