As fall begins and temperatures start to cool, new data show the health impact this summer’s extreme heat had on Missourians.
Nearly 4,200 Missourians went to emergency rooms for heat-related illnesses this year — already nearly 200 more than last year, despite two fewer months of available data.
The 2026 Heat Visits and Temps surveillance dataset, acquired by KBIA from the Missouri Department of Health and Senior Services, tracks heat-related illness emergency department visits over a 91-day period from June 18 through Sept. 16, 2026. The dataset compares daily heat-related illness emergency department visits with the day’s highest and (overnight) lowest heat index values, which account for both temperature and humidity.
An analysis of the data shows emergency room visits rose as temperatures spiked. The highest numbers of visits occurred when severe daytime heat was combined with unusually warm nighttime temperatures.
Emergency room visits did not increase gradually as temperatures rose. Instead, visits often surged during periods of sharp temperature increases and prolonged heat alerts. During peak heat waves, hospital emergency departments experienced a 10%-30% increase in patient volume within 48 to 72 hours compared with regular, non-heat-alert days, creating sudden demands on local health care resources.
From late June through mid-September, Missouri experienced four periods when daytime heat index values exceeded 100 degrees and severe heat alerts were in effect.
Late July brought the most severe 24-hour heat wave of the summer. Daily maximum heat index values exceeded 100 degrees on each day during the ten days of July. The heat index reached 111 degrees on both July 26 and July 27.
That prolonged period of extreme heat coincided with more than 100 heat-related emergency room visits per day for several consecutive days, reaching a seasonal peak of 151 visits on July 27.
Lack of cooling time pushed people to the brink
The data also show nighttime heat played a role in emergency room visits.
When extreme daytime heat was followed by warmer nights, average daily emergency room visits nearly doubled, compared with days followed by cooler nights.
The largest increases occurred when overnight heat index values stayed above 80 to 85 degrees, according to the data.
Christopher Sampson, an MU Health Care emergency room physician, said nighttime cooling gives the body an opportunity to recover from prolonged heat exposure.
“If you're in a position where you don't have that or it's not even cooling down, then your body's not really getting a chance to kind of rest,” Sampson said. “So you're exposing yourself to that constant heat, which then again goes back to exacerbating all your medical problems.”
Sampson said prolonged heat exposure can contribute to people feeling ill and seeking emergency care.
The data also show that prolonged daytime heat combined with warmer nights increased hospital workloads between 6% to 8% in 2025 and 2026, according to a DHSS calculation.
Climate change linked to rising heat-related ER visits
Missouri’s increase comes amid a broader national trend.
Climate Central, a nonprofit research organization, found that nearly one-quarter of U.S. heat-related emergency room visits between 2018 and 2025 were linked to climate change.
Southern and Western states had the highest average rates of heat-related emergency room visits, at about 225 and 230 visits per 100,000 people, respectively, according to Climate Central. The Midwest followed, with about 199 visits per 100,000 people.
Christina Dow, Climate Central’s vice president for science, said people in the Midwest may seek medical care at lower temperatures because they are less accustomed to extreme heat than people in hotter regions.
“People in those regions tend to be much less adapted to it,” Dow said. “And so you actually see big spikes at lower temperatures of people starting to seek care.”
Dow said the period of the year when extreme heat can affect health is also expanding.
“Typically, we think of summer as June, July, August, but we know with climate change, we're seeing an expansion of the season in which we experience the kind of heat that can send people to the emergency room,” Dow said.
