Modification of temperature-morbidity associations by social determinants of health

Exposure to high ambient temperature is responsible for more than 11,000 deaths and over 230,000 disability-adjusted life-years in the United States each year. However, which individuals and populations are most at risk, and why, is still not well understood. In 2015, a subset of "Z" diagnosis codes (or "Z-codes") were introduced as a standardized option for healthcare providers to document the social needs and conditions of their patients. To assess heat-related risk across social determinants of health (SDoH), we leverage these codes using a dataset of patient-level emergency department (ED) visits from seven US states. Using a bi-directional, time-stratified, case-crossover design and conditional logistic regression, we compared hospital encounters for seven different health outcomes with SDoH Z-codes at discharge to a reference group matched on age, sex, race, ethnicity, year and hospital. We investigated the following Z-code domains: inadequate housing (Z59.0, Z59.1), poverty-related (Z56.0, Z59.5-Z59.7), living alone (Z60.2), institutional living (Z65.1, Z59.3), and other problems with the social environment (other Z60 sub-codes). We calculated cumulative odds ratios (ORs) for a 3-day lag change in temperature from the 95th to 50th percentile, using ZIP code-specific temperature percentiles. Among 60,557,958 ED visits with available demographic and meteorological data, 461,468 (0.8%) included a SDoH Z-code. Across temperature metrics and outcomes, patients with SDoH Z-codes consistently showed higher associations with heat than the matched reference group without SDoH Z-codes. The largest difference was for acute kidney injury, with a ratio of ORs of 1.21 (1.10,1.33) for daily mean temperature. Notable subgroup findings included elevated kidney-related risks in patients with inadequate housing or poverty-related SDoH, increased mental health risks among those living alone, and elevated cardiovascular risks in people with other problems related to the social environment.