PROJECT SUMMARY Acute kidney injury (AKI) is a sudden decrease of kidney function that is almost always reversible. While reversible, AKI is serious and often requires hospitalization. The economic burden is also substantial, estimated at $5-24 billion annually in the USA. Although AKI can arise within already-hospitalized patients (“hospital- acquired AKI”), an unknown proportion of AKI initiates outside of the hospital (“community-acquired AKI”). One potential contributor to the incidence of community-acquired AKI is exposure to high ambient temperatures, which is thought to impact kidneys largely through volume depletion. Motivated by increasing concerns about climate change, population-level studies using administrative health data have consistently reported positive associations between outdoor heat (high temperatures and heat waves) and emergency department (ED) visits and hospitalizations for AKI. A key limitation of existing studies is the identification of AKI cases through International Classification of Disease (ICD) discharge diagnosis coding. Code-classified AKI, while specific, is not a sensitive marker of AKI; it misses a high percentage of total cases. Moreover, it does not distinguish community-acquired from hospital-acquired AKI, cannot assess AKI severity, and does not provide information on patient context. In addition to these limitations, the majority of population-level studies have relied on spatially crude measures of temperature (e.g., from one or a few monitoring sites) that do not fully capture important urban heat dynamics. As a whole, the misclassification in both AKI outcome and heat exposure contribute to a lack of understanding of the true effect of outdoor heat on AKI. This project is motivated by the need for (1) improved outcome assessment, (2) improved exposure assessment, and (3) the identification of heat-vulnerable populations that can be protected via targeted interventions. To address these needs, we will analyze a highly detailed clinical database of over 1.5 million electronic medical records in Atlanta, Georgia for 2013-2021 linked to a high-resolution, state-of-the-art exposure product that will developed as part of this project. For ascertainment of AKI cases, we will consider ICD diagnosis codes - reflecting what has been used in the heat- AKI literature – as well as via assessment of a KDIGO (Kidney Disease Improving Global Initiative)-based serum creatine (SCr) definition comparing ED ‘first-measured’ SCr to baseline values. The KDIGO definition will enable a specific assessment of community-acquired AKI and severity (i.e., stage). In Aim 1, we will develop of a rich database of ED visits, heat exposure metrics at patient residential address, and individual- and area-level sociodemographic and health risk factors. In Aims 2 and 3, we will estimate associations of short-term outdoor heat exposure and AKI, and will identify individual- and area-level risk factors that increase vulnerability to outdo...