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.