CHICAGO — When temperatures spike, emergency physicians expect to see more cases of dehydration and heat stroke. But a new Northwestern University study suggests that extreme heat also is linked to a much wider range of health problems seen in emergency departments and urgent care clinics, including skin conditions, multiple sclerosis and even injuries from traffic accidents.
The findings could help emergency departments and health care systems better prepare for heat waves, which are becoming more prevalent with climate change.
Heat-related illness is difficult to track because medical records often capture the condition being treated without noting that heat may have contributed. This study used an outside-the-box framework, inspired by methods used in genomics research, to examine 1,803 diagnoses recorded across nearly 1 million emergency department and urgent care visits in Chicago between May and September 2011-2023. It identified 33 diagnoses associated with extreme heat exposure, many of which were surprising or understudied.
"It is a public health obligation to protect the individuals and communities most vulnerable to the harmful effects of climate change," said corresponding author Hyojung Jang, a Ph.D. candidate in biostatistics at Northwestern University Feinberg School of Medicine . "If we're only tracking the more commonly known heat-related illnesses, we'll underestimate the true heat-related health burden, which could result in vulnerable people and communities being overlooked."
The study will be published Aug. 26 in Science Advances.
From skin rashes to accidental firearm discharge
Some of the heat-related diagnoses included skin rashes and swelling, acute kidney failure, multiple sclerosis, mental and behavioral disorders, insect bites and stings, traffic accidents, assault and firearm-related injuries.
"Previous research has shown that when people are exposed to extreme heat, they often act more aggressively, so that may be playing a factor here," said co-author Daniel Horton , associate professor of Earth, environmental and planetary sciences in Northwestern's Weinberg College of Arts and Sciences. "When it's hot out, your body is stressed, and your mind is stressed, so that may be distracting or aggravating, and it could lead to more accidents."
What's new?
The study is the first to employ a data-driven approach that borrowed methods from genome-wide association studies to gain a fuller understanding of the range of diagnoses associated with extreme heat exposure. Prior studies examining heat and health typically began with a predetermined list of diagnoses of interest, such as heat exhaustion, heat stroke or heart disease. While valid, that approach narrows the focus of heat-related illness research to what is generally already known.
"By using novel methods to link clinical data from our Chicago-wide CAPriCORN data network with climate data by place and time, we can better understand how extreme heat may affect the health of at-risk individuals," said senior author Dr. Abel Kho , director of the Institute for AI in Medicine at Feinberg and co-director of the Northwestern Network for Collaborative Intelligence .
While this study only looked at Chicago, the authors said their study framework can be extended for use in any city, state or country, and also can be used with other types of health data, like death records or ambulance calls.
What can be done?
An extreme weather expert focused on the link between human-caused climate change, extreme weather events and public health, Horton co-leads the Roberta Buffett Institute for Global Affairs' Defusing Disasters Working Group , which aims to mitigate the adverse effects of extreme weather events in Chicago and beyond. Knowing the catastrophic effect extreme heat can have on Chicago residents — such as the heat wave of 1995, which caused 739 excess heat-related deaths — the working group developed the Chicago Heat Vulnerability Index (HVI) based on death records.
The Chicago Park District, for example, currently uses the HVI to prioritize air-conditioning upgrades at park fieldhouses in neighborhoods most vulnerable to extreme heat. The findings of this study suggest climate preparedness efforts may need to take many forms and account for a broader range of health impacts than is currently recognized, the authors said.
"Since we know there's an association between extreme heat and a number of different health outcomes, reducing exposure is critical," Horton said.
Co-developed with community members and organizations, the Defusing Disasters Working Group has provided Chicago with 30 policy proposals, such as planting more trees, installing more shade at bus stops, creating a heat-protection ordinance for outdoor workers and proposing a city mandate for landlords to supply apartments with adequate cooling.
More study findings:
Among 372,140 adults with emergency department or urgent care visits, females accounted for 57.9% of patients, and the average age of patients was 52 years old. Adults aged 25–44 years old represented the largest single age group, Black or African American patients comprised the majority by race/ethnicity, and residents of Chicago's West Side contributed the highest share of emergency department or urgent care visits. Over the study period, there were 100 extreme heat days (92.6 degrees Fahrenheit and higher). These were concentrated in mid-summer, occurring most frequently in July (36%), followed by August (25%), June (23%), September (12%) and May (4%).
