Cold exposure and urban opioid risk: A spatial regression discontinuity analysis in Chicago

Opioid mortality in the United States exhibits pronounced spatial variation within major metropolitan regions. Whether these differences shift sharply at municipal boundaries where governance structures and service systems change remains poorly understood. This study examines whether opioid involvement in drug-related deaths changes discontinuously at the boundary between the City of Chicago and suburban Cook County, Illinois and assesses whether cold-related deaths display similar spatial patterns. Mortality microdata from the Cook County Medical Examiner's Office were linked with geocoded death locations, and a spatial regression discontinuity design was applied using the Chicago municipal boundary as a sharp institutional cutoff. The analysis reveals a consistent increase in opioid involvement on the Chicago side of the boundary, indicating that localized risk conditions may extend beyond the urban core and align with jurisdictional transitions. Cold-related deaths show a comparable boundary pattern and display additional concentrations in areas characterized by prolonged outdoor exposure. Although interaction estimates suggest that cold exposure may heighten contrasts across the boundary, this pattern is less meaningful due to the smaller number of cold-related cases. These findings illustrate that mortality risk can shift at administrative boundaries even when environmental and socioeconomic conditions remain spatially continuous. The results highlight how institutional capacity and anthropogenic environmental factors, such as inadequate shelters and warming facilities, structure localized patterns of overdose harm within a shared metropolitan region and demonstrate the value of boundary-focused approaches for understanding spatial disparities in opioid-related mortality.