Heat-related 911 emergency calls are growing fast, Arizona study finds

Joan Meiners, Arizona Republic

Thu, February 5, 2026 at 12:02 PM UTC

11 min read

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Extreme heat is increasingly the culprit behind 911 emergency calls among older and lower-income individuals and those living in the Intermountain West, new research out of the University of Arizona has found.

The study, published Feb. 4 in JAMA, the peer-reviewed journal of the American Medical Association, looked at the recorded cause for EMS activations (similar to 911 calls) placed across the country between 2019 and 2024. The researchers acquired public data on tens of millions of emergency calls, separated the incidents based on whether they had been categorized as heat-related, then looked at population-controlled trends across census regions and demographic groups.

Overall, they found the number of patients requesting emergency medical services per 100,000 residents had increased drastically in both categories over the six years evaluated — by 65.1% for heat-related calls and by 68.8% for non-heat-related calls.

Lead study author Chris Lim, an assistant professor in the Mel and Enid Zuckerman College of Public Health at the UA, attributed this to an aging population, chronic disease burden and issues stemming from the pandemic.

When they looked closer, they found evidence of a more serious heat-specific trend in the Mountain (intermountain region between Arizona and Montana) and West South Central (Texas area) census divisions, among adults ages 35 to 64 and in zip codes with a median household income between $25,000 and $35,000.

In the Mountain division, heat-related emergency calls rose by 41.3% between 2019 and 2024 compared to 25% for non-heat-related calls, a relatively large and statistically significant difference.

In contrast with those results, households with a median income between $100,000 and $150,000 saw a shift in the opposite direction, with proportionally more EMS calls unrelated to heat. Several census divisions including the Pacific and the Middle Atlantic also saw bigger increases in non-heat-related demand on emergency services.

These findings underscore the need for regionally tailored heat preparedness and expanded access to air conditioning for middle-aged and less financially stable groups, Lim said. With new records for heat waves and extremes now set each year, even states that have long dealt with high temperatures will need more tools to manage the toll on medical systems and human well-being.

Heat: While global climate action stalls, cities can adapt to ease heat effects, study finds

Emergency medical systems under strain from uneven heat impacts

The researchers had known about the massive modern strain on the EMS system from prior work. A study by different scientists in 2023 also pointed to a likely overload of Phoenix's medical system if a five-day heat wave coincided with a power blackout. That analysis, conducted in part by climate scientists at Arizona State University, estimated the unlikely but not impossible scenario could send almost half of the city's 1.4 million residents to emergency departments that collectively only had about 3,000 beds.

Given intensifying heat waves and energy demands, that study made it clear Phoenix should consider additional heat mitigation strategies for blackout events.

What was less clear when Lim and colleagues started their analysis was what role worsening extreme heat played in the increase of more individual, routine medical emergencies.

"Going into this study, I expected heat-related EMS activations to be increasing faster than non-heat-related activations overall, given rising temperatures and the growing public attention to heat-related illness," Lim said.

Finding the opposite in the nationwide results surprised him, he said, while highlighting the importance of comparative analyses. If they had only looked at patterns in heat-related calls, they would have missed the detail about which groups and regions really are experiencing a shift in emergency care burden due to extreme temperature exposure.

A heat warning sign stands at the entrance of the popular hiking spot at Camelback Mountain in Phoenix on Thursday, June 10, 2022. City officials restricted access to some hiking trails throughout the city due to the extreme temperatures.

"Without that context, it would be easy to overinterpret headline increases," Lim said. "The study identified specific subgroups and regions where heat-related EMS use did increase faster. Those patterns were both unexpected and policy-relevant."

The key takeaway, Lim said, is that overall EMS demand is rising, and heat is contributing to that load in uneven ways. This means EMS systems will need more support for capacity and surge planning, and that heat-specific interventions that focus on outdoor workers and people with limited access to cooling options may help the most.

States like Arizona and Texas, where heat-related calls are increasing faster despite existing heat relief programs and growing risk awareness, may also need to explore new solutions. Without more expansive strategies, medical providers and infrastructure could become dangerously overwhelmed as temperatures continue to rise due to both climate change and urban development.

2023 study on Phoenix heat emergency: Phoenix is not prepared for a simultaneous heat wave and blackout

What is Phoenix already doing for various groups exposed to heat?

Some existing interventions have helped Phoenix fend off worsening reports of heat deaths in recent years. After a decade of sharply increasing heat-related fatalities, Maricopa County recorded a slight drop in 2024 and a larger one in 2025.

Dave Hondula, director of the Phoenix Office of Heat Response and Mitigation and a former climate scientist at ASU who co-authored the study about a five-day heat wave and blackout, thinks this is partially due to efforts undertaken by his team to open more cooling centers, distribute water bottles and other heat relief supplies on especially hot days and to work with Phoenix EMS on solutions like bringing patients' body temperatures down quickly on site by submerging them in body bags filled with ice. These actions may have helped with recent dips in total heat-related deaths among the unhoused community in particular.

Innovations like Phoenix's "Cool Callers" program tested other ways of managing heat impacts before body bags, ice filled or not, became necessary. In 2023, Hondula was running this program himself, calling vulnerable, often elderly residents during heat waves to check that they had access to air conditioning. He told The Republic he wasn't sure why more people weren't signing up as volunteers or to add their loved ones to the list. The website for that program has since gone dark. Meanwhile, adults over the age of 65 made up 30% of heat-related deaths in 2025.

For low-income households, Phoenix area utilities offer energy bill assistance, and Gov. Katie Hobbs' Efficiency Arizona program has helped residents across the state install new heat pump cooling units using funds from former President Joe Biden's Inflation Reduction Act.

