In Chicago ERs, Extreme Heat’s Toll Goes Far Beyond the Obvious

CHICAGO—Extreme heat is filling up emergency rooms, and not just with heat stroke victims. 

Periods of extreme heat in Chicago in recent years were linked to more emergency department visits for a range of health harms, from kidney dysfunction and multiple sclerosis symptoms to injuries from some kinds of accidents and violence, according to a study out Wednesday from Northwestern University. 

Researchers analyzed more than 900,000 visits by adults to emergency rooms and urgent care facilities for all reasons across the city during its hottest days from 2011 to 2023. They found that heat was associated with spikes in a number of conditions that previous research may have missed. The study came from the Defusing Disasters Working Group, an effort to mitigate harm from extreme weather in the city.

This July was the hottest month on record in the contiguous U.S., and Chicago endured multiple dangerous heat waves this summer. As the city suffers through rising temperatures due to climate change, the researchers’ results illustrate the importance of preparing emergency facilities for wide-ranging health fallout, said Daniel Horton, co-lead of the Defusing Disasters Working Group and a co-author on the study. 

“The healthcare system needs to prepare for things like this,” Horton said. “Heat waves cause all sorts of public health impacts, and with continued warming we’re going to only exacerbate that.”

The researchers found that heat was associated with increases in visits for conditions like kidney failure, edema, varicose veins, cannabis-related mental and behavioral issues and a dangerous lack of salt and fluids known as volume depletion. They also saw spikes in some kinds of accidents, including leg injuries and accidental firearm discharge. 

As they looked at distributions across age, sex, race and geographic region of the city, they found some key differences. For instance, edema—swelling as fluid builds up in tissue—varicose veins in lower extremities and pressure ulcers were associated with heat in older patients. And the West Side of Chicago—an area that sees higher temperatures and compounding environmental injustices—saw the broadest range of conditions associated with hot days. 

In 1995, a heat wave in Chicago killed more than 700 people over five days, disproportionately Black residents in poorer neighborhoods, especially elders. Three decades later, many of the social inequities driving that disparate impact persist. 

It’s well documented that heat affects every system in the body and can cause wide-ranging harms, from dehydration to exacerbation of chronic disease and mental health problems. Prior research has found links between hot days and increased irritability, aggression, irrational behavior and violence.

Carlos Gould, an environmental health scientist at the University of California San Diego who studies extreme temperature hazards, said the new study bolsters what scientists and health professionals already know. 

“It’s an interesting way of confirming what we have been seeing more and more, which is that extreme heat is associated with a wide range of changes in healthcare utilization across a wide range of specific underlying causes,” Gould said. 

Gould and others pointed out that while most prior research tracked spikes in problems already associated with heat, this study took a novel approach—similar to a process used to link genomic variants with diseases. The researchers looked at all 1,800 diagnostic codes used in emergency visits and zeroed in on whichever rose significantly during the city’s hottest days and their immediate aftermath.

“Historically, we’ve thought up what diagnoses have been important, then looked for those effects,” said Marshall Burke, a professor of global environmental policy at Stanford University. “This paper takes a step back and says: a lot of other things could have been affected.”

Burke said the study was well done and noted that it was aided by the rich availability of health data in Chicago. The city’s devastating 1995 heat wave, he added, spurred significant public health and scientific interest in the effects of heat. 

Burke called the study’s criteria for linking a condition to heat conservative: They required at least 30 percent of diagnoses to take place on relatively hot days, which could miss some more common conditions. 

“That’s just a pretty high bar,” Burke said. “If anything, it just understates the number of things that heat is actually affecting.” 

The study didn’t find an association between heat and cardiovascular or respiratory diagnoses—both known to be worsened by heat—but researchers cautioned that doesn’t mean there was no link in Chicago. 

Abel Kho, an internist and professor of medicine at Northwestern’s Feinberg School of Medicine and senior author on the study, said some emergencies linked to underlying lung or heart issues could be coded for secondary complications.

“A person might have had heart failure, but they might have come in because their kidney function got worse, because they got dehydrated,” Kho said. 

Kho said the study provides useful information for public health officials—like the need to provide heat resources on days following heat waves as well as during periods of extreme heat—and for doctors looking to inform their patients about the growing hazards. He sees several patients with multiple sclerosis, for example, but the association with heat isn’t at the top of his mind the way it is for conditions like asthma or lung disease. 

“It does remind us that there’s other conditions that we should be thinking about,” Kho said. “If there’s going to be a particular hot spell, and you happen to see them in clinic, that’s something that you should be counseling people on.”

Lisa Patel, a pediatric hospitalist and the executive director of the Medical Society Consortium on Climate & Health, an organization of health professionals, said the study emphasizes the need to prepare emergency departments to serve patients impacted by heat while also investing in preventative measures like cooling and hydration centers, public awareness and regular patient care. 

“Health systems [and] public health departments need to be partnering with community organizations to provide the resources that community members need to keep them safe and keep them from ending up in the emergency room in the first place,” Patel said. 

“Unfortunately,” she added, “most people really underestimate the risks of extreme heat.”