Temperature and Emergency Department Visits: Present-Day Associations and Future Projections.

High ambient temperature (i.e., "heat") is associated with increased rates of death. The association between heat and illnesses that result in emergency department (ED) visits is less well characterized: most studies have examined heat impacts in the summer months, for a subset of health outcomes, or at limited spatial scales. This work characterizes the relationship between temperature and all-cause ED visits across the entire year and at large spatial scale, and projects ED visits associated with future temperature change. We analyzed health data from 21,090,141 commercially insured individuals across 2,161 counties in the contiguous US from 2010 to 2021. We used distributed lag non-linear models (DLNM) to fit the exposure-response relationship in the historical period (2010-2021), then used projected mid-century (2040-2050) temperatures from global climate models across three Shared Socio-Economic Pathways to project future all-cause ED visits. Increasing daily maximum temperature were associated with increased ED visits throughout the year and across the US. ED visits increased 1.3% per year under the highest emission scenario and more than half of the counties in the study experienced 27 or more days per year of high healthcare utilization under future climate scenarios, an increase of 50% over present day conditions. These findings clarify the exposure-response relationship between daily maximum temperature and ED visits as distinct from the relationship between temperature and mortality. They also emphasize that future healthcare systems will need to accommodate more baseline ED visits throughout the year and a higher frequency of days with extremely high ED visit rates.