The disaster no major U.S. city is prepared for
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In the early hours of July 8, Tropical Storm Beryl headed for the city of Houston after causing widespread devastation in the Caribbean.
Winds up to 90 miles per hour flooded the city’s highways and knocked trees and debris into power lines.
Within eight hours, two thirds of homes in Houston and its suburbs were without power. Some of them would remain in the dark for more than a week.
Then came the heat.
For days, residents of Houston struggled to survive as temperatures rose. They shared generators, filled buckets and bathtubs with ice, packed air-conditioned hotels and emergency rooms. The most vulnerable struggled to get the care they needed. Many died.
But in some ways, Houston was narrowly spared. Temperatures rose to the high 90s, but only for a couple of days. If the heat had stayed, the human toll could have been far worse.
Experts warn this type of catastrophe — a combined power outage with a heat wave — is a scenario that cities and states are unprepared for.
“I don’t think it’s likely — I think it’s an absolute certainty,” said Brian Stone, a professor and director of the Urban Climate Lab at the Georgia Institute of Technology. “I think it’s an absolute certainty that we will have an extreme heat wave and an extended blackout in the United States.”
The Washington Post analyzed the risks of a prolonged, citywide blackout coinciding with a more severe heat wave. The results show that such a heat wave could kill between 600 and 1,500 people in the Houston metro area over five days. With the power grid working normally, the same heat wave would lead to around 50 deaths.
To estimate the number of deaths, The Post created a statistical model that follows a peer-reviewed study from 2023 with some simplifying assumptions. The analysis incorporates detailed models of how indoor heat exposure would rise in a blackout, developed by Amir Baniassadi, an expert on environmental health and indoor heat at Harvard Medical School.
The Post verified key assumptions of the model with three independent environmental health researchers.
Houston is particularly vulnerable. In the Southeast United States, all the ingredients for a monster heat wave and outage are already in place: an aging electricity grid, damaging hurricanes, and temperatures that break records every year. According to data from Whisker Labs, a company that uses sensors to monitor the electrical grid for fire risks, Houston residents are 2.7 times more likely to experience an outage than the average American.
As the climate changes, strong hurricanes are forming earlier and earlier in the year, and increasingly overlapping with the hottest months of the summer. Those summer months are, themselves, getting hotter — Houston already has around 50 days of dangerous heat every year. And while the immediate aftermath of a hurricane may be slightly cooler air, thanks to wind and rain, tropical cyclones are often followed by a band of hot, dry air that raises temperatures just when residents are most vulnerable.
“It’s a recipe for a pretty bad situation,” said Matt Lanza, a Houston-based meteorologist.
The Post spoke to dozens of residents and heat wave and electricity grid experts to understand what a combined power outage and heat wave looked like after Hurricane Beryl — and how it could strike the city again.
When a power outage hits a city, some services immediately disappear. After Hurricane Beryl, stoplights ceased working; some gas stations, which provided critical services to anyone running a generator, lost power themselves and closed. Cell towers not equipped with their own backup power went down, severing thousands of people from communication with the outside world.
At the same time, experts say, the population instantly becomes divided: Those with power — or the money and means to get it — and those without.
Eric Klinenberg, a sociologist at New York University who wrote a book about the 1995 Chicago heat wave, says that for a brief time there will be a flurry of activity. People will socialize in the streets and then hunt for supplies: nonperishable groceries, gas to run their cars, ice to cool off.
Then the calls to 911 will start. After Beryl, early calls for help came from the users of medical devices, like ventilators, oxygen concentrators and CPAP machines for sleep apnea. There are 2.5 million Americans who rely on some kind of powered electricity device; their options in a power outage are to stay put and suffer the consequences, or try to get to a hospital or clinic.
Text messages sent by residents hit by Beryl to The Metropolitan Organization, a group of churches and nonprofit organizations, and shared with The Washington Post, show the toll for these residents. Many wrote in about medications spoiled in the heat, medical devices unpowered, elderly family with nowhere to go.
