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A sweltering 112 degrees Fahrenheit in North Texas. 97 in Minneapolis. 103 in Nebraska. A record 104 in London. Temperatures across the United States and other parts of the world are soaring in the kind of extreme heat waves that are expected to become more common with climate change.

Officials are warning the public to do whatever they can to stay cool. But the dangers from extreme temperatures go beyond dehydration, heat exhaustion or heat stroke. Extreme heat holds special risk for people with chronic diseases — an enormous group that has only been made larger by Covid-19.


Nearly half of American adults live with chronic disease, and rates are rising, just as intense, climate change-related shocks — droughts, floods, hurricanes, wildfires, and polar vortexes — are becoming more frequent and dangerous.

Extreme heat, as defined by many researchers, is a temperature of 90 degrees Fahrenheit or higher over the span of two or more days. But the threshold — what extreme heat means to different people living in different places — varies wildly. The health impacts vary even more. Ninety degree days in New Orleans are apples to the oranges of 90-degree days in Portland, Me. The temperature at which heat-related hospitalizations peak can be vastly different, even in states that share a border.

From 1998 to 2017, the World Health Organization estimates 166,000 people died from heat waves globally, and that is likely an undercount. The problem is getting worse, too. Exposure to extreme heat has tripled in the last few decades, and now afflicts nearly a quarter of people on earth, an analysis by the Associated Press found. Heat kills more people each year, an estimated 1,300 Americans, than any other weather-related event. By 2050, that number could be closer to 60,000 deaths each year.


When it doesn’t kill, heat harms, pushing more people into emergency rooms for all kinds of reasons, not just heat stress or heat stroke. Hot days worsen mental health, and can increase the odds of being injured at work, or having a heart attack or an infection. Heat can set off flare-ups in a vast array of chronic health conditions, from migraine and rosacea, to lupus, rheumatoid arthritis, chronic obstructive pulmonary disease, asthma, chronic kidney disease, hypertension, type 2 diabetes and cardiovascular disease.

Even a healthy heart is strained by hot days. Everything takes more work. Remove some of the heart’s power or steadiness or architecture and heat is an even bigger hurdle. The lungs are perhaps among the most afflicted by heat, which traps smog and other pollutants at ground level. All kinds of respiratory conditions, from asthma to COPD and pneumonia, are made worse by the lower-quality air.

Sweating, one of the body’s best tools for regulating body temperature, can lead to dehydration, a major culprit in heat-related illnesses. A disruption to the balance of water and electrolytes can trigger headaches, cause elevated blood glucose levels, and reduce kidney function and blood pressure.

Research is finding that exposure to heat over time, or even a few months, can cause long-term damage to the human body, and maybe even increase the likelihood of developing some chronic conditions, such as kidney disease or respiratory diseases. Already, one in four adults in the U.S. has at least two chronic conditions. More than two-thirds of all deaths in this country are from chronic diseases. Over the last 20 years, heat-related deaths among people 65 and older have increased by 50 percent.

Many people don’t realize how vulnerable they truly are, researchers say. Part of that falls to the messengers — clinicians, public health officials, weather services and even local governments, who for decades have not clearly communicated the harms associated with heat, or proffered solutions.

People of all ages and most geographies will be stressed by heat. Children wind up in the ER much more often on hot days in the warm season than on moderate and cool days, largely due to infections, injuries and neurological concerns. Kids with chronic health conditions, and child athletes are especially at risk during heat waves. “We’re warm blooded. Our whole body is designed to operate within a narrow range of temperatures,” said Aaron Bernstein, interim director of The Center for Climate, Health, and the Global Environment at Harvard T.H. Chan School of Public Health.

And risks often vary greatly by intersecting vulnerabilities.

A white, middle-aged American living in New England might be more susceptible during a heat wave than a white, middle-aged American in the South. But what if the Southerner is Hispanic, and lives in a low-income neighborhood with heavy air pollution and few trees? What if he’s a construction worker, and the New Englander has an office job?

How about if the Northeastern office worker has multiple sclerosis, an autoimmune condition affecting the central nervous system, worsened by overheating? What if both of them have high blood pressure, and have been prescribed beta blockers, which can make people more sensitive to heat? Every factor can alter the risk profile.

The Covid pandemic vividly illustrated how longstanding inequities widen into chasms during crises.

