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Illinois Institute of Technology and , a nonprofit organization that works to implement equitable climate solutions, published a report showing that families without central air conditioning are exposed to dangerous indoor temperatures during increasingly hotter summers.

The research showed that many homes without central air conditioning reached dangerously high temperatures during a severe heatwave in 2023 and remained high overnight even after outdoor temperatures dropped. Although most of the participants reported feeling uncomfortably warm, only half of the participants perceived their homes had reached unsafe temperatures.

Elevate and Illinois Tech recently released these findings in titled, “Summer Indoor Thermal Conditions and Heat Adaptation in �鶹APP Residences.” The publication includes contributions from �鶹APP’s Department of Environment (DOE) and the �鶹APP Department of Public Health (CDPH). It was funded by Northwestern University’s Buffett Institute for Global Affairs through the which is developing �鶹APP’s first public health-informed Heat Vulnerability Index.

“This collaborative study is among the first to quantify the indoor temperature extremes using actual data measurements. Until now, we have had limited information on indoor temperature extremes during heatwaves, as most of our understanding was based on model results or focused mostly on outdoor measurements. Our study shows that each space within a home can respond differently to a heatwave, highlighting the need to provide a safe cooling space in every home,” says Mohammad Heidarinejad, associate professor of architectural engineering at Illinois Tech. 

The study’s findings indicate many families are unknowingly exposed to extreme heat, which the National Oceanic and Atmospheric Administration (NOAA) identifies as the leading cause of weather-related deaths across the United States. Extreme heat can also cause heat stroke and exacerbate existing health conditions, leading to deadly complications.

“The findings in this report were very powerful and confirm what is anecdotally expressed by many residents throughout the summer,” says Elevate CEO Anne Evens. “If homes are retaining extreme heat overnight, it’s dangerous. Simply put, �鶹APP residents need greater access to affordable cooling in their homes to stay safe during increasingly frequent and intense heatwaves. Thankfully, there’s an influx of city, utility, and federal funding to help homeowners and renters increase energy efficiency and install heat pumps that provide both heating and cooling in homes. This is an opportunity to prioritize seniors who live alone, occupants that have chronic health conditions or may be vulnerable, as well as homes that lack central cooling. It’s our job to ensure that these upgrades don’t exacerbate energy burden.”

“Social isolation is one of the biggest risk factors in heat-related deaths, which is why the �鶹APP Department of Public Health, in collaboration with the Department of Environment, community partners, and academic partners, is developing the city’s first community-driven, public health-based heat vulnerability index. We know heat impacts neighborhoods and individuals differently; what we want to learn now is who exactly is being impacted, so we can protect them,” says CDPH Commissioner Dr. Olusimbo “Simbo” Ige.

The report’s insights on the duration and severity of high indoor temperatures in �鶹APP confirm residents’ varied experiences with extreme heat. The findings stress the importance of policies and programs to protect people when air conditioning or hydration is unaffordable or unattainable. In addition to increasing access to affordable cooling in homes, the report calls for further research and more education about extreme heat risks, as well as additional public communications before and during heat events so people can take precautions.

Researchers collected data on the indoor temperature and humidity of 10 �鶹APP homes for about four weeks during summer 2023, which included one of the city’s hottest heatwaves on record. The 10 homes represent the five most common housing types in �鶹APP. During the summer’s worst heatwave, all the homes monitored during the study reached the National Weather Service’s “Extreme Caution” heat index threshold of 90 degrees. Eight (80 percent) of the homes met the “Danger” heat index threshold of 103 degrees and many did not cool down in the evening, even as outside temperatures began to drop. The heat index findings show extreme conditions in many of the homes, with a maximum heat index of 120.1 degrees and a maximum heat index differential of 32 degrees within one home, between the basement and second floor. Although all participants reported using multiple strategies to cool off, many still reported feeling uncomfortably warm despite their best efforts.

“Recent decades have seen outdoor summer nighttime temperatures increase at twice the rate of outdoor summer daytime temperatures, with human-caused climate change partly to blame,” says Northwestern Professor Daniel Horton, co-lead of the Defusing Disasters HVI development project. “Warmer nights limit the body’s ability to recover from daytime temperature extremes which exacerbates health risks. These hazardous climatic patterns, coupled with the indoor temperature findings presented in this study, suggest that heat mitigation efforts are needed to protect society’s most vulnerable, both indoors and out.”

As climate patterns shift, average summer temperatures in �鶹APP are expected to rise higher and sooner in the season. Extreme heatwaves are projected to become more frequent, intense, and prolonged. Elevate and its partners’ research empowers community leaders to implement better informed strategies to increase the city’s resilience to severe weather events.

With Northwestern University’s Buffett Institute for Global Affairs Defusing Disasters Working Group, community organizations, CDPH, and DOE are working together to address heat vulnerability due to climate change.

Last year Elevate and Illinois Tech coordinated the indoor air temperature study with HeatWatch �鶹APP, NOAA’s Urban Heat Island Mapping Campaign to measure outdoor air temperatures across all 77 �鶹APP community areas. One hundred community scientist volunteers helped measure air temperature and humidity across 228 square miles to create urban heat island maps to visualize the disparities of how different neighborhoods experience heat differently. The results are published .

After HeatWatch �鶹APP, the Defusing Disasters Working Group began collecting climate data, �鶹APP death certificate data from the last 30 years, as well as health, geographic, and demographic data to better understand who is most vulnerable to hot weather. This data will be used to co-develop a community-driven, data informed, public health-based heat vulnerability index (HVI) that will assist the city in developing policies, programs, and strategies to improve heat resiliency in �鶹APP communities and prevent hospitalizations and deaths of at-risk residents.

Access to data is still a challenge to overcome, especially data on who has air conditioning or can’t afford to use it, that could improve the HVI to better prioritize and allocate resources during extreme heat events.