Although extreme heat is a leading cause of weather-related illness and death, scientists have conducted limited research on its impacts indoors, where people spend most of their time and may suffer severe health effects if their homes are inadequately cooled. 

Now an interdisciplinary team of scientists is addressing this issue by developing an innovative method to estimate health risks of older adults indoors. The method, detailed in a new paper and available for other researchers to adopt, suggests that exposure to extreme indoor heat may be connected to a greater number of heat-related deaths than previously believed, especially among the elderly.

“People in the United States routinely spend as much as 90 percent of their time indoors, yet we know very little about indoor exposure to extreme heat,” said Cassandra O’Lenick, a scientist at the National Center for Atmospheric Research (NCAR) and lead author of the paper. “To get a better picture of how heat is affecting health, we need to consider the indoor environment, especially as temperatures warm with climate change.”

O’Lenick, an environmental epidemiologist, worked with a team of researchers specializing in a broad range of disciplines, including the atmospheric sciences, social sciences, building science, and public health, to measure the impacts of indoor heat.

The researchers developed a method to estimate hour-by-hour indoor temperatures by drawing on models of indoor thermal comfort and building energy, and by incorporating such data sources as weather records and the characteristics of different types of buildings. They then applied statistical techniques to correlate indoor heat with emergency room admissions and mortality rates.

The team focused on the city of Houston because of its aging population, socioeconomic disparities, and extreme heat during summer months. But the method they created could also be adapted to other cities.

Although the main focus of the article was to demonstrate the method’s effectiveness, the study noted that indoor heat exposure may be responsible for a substantial number of heat-related deaths not captured in official records. The authors found that people living in lower-income neighborhoods that were predominantly nonwhite and had less central air conditioning faced the greatest risk of dying from health conditions caused by indoor heat. Residents of east and north central Houston were particularly vulnerable.

“These findings in Houston show that we need to learn more about the influence of built environment and outdoor temperatures on indoor heat and population health risks in other places as well,” said NCAR scientist Olga Wilhelmi, the senior author of the study. “This can help guide strategies to better safeguard human health through building design and management.”

The study, funded by the U.S. Environmental Protection Agency (EPA) through an EPA STAR grant, was published in Environmental Health Perspectives. It is part of a larger project, conducted by Arizona State University and NCAR in collaboration with Houston Health Department, looking at the impacts of indoor and outdoor exposure to ozone and extreme heat in a warming climate and the health risks for an aging population.

“This work represents a fundamental shift in how we think about modeling the health implications of extreme heat,” said David Sailor of Arizona State University, the project’s principal investigator. “The old approach of using fixed-location weather stations as a proxy for total heat exposure is outdated. By integrating information about indoor and outdoor exposure, we are better able to assess heat-related health risks and propose mitigative actions.”

Alternative approaches to air conditioning

Heat is dangerous to human health because it can exacerbate cardiovascular and respiratory disease, as well as lead to dehydration and heat stroke. The elderly, very young, and those with chronic health conditions are especially vulnerable.

Although air conditioning can make a substantial difference, many residents cannot afford to keep their homes sufficiently cooled. Even those with air conditioning may not be able to pay utility bills or the cost of maintaining or repairing a faulty unit. In addition, power outages can knock cooling systems offline. 

Instead of relying solely on air conditioning, society may be able to adapt construction and design approaches to prevent indoor spaces from getting too hot, O’Lenick said. These can include changes to insulation, windows, and roofs, as well as ensuring that homes are well shaded.

“We're going to have warmer nights and that's going to put more pressure on infrastructure,” O’Lenick said. “If air conditioning is the only safeguard, a lot of people will be at risk. There are definitely opportunities for innovative design and construction so we're not completely dependent on air conditioning.”

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