“When we allow disparities to fester in our country and around the world, these are the fissures that things like pandemic sprout from. They exploit these gaps,” said Bernstein. “With climate shocks, we see the same thing.”

Heat Wave Death Valley
Death Valley in southeastern California John Locher/AP

In the spring of 2021, researchers at the University of North Texas began asking people about the effects of heat on their health, especially those with chronic diseases such as heart diseases, diabetes, asthma and long Covid.

They’ve found a series of inextricable links between environmental issues and health. In Dallas-Ft. Worth, home to 7.5 million people, neighborhoods that experienced the highest Covid death rates were working class, and communities of color, researcher Courtney Cecale told STAT in an email.

“Those same communities are also disproportionately burdened with environmental health hazards, including both high levels of contamination (from racist zoning policies) as well as heat island effects,” she said. Heat island effects occur when developed areas, usually in cities, are hotter than surrounding areas because of an abundance of roads and buildings that absorb and regurgitate heat, and make it linger long after sunset. Formerly redlined communities tend to suffer most from lack of heat-abating tree cover and green space.

Those with long Covid, an umbrella term for a multitude of symptoms that linger months to years after infection, have reported the negative effects of heat to Cecale’s team. Some people who had severe Covid said they developed chronic problems, such as heart disease, lung damage or the tricky pain condition fibromyalgia — all of which can make them more vulnerable to heat. “Some people have shared how they have had to radically alter their lives to avoid the heat because it causes flare-ups, pain, or danger to them,” said Cecale, an assistant professor of anthropology.

Others told the Texas researchers they worry about taking or storing heat-sensitive medications. Some common drugs, including certain ACE inhibitors, antidepressants, antihistamines and antipsychotics, can increase the risk of heat stroke by interfering with the body’s ability to regulate its temperature. Many medications are meant to be stored at cool temperatures, away from moisture and heat, which can pose problems for people who don’t have air conditioning.

Long Covid is still an emerging illness, puzzling in its many manifestations and urgent in its prevalence. Some estimates suggest up to 30% of people infected with Covid develop long-lasting symptoms — a mass disabling event for millions of Americans, and millions more around the world.

“The pandemic has worsened climate vulnerability for millions of people in ways that we don’t yet grasp the extent of yet,” Cecale said.

Climate shocks, and especially heat, hurt people’s mental health, too.

On extreme heat days, ER visits for mental health diagnoses increase, and specifically for people with substance use disorders, anxiety and stress disorders, mood disorders, schizophrenia, self-harm, and many others, a recent JAMA Psychiatry paper reported. Other studies have made similar findings.

One way heat disturbs mental health is by interrupting sleep, researchers theorize. But there could be other, unexplained biological and social reasons.

To avoid heat stress and shock, people with A/C at home can access air conditioning at private businesses like malls and movie theaters, at libraries, or at government-run cooling centers. But, “just because the cooling center is there, we don’t necessarily know that people are using it,” or that the most vulnerable people are accessing it, said Amruta Nori-Sarma, an assistant professor at Boston University, and lead author of the JAMA paper. Some high-risk individuals, people with limited mobility, those who are immunocompromised or who live in rural settings, might not be able to go to cooling centers. Others might not want to stay at facilities with strangers. Those concerns are amplified during Covid surges, like those happening across the country in recent weeks.

Social networks are very valuable in this context, Nori-Sarma said. Checking in on family, friends and neighbors, especially those over 65 years old and those with chronic health conditions, and sharing resources (drinking water, electrolytes) can be a mighty protective force when communities face extreme heat, she said.

Northwest Heat Wave
A man reads an informational handout about heat stroke provided by the Cascadia Behavioral Healthcare’s street outreach team in Portland, Ore. Nathan Howard/AP

Mutual aid groups organized during the pandemic have also retooled efforts to combat heat. Across the U.S., volunteers have built benches, shade structures and misting stations, and distributed drinking water, fans and A/C units.

State and local governments in places like rural western Arizona use police or other employees to check on high-risk people during extreme heat. This part of the country has some of the highest heat-related illness and mortality rates, an analysis by The Arizona Republic and Columbia Journalism Investigations found.

Heat illness and death often strike unhoused populations, but also burden those with low incomes, stranded in places without access to basic services or air conditioning, or unable to afford high energy bills. Deaths from heat are expected to increase especially in places like Arizona, Southern California and Southwest Texas — all regