Ambient temperature and intentional homicide: A multi-city case-crossover study in the US

Background: There has been an increasing interest in the association between ambient temperature and violence and crime, in the context of global warming. We aimed to evaluate the association between daily ambient temperature and intentional homicide-a proxy for overall inter-personal violence. Methods: We collected daily weather and crime data from 9 large US cities (Chicago, Detroit, Fort Worth, Kansas City, Los Angeles, Louisville, New York, Tucson and Virginia Beach) from 2007 to 2017. A time-stratified case-crossover design was used. The associations were quantified by conditional logistic regression with distributed lag models, adjusting for relative humidity, precipitation and effects of public holidays. City-specific odds ratios (OR) were used to calculate the attributable fractions in each city. Results: Based on 19,523 intentional homicide cases, we found a linear temperature-homicide association. Every 5 degrees C increase in daily mean temperature was associated with a 9.5% [95% confidence interval (CI): 4.3-15.0%] and 8.8% (95% CI: 1.5-16.6%) increase in intentional homicide over lag 0-7 days in Chicago and New York, respectively. The association was not statistically significant in the other seven cities and seemed to be stronger for cases that happened during the hot season, at night (18:00-06:00) and on the street. During the study period, 8.7% (95%CI: 4.3-12.7%) and 7.1% (95% CI: 1.4-12.0%) intentional homicide cases could be attributed to temperatures above city-specific median temperatures, corresponding to 488 and 316 excess cases in Chicago and New York, respectively. Conclusions: Our study suggests that the interpersonal violence might increase with temperature in some US cities. We also provide some insights into the mechanisms and targeted prevention strategies for heat-related violence.