The Deadly Mystery of Indoor Heat

If the last few weeks are any indication, this summer is going to be a scorcher.

In Spain and Portugal, April temperatures reached record highs. A heat wave swept through Asia, killing dozens on the Indian subcontinent; temperatures in the region hovered around 110 degrees Fahrenheit for days. The United States saw records break throughout the Northeast and Midwest, with temperatures into the 90s.

And that’s just how hot it was outside. Inside is a completely different story — one we know far less about.

Heat is the deadliest extreme weather phenomenon in the United States, and when the outside world is boiling, the advice is often pretty simple: get inside. But the majority of heat-related deaths happen indoors, and, unlike the satellites and weather stations that can measure outdoor temperature, we have very little data on just how hot our homes are getting.

That’s a major blindspot. Without knowing exactly how hot buildings are getting, lawmakers have little, if any, data to rely on when it comes to crafting policies around indoor heat. A WHO report from 2018, which lays out a strong recommendation for a minimum heat threshold of 18 degrees Celsius (about 64 degrees Fahrenheit), simply suggests that, when it comes to heat, “strategies to protect populations from excess indoor heat should be developed and implemented.”

“Humans spend the majority of their time indoors, and we have entire building stocks across our cities where we haven’t taken into account what the weather systems around those buildings are going to look like,” said Vivek Shandas, a professor at Portland State University who studies heat in urban environments and advisor to CAPA Strategies, a climate data consultancy. Regional architecture gave way to cheap steel and concrete around the country, and the result has been residents being put at risk by the very nature of their homes.

A new study from the city of Portland, Oregon, one of the first of its kind, goes a little way towards closing the indoor temperature data gap. In the wake of an intense, deadly heat wave that killed 123 Oregonians in June 2021 — locals called it a heat dome, for the hot air mass that parked itself over the region for days — the Portland Bureau of Emergency Management (PBEM) commissioned CAPA Strategies to find out just how hot the homes of the city’s residents were getting. In particular, they looked at three properties managed by Home Forward, the city’s housing authority, which had each seen resident deaths from heat-related illnesses.

The setup was simple: Residents volunteered to have temperature sensors placed in their units — usually away from an air conditioner, if they had one. The sensors then monitored indoor temperatures over the summer of 2022, which while not quite as hot as 2021’s heat dome, still brought intense heat to the region. If indoor temperatures got above 80, 85, or 90 degrees Fahrenheit, residents got an alert that would, ideally, nudge them into taking action to protect themselves from heatstroke.

And the apartments did get hot, though not quite as hot as the outside world: Interior temperatures maxed out in the low to mid-90s on 100-degree days, and every apartment in the study tipped over 80 degrees on multiple days. Units in two of the residences, which were built with concrete, stayed hot for longer even as nighttime temperatures fell outside. (Units in the third residence, which was built out of wood, were far better at cooling down.)

That kind of heat is striking: Prolonged exposure to temperatures that high can be dangerously hot, especially for elderly people or anyone with a medical condition that makes them susceptible to heat, though none of the residents who participated in the study suffered any serious medical impacts.

To get an idea of how that indoor heat affected residents in less life-threatening ways, the researchers also periodically sat down with them to conduct surveys and workshops. They found that residents experienced some sort of heat stress — difficulty sleeping, headaches, or even just heightened irritability — throughout the summer, not just during heat waves.

“It was disheartening to see how much heat stress many building residents are putting up with all the time,” said Jonna Papaefthimiou, who was the city’s chief resiliency officer at the time of the study and recently left for the same role at the state level. The residents of the Home Forward buildings dealt with particular obstacles that might not have been present in other houses, like a lack of mesh screens that discouraged residents from opening their windows at night for fear of intruders, whether insect or human. “There were a lot of barriers for people to just do basic things to cool off,” Papaefthimiou told me.

But they also tried to take care of each other, she said. Many of the residents signed up for the study out of a desire to help their neighbors and better understand heat risks in their building, including a person whose apartment had previously been the home of one of the victims of the 2021 heat dome. Mutual aid is a simple, if underappreciated, climate-adaptation practice, and this kind of community involvement can save lives: Over the course of the study, the researchers found that residents were eager to learn how to check in on and help each other during heat waves.

While there’s certainly a lot of work that governments need to do to help their citizens deal with extreme heat, Papaefthimiou thinks this desire to help is an encouraging sign. “Neighbors helping each other does not represent a failure of government to me. It actually means that something's going well in the community as a whole,” she told me.

For the most part, cities across the country have dealt with heat by letting developers and residents throw air conditioning at the problem. It’s an effective, if blunt, tool — the best one we have in a heat wave, really — but it’s by no means perfect. Air conditioners are energy-hungry, which makes them expensive to run, often out of reach for lower-income residents, and vulnerable to black outs when everyone turns them on. They also struggle to cool buildings on particularly hot days. That’s especially true if they’re, say, window AC units in buildings that were never designed with cooling in mind, as is the case with many cities in the northeast.

Most of the buildings in Portland were built for a different climate than the one that exists today and will need to be retrofitted to adapt for a changing climate, Papaefthimiou told me. This is true of cities across the country, and each one will be forced to reckon with an associated host of questions as a result, from what the best approach to retrofitting is (passive cooling might be a better investment than air conditioning in some instances, for example) to whether that process will end up pricing people out of the places they live in now.

The Portland indoor heat report includes a number of recommendations for what the city’s government can do to help its citizens, from the short-term (distributing things like thermal curtains and magnetic window screens) to the medium- and long-term (retrofitting buildings with central AC or providing professional insulation services). But the study is limited — only 53 residential units participated over three months — and researchers at CAPA are hoping to secure funding from Multnomah County, which was one of the partners of this year’s report, to conduct a second study later this year.

More study is needed either way, and not just in Portland: The more information we have about how extreme heat affects people who are trying to shelter from it, the better prepared we are to make policies that can mitigate it. Some activists, for example, are calling for cities to institute summer maximum heat thresholds similar to how many northeastern cities mandate minimum temperatures in the winter — something that the Arizona cities of Phoenix and Tempe have already implemented. But every city, and even every building in every city, is different, and data collection will be key to moving from a one-size-fits-all policy of air conditioning to more targeted, productive solutions that take into account the way people interact with the buildings they live in.

“I tend to think that often what we're doing is throwing lots of money at things that we intuitively believe will work,” Shandas told me. “But what we think works may not always be the thing that works well. People inhabit spaces in very different ways, and I think we need to get a better handle on designing for their behaviors instead of throwing a bunch of money at our assumptions.”