Heat index and reported crime incidence in Baltimore, MD and Washington, DC

Extreme heat events are increasing in frequency and severity across the United States, with growing evidence linking elevated temperatures with increased crime incidence. Most existing studies rely on ambient air temperature rather than heat index, a composite metric that accounts for both temperature and humidity to better approximate physiologically experienced heat stress. Using an ecological time-series design, we analyzed daily urban reported crime counts in Baltimore, MD and Washington, DC, from January 1, 2020, through December 31, 2024, examining associations between daily maximum heat index and total, violent, and non-violent reported crime incidence. Heat exposure was categorized as High Heat Index (85th–95th percentile), Extreme Heat Index (≥95th percentile), and reference days (<85th percentile). City-stratified negative binomial regression models were used to estimate incidence rate ratios (IRRs), adjusting for day of week and month. In Washington, DC, violent crime was 10.4% higher on High Heat Index days (IRR = 1.104, 95% CI: 1.018–1.198) but not significantly higher on Extreme Heat Index days (IRR = 0.989, 95% CI: 0.889–1.100) compared to reference days. No significant associations were observed for non-violent crime in Washington, DC or any crime category in Baltimore, MD. High Heat Index days were associated with increased violent crime in DC but not Baltimore, suggesting city-specific contextual factors modify heat-crime associations. These findings underscore the need for public health and safety planning to account for local urban context as extreme heat events increase in frequency.