“No food, no water, no lights”
“My father-in-law is 70 years old and needs electricity for his oxygen machine”
“I sleep with a CPAP machine. I have not been able to sleep for 5 days”
“My biggest concern is my mother. She has a defibrillator that needs to be monitored daily”
Messages from Houston residents during the July blackout
Most local medical clinics aren’t equipped with generators, sending patients with medical devices flooding into ERs. The system will slow to a crawl, with dialysis patients clustered in hospital auditoriums and hallways.
“The hospital system in the region is gridlocked,” Owais Durrani, an emergency medicine physician in Houston, said almost a week after Beryl struck Houston. “We have no beds. Patients are stuck in the ER.”
Hospitals, which have digitized most of their records, also suffer from another problem: their internet connection going down. And with it, the ability to access most patient paperwork, medical records, X-rays and other scans. After Beryl, hospitals kept running by generators were slowed by a lack of internet. Without those functioning systems, doctors were transported back to paper charts and whiteboards.
“The entire medical structure lost internet access,” said Chris Ziebell, an ER doctor in Houston.
That first wave of pressure on emergency rooms is followed by another.
By the third day, the body begins to collapse under the strain of relentless heat. “After about 36 to 48 hours, you really see the human body break down,” Klinenberg said. “Health problems start to accumulate.”
The body can withstand even severe temperatures for a short time. Sweat glands push salty liquid to the surface of the skin, where it evaporates, cooling the body. The heart pumps faster, circulating blood away from the warm center of the body and toward the cooler extremities. Depending on health, fitness and humidity, even eight to nine hours daily in sweltering heat is tolerable — provided one can spend the night in cooler, ideally air-conditioned temperatures.
But in a power outage under severe heat, that respite never comes.
Once the power is out, some homes withstand the heat better than others. With modern insulation and shade from trees, a single-family home could stay at around 80 degrees even if the outside temperature is a scorching 99 degrees.
A similarly sized home without insulation that is fully exposed to the sun will heat up until the temperature is virtually the same as outside.
At night, temperatures in both homes are still in the mid-80s, making sleep difficult. Some people open their windows for relief, some keep them shut for fear of a break-in.
During an extreme heat wave, the outside temperature may be so hot that open windows bring no improvement.
Bodily systems work overtime to try to continue that cooling: the heart pumps, sweat evaporates rapidly. With enough hours of uninterrupted heat, the body can no longer cool itself down. Body temperature rises, eventually causing heatstroke — or death. Calls to 911 for heat-related illnesses will climb.
“You have less runway” with each subsequent day of dangerous heat, said Daniel Vecellio, a professor of geography at the University of Nebraska at Omaha who studies heat stress. “The longer you’re exposed, the more stress it puts on the body.”
The second night of the Houston blackout, 40-year-old Jaymar Wilson spent all night in a chair next to his screened front door with his dog Bandit, trying to get brief gusts of cooling air. Wilson, who has lupus and diabetes, normally does home dialysis every night — he had been without it for two days.
Without treatment, his legs had swollen to double their normal size; he felt nauseous and dizzy. He had been forced to throw out his insulin and was trying to keep his blood sugar from climbing too high. Every few hours, he and Bandit would climb into Wilson’s car and run the engine and air conditioning for a few minutes. Then they would stumble back into the heat.
“My legs started to swell up really bad, it was hot. I couldn’t find any hot food, couldn’t find ice. I had to throw all my insulin away.”
— Jaymar Wilson, 40
Across town, Rosie McCutcheon was trapped. McCutcheon, who at 66 has had five back surgeries, often struggles to stand and get around. Unable to afford or get insurance to pay for a wheelchair, she moves around her house with the help of rolling office chairs.
Her youngest grandchild, an 11-month-old, cried constantly as the heat increased. McCutcheon’s neighbors, who had a generator, tried to run an extension cord to power a large fan for the family — but the generator buckled under the load. On Wednesday, two days before the power came back on, the family ran out of food.
The Post’s modeling shows that, in just a slightly more prolonged heat wave, deaths from an outage like that in Houston could climb dramatically. Some of the most vulnerable, like McCutcheon and Wilson, could be pushed to their breaking point.
