PROJECT SUMMARY Exposure to non-optimal outdoor temperatures is responsible for an estimated 125,000 deaths and 1.8 million DALYs in the United States each year. However, it is still not well known which individuals and populations are most vulnerable. In particular, there is a shortage of evidence linking social determinants of health to temperature vulnerability, which constrains the ability of decision makers to protect high-risk groups through targeted policy interventions. Here we leverage the recent introduction of ICD-10 Z-codes – which encode the social determinants of health – to investigate potential risk factors for temperature-related morbidity. The study will focus on the period 2016-2020 in six states: Arizona, California, Georgia, Missouri, New York and Oregon. Temperature data will come from a national 1km2 meteorological product and health data from state-level government health agencies or hospital associations. We will use a case-crossover study design to estimate short-term associations between outdoor temperature and emergency department visits for all-causes and cardiorespiratory causes. Assessment of effect modification of these associations by individual-level social determinants of health (derived from Z-codes) will focus on social domains related to housing, income/deprivation, social isolation, and institutional living (Aim 1). In addition, we will explore potential interactions between individual-level and area-level social determinants of temperature vulnerability, with the latter derived primarily from the American Community Survey (Aim 2). Stratifying on area-level indicators of the social determinants of health will allow us to explore how any individual-level effects are dependent on the context within which they occur; it is possible that some effects will vary with respect to characteristics of the broader living environment. Timely knowledge of temperature vulnerability can help tailor interventions including public health messaging, warning systems, infrastructure investments and neighborhood outreach programs. As climate change impacts are expected to disproportionately affect already-disadvantaged populations, it is essential to better understand climate-sensitive risks across sub-groups in order to prevent the exacerbation of health disparities. Results from these aims will also provide methodological directions for future use of Z-codes in environmental epidemiology, can be expanded to include other US states and other environmental exposures, and can guide other types of studies exploring vulnerability to temperature.