The housing environment and ambient temperature (HEAT) study

Abstract Without intervention, by the end of this century, extreme heat will cause tens of thousands of excess deaths, particularly from respiratory and cardiovascular causes. Beyond the impacts of short-duration heat events, such as heat waves, it is critical to understand the effects of chronic heat exposure, assessed where most

heat-related deaths occur: indoors. Improving understanding of individual and neighborhood characteristics that heighten or reduce heat vulnerability is also crucial, for informing the design of effective heat adaptation strategies. Yet, current knowledge of heat-related mortality and vulnerability remains limited in at least three

ways. First, most research on heat-related death has quantified associations between short-term temperature spikes and acute mortality outcomes, leaving uncertainty about the total mortality burden of chronic heat exposure. Second, most research has calculated associations with outdoor rather than indoor temperatures,

thus potentially under-estimating the mortality impacts of heat and leaving critical gaps in knowledge about safe maximum indoor temperature thresholds. Third, little is known about the extent to which housing interventions that promote thermal comfort and conserve energy, such as improving insulation or altering

roofing material, may prevent excess deaths from chronic indoor heat exposure. There is an urgent need to fill these gaps since most people, and especially heat vulnerable subpopulations including individuals over age 65, spend the majority of their time indoors. We propose the first ever population-based, nationally

representative, quasi-experimental, longitudinal cohort study of the effects of chronic indoor heat exposure on mortality in the United States. We will use data on age 65 and older adult participants of the Mortality Disparities in America Communities (MDAC) study linked with Medicare and National Death Index data. This

remarkably rich data set, which contains follow up on individuals for up to eight years, will be combined with indoor temperature and humidity variables calculated using rigorous, extensively validated, physics-based simulation models; individual-level housing characteristics; and high-resolution land cover data. To improve

understanding of associations between the housing environment, chronic indoor heat exposures and mortality, and person- and neighborhood-level determinants of heat vulnerability, we pursue three aims. In Aim 1, we quantify associations between chronic indoor heat exposures and all-cause and cause-specific mortality, and

identify climate-zone specific, safe upper thresholds for hot-season indoor temperatures. In Aim 2, we elucidate person- and neighborhood-level factors that enhance or reduce vulnerability to chronic indoor heat exposure. In Aim 3, we quantify the total excess deaths that may be prevented through housing interventions

that improve thermal comfort, under current and future greenhouse gas emission scenarios. This impactful project will provide information that is critically needed to guide interventions that balance heat adaptation for health protection in the most vulnerable subgroups with climate change mitigation goals.