“I’m a senior with health issues and this heat is just unbearable. I’ve experienced nausea, vomiting and extreme loss of energy and appetite”
“M. D. has taken me off the cancer medication because I can’t tolerate the medication in this heat”
“Mis dos bebés están sufriendo mucha calor. No pueden dormir bien”“My two babies are suffering from the heat. They can’t sleep well”
Messages from Houston residents during the July blackout
The modeling assumes a 48-hour blackout for the entire Houston metro area, and a loss of power for up to five days for the worst-affected homes. The model shows that the increase in health risks is highest for those accustomed to spending their days in air-conditioned environments.
The estimates include only deaths that are directly related to increased heat exposure. They don’t account for other medical emergencies that could arise from a blackout — elderly people who fall and are unable to call for help, or those with medical devices that become inoperable during the loss of power.
Another study has shown that a power outage combined with a heat wave in Phoenix could kill around 13,000 people and send about half the population to the emergency room. A similar event in Detroit could kill over 200.
“What if I get sick? There’s no way that I can call medical, the paramedics or nothing to get me.”
— Rosie McCutcheon, 66
Many cities in the United States lack heat response plans; those with them often don’t go into detail about how a power outage might affect those plans.
City cooling centers, for example, often aren’t equipped with generators. The day after Beryl, Houston had only two city-run cooling centers open and powered. And these centers have only a modest effect: Scientists have also shown that visiting a cooling center for only a few hours doesn’t significantly reduce heat stress.
Houston public officials have started investigations into the city’s utility, Centerpoint, to find out why it took the utility so long to get the power back on.
Mayor John Whitmire’s senior adviser Mary Benton said in an email that the mayor “inherited many challenges following years of neglect and deferred maintenance” but the city is working to ensure citizens have safe places to go in the event of power outages and heat waves.
During the storm, she added, a regional medical center was established “for those who could not be safely discharged from hospitals to their homes due to a lack of power” and as many as 80 cooling centers “were available to residents as power returned across the city.”
But Houston — as well as other cities throughout the United States — is still vulnerable to another big storm.
Seven days after his power went out, Wilson’s lights finally flickered back on. But the surge of power knocked out his air conditioner. He still would not have cooling for days afterward.
“This hurricane isn’t even the worst we’ve had,” Wilson said. “If you all can’t get the power together for this, what’s going to happen moving forward?”
Osaka reported from Houston.
correction
An earlier version of the map showing Tropical Storm Beryl heading over Houston displayed incorrect times for the storm. The story has been corrected.
About this story
This report was edited by Monica Ulmanu and Juliet Eilperin. Copy editing by Jeremy Lang.
The animation of power outages after Beryl uses data provided by Ting fire prevention sensors, courtesy of Whisker Labs.
Resident text messages were collected by The Metropolitan Organization, a part of the West/Southwest Industrial Areas Foundation, and shared with The Washington Post.
To estimate the number of deaths in a combined heat wave and blackout scenario, The Post adapted and simplified the methodology of Stone et al., 2023.
Because of a lack of publicly accessible measurements of indoor temperatures, the analysis relies on computer models of indoor heat in Houston single-family homes under various assumptions, provided by Amir Baniassadi of Harvard Medical School.
The Post used survey data to estimate the prevalence of air conditioning in Houston and the amount of time that people typically spend indoors or outdoors.
While Stone et al. modeled temperature variations within the cities they examined, The Post did not try to model differences in outdoor temperature within the Houston metro area.
To estimate a range of possible outcomes, The Post ran the analysis for many combinations of assumptions. We report the bottom and top quartile of our results.
Brian Stone and Carina Gronlund offered detailed insights into the methodology of their 2023 paper.
Climate and health experts Drew Shindell (Duke University), Kevin Lanza (UTHealth Houston School of Public Health) and Kristie Ebi (University of Washington), none of whom are affiliated with the authors of the original paper, provided informal feedback on The Post’s methodology.
A computational notebook of The Post’s mortality model can be found here.