Dehydration and heatstroke are the health problems most people picture when the mercury climbs. New research from Northwestern University suggests that picture is far too narrow. Analyzing nearly a million emergency and urgent care visits in Chicago, scientists found that extreme heat is tied to a much broader set of illnesses and injuries, including skin disorders, multiple sclerosis flare-ups and traffic crashes.
The study was published on August 26, 2026 in Science Advances and identified 33 diagnoses that spiked when temperatures reached 92.6°F (33.7°C) or higher. Researchers pulled from 916,404 visits recorded between May and September in the years 2011 through 2023.
The team borrowed a method from genetics, running what they call a heat-wide association study, rather than starting with a list of illnesses they expected to see rise. That approach let unexpected connections surface.
What showed up beyond heatstroke
The list stretches well past the classic warnings. Acute kidney failure and skin rashes climbed, as did insect bites, mental and behavioral disorders and injuries from assault and firearms.
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Traffic accidents rose on the hottest days. Even varicose veins and cannabis-related mental health visits appeared more often, according to reporting on the study.
"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," said Hyojung Jang, a biostatistics doctoral candidate at Northwestern University Feinberg School of Medicine and lead author of the study.
Senior author Dr. Abel Kho, who directs Northwestern's Institute for AI in Medicine, put it more plainly. "Hot days are just like, something's going to happen," he said.
Why heat can worsen MS and other conditions
The multiple sclerosis link may surprise readers, but neurologists have documented it for more than a century. Heat sensitivity, known as Uhthoff's phenomenon, is a temporary worsening of neurological symptoms when core body temperature rises. Nerves already damaged by MS conduct signals more slowly at higher temperatures, and symptoms typically ease again once the body cools down.
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Skin is the largest organ exposed to the environment, and hot, humid conditions can trigger rashes, heat swelling and infections. Kidneys strain when the body sweats out water it cannot easily replace, and repeated dehydration is a known driver of acute kidney injury.
Cardiovascular problems did not stand out in the Chicago data, which the researchers attributed to how emergency departments code overlapping issues rather than to any protective effect.
Injuries also climbed, and that fits a broader pattern. "When people are exposed to extreme heat, they often act more aggressively, so that may be a factor here," said co-author Daniel Horton, an associate professor of Earth, environmental and planetary sciences at Northwestern, in comments reported by the Associated Press. In separate remarks to Euronews, he added: "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 it means as US summers keep breaking records
Horton also framed the wider context for the findings. "We know that heat waves are becoming more frequent, more intense, and lasting longer," he said. A record-breaking heat dome pushed temperatures across the Desert Southwest and Intermountain West above 110°F earlier this month, with Extreme Heat Warnings issued by the National Weather Service through August 9.
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For readers with chronic conditions, the practical takeaway is to plan for hot days the same way patients with asthma plan for high-pollen days. Common advice for people with MS includes cool showers, cooling vests or wristbands, breathable clothing and lighter meals to help lower core temperature during hot spells.
Similar guidance for people with kidney disease or heart failure emphasizes tracking fluid intake, watching for early signs of dizziness or confusion and speaking to a doctor before adjusting any diuretic medication.
You can find a plain-language rundown of the physical toll of hot weather in our guide to how extreme heat can affect your body.
Anyone with symptoms that go beyond mild fatigue during a heat wave, whether it is confusion, chest pain, sudden weakness or worsening MS symptoms that do not ease with cooling, should seek medical care rather than wait it out.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.