New JAMA research letter identifies increase in Emergency Medical Services calls related to extreme heat among vulnerable populations

Tucson, AZ — February 2026— Emergency medical services (EMS) activations increased substantially across the United States between 2019 and 2024. In a new Research Letter in the Journal of the American Medical Association (JAMA), researchers examined how heat-related EMS activations changed relative to non–heat-related activations, identifying population groups and regions where heat-specific increases outpaced broader EMS growth.

Using nationwide EMS data, the study found that population-adjusted heat-related EMS activation rates increased by 65% from 2019 to 2024. Over the same period, non–heat-related EMS activations increased by 69%, indicating that heat-related EMS use rose slightly more slowly than overall EMS demand nationwide. Several population groups and regions experienced heat-related increases that outpaced non–heat-related emergency calls—signaling emerging, heat-specific vulnerabilities.

“Emergency medical services are often the first point of contact for people experiencing acute heat illness,” said lead author Chris C. Lim, PhD, of the Mel & Enid Zuckerman College of Public Health at the University of Arizona. “Our analysis helps distinguish where heat-related emergencies are increasing faster than overall EMS demand, which is important for targeting prevention and preparedness efforts.”

Key findings include:

• Middle-aged adults (ages 35–64) experienced heat-related EMS activations that increased faster than non–heat-related calls, suggesting heightened occupational or environmental exposure.
• Lower-income areas, particularly ZIP codes with median household incomes between $25,000 and $35,000, showed accelerated growth in heat-related EMS use compared with other emergencies, while higher-income areas showed slower heat-related growth relative to overall EMS demand.
• Mountain and West South Central regions of the U.S. saw heat-related EMS activations rise more rapidly than non-heat-related activations, highlighting geographic hot spots of growing risk.
• Adults aged 75 and older had the highest absolute rates of heat-related EMS activations, though their increases closely mirrored overall EMS growth rather than heat-specific acceleration.

This contrast underscores the importance of distinguishing heat-specific risks from broader trends in healthcare utilization.

“As heat exposure increases, public health and emergency response systems will need to address rising overall EMS demand while also identifying where heat-specific risks are emerging, including through occupational protections, cooling access, and region-specific planning,” Lim said.

The authors note that the study is limited by reliance on EMS provider coding and the inability to track patient outcomes after transport. Still, the findings offer one of the most comprehensive national assessments to date of heat-related EMS trends and disparities.

Additional University of Arizona co-authors included Ladd Keith, PhD, School of Landscape Architecture and Planning, and Brian Drummond, MD and Noah M. Tolby, MD, College of Medicine – Tucson. 

The research letter, Trends and Disparities in Heat-related Emergency Medical Services Activations in the US, 2019–2024, appears in JAMA.

Media Contact: 
Shipherd Reed 
Mel & Enid Zuckerman College of Public Health 
The University of Arizona 
Email: shipherd@arizona.edu 
Phone: 520-626-9669

Research Contact: 
Chris C. Lim, PhD 
Mel & Enid Zuckerman College of Public Health 
The University of Arizona 
Email: chrislim@arizona.edu 
Phone: 512-913-6114