Still, 111 people, 27% of the 2025 total, died from heat-related causes while indoors in Maricopa County last year, often in spaces without functioning or running air conditioning units. A national utility assistance program with longstanding bipartisan support that had helped more than 70,000 households in Arizona cover cooling expenses since 2022, LIHEAP, was recently defunded by the Trump administration.

Will Humble, who previously served as ADHS executive director and now directs the Arizona Public Health Association, worries most about Phoenix's unhoused population, many of whom are homeless for reasons related to untreated serious mental illnesses.

This group made up 48% of heat-related deaths in Maricopa County in 2025. Humble was disappointed the JAMA study didn't mention homelessness as a central cause of heat-related EMS incidents and emphasized that this ongoing problem ultimately affects the safety and well-being of everyone in the area.

"In the EMS world, all roads lead back to response time," Humble said. "If the ambulances are overloaded with heat calls, it blows over into other sectors to make the response for a heart call or a stroke call a few minutes longer. And for those patients, a few minutes can be the difference between a good outcome and death."

The Arizona Department of Health Services released its own report in 2025 showing sharp increases in heat-related emergency room visits and hospitalizations between 2020 and 2024. The report was criticized, including by Humble, for not doing enough to examine trends alongside data on homelessness and occupational exposure to help identify causes and solutions.

As part of an investigation of progress on statewide heat responses, The Arizona Republic recast the department's data in graphs instead of tables and spoke with experts about what the report was missing. Part of the issue, The Republic found, is that Arizona's focus on mitigating heat health impacts has been largely on reducing the number of deaths rather than on keeping heat patients from arriving at hospitals in the first place.

The Republic requested access to the full public dataset on heat-related visits to Arizona hospitals in June 2025 in order to further evaluate heat illness trends, but has not yet received any substantive response from ADHS. Seeing the same gap in understanding of heat risk patterns, researchers like Lim have stepped into that space with other data to offer new insights and directions — while they still have the funding to do so.

From a practical, on-the-ground standpoint, Hondula also emphasized the value of research efforts and data access in predicting and addressing heat-related medical emergencies.

"Our local EMS call data are one of our most important assets for crafting the City of Phoenix Heat Response Plan," he said. "Each year, we review patterns and trends in our EMS data, along with other community health indicators, to guide our decisions about when, where and how to best allocate resources. We hope that work of Lim and colleagues encourages and inspires other municipalities across the United States to do the same."

Heat: As heat-related hospital visits rise, Arizona's focus is on deaths and quick fixes

Rising heat illness makes climate health research funding critical

The study published Feb. 4 in JAMA was not designed to directly attribute regional patterns in heat-related EMS activations to climate metrics or adaptation capacity, Lim said. It fits into the "letter" category of research papers that target a focused question concisely. So he remains scientifically careful to not implicate a specific cause of the increase in heat-related emergencies until he can pursue follow-up analyses.

"That said, several plausible mechanisms are consistent with the observed patterns," he said, noting that the Mountain and West South Central regions experiencing greater increases in heat-related EMS demand tend to have more outdoor workers in construction, agriculture and energy-related jobs that result in exposure to heat extremes.

While the study controlled for number of residents, Lim also said that the rapid population growth in these regions may indicate that heat relief options and heat-ready infrastructure isn't keeping up.

The Republic examined in 2023 how lacking housing availability, affordability and quality put vulnerable residents at greater risk as summer temperatures tilted more deadly due to climate change. In the Phoenix area, construction of mostly luxury apartments and million-dollar homes does little to bring down escalating heat-related deaths among unhoused, elderly and low-income residents. Likewise, Scottsdale's sustainable building policies count for little if the city blocks housing projects, The Republic found.

In Yuma, The Republic reported farmworkers struggling to recover from long, hot days in the fields in inadequately cooled mobile homes while the city lagged on affordable housing development.

Farmworkers and heat: In Yuma, farmworkers' struggle with heat is worsened by inadequate housing

Lim plans to next explore connections between the increase in heat-related EMS activations and specific "meteorological, land use and occupational exposure data." He theorizes that surges in 911 calls may coincide with worsening heat waves — many of which scientists have determined, urban heat island aside, would be unlikely to occur without the warming influence of specifically human-caused climate change.

"(This study) naturally sets up several next steps, including linking EMS activations to temperature extremes, examining finer geographic and temporal resolution and evaluating associations with other health endpoints," Lim said.

He also wants to evaluate connections to heat-caused cardiac arrest and air pollution-caused asthma. And there's work to be done teasing out how the results may be affected by what protocols providers are following when coding a medical emergency as heat-related, which The Republic reported are being developed and adopted at different paces across the country.

Though the study shows restraint in connecting heat-related medical emergencies directly to climate change, Lim did not hold back in stating the need for continued research funding to clarify that relationship.

Together with his colleagues at the UA and his collaborators across the country, Lim has spent the past year working furiously on these efforts while navigating uncertainty around federal funding. Grants that explicitly reference climate change have been particularly vulnerable to cuts under the Trump administration.

“The end result will just be the dismantling of public health safety nets, discouraging researchers from going into these fields and a loss of scientific productivity," Lim said. "And I worry it could worsen as the administration expands efforts that restrict or discourage climate-related research."

Joan Meiners is the climate news and storytelling reporter at The Arizona Republic and azcentral.com. Her award-winning work has also appeared in Discover Magazine, National Geographic, ProPublica and the Washington Post Magazine. Before becoming a journalist, she completed a doctorate in ecology. Follow Joan on Twitter at @beecycles, on BlueSky at @joan.meiners.bksy.social or email her at joan.meiners@arizonarepublic.com.

Sign up for AZ Climate, The Republic's weekly climate and environment newsletter. Read more of the team's coverage at environment.azcentral.com.

This article originally appeared on Arizona Republic: UA researchers find the number of heat-related 911 calls is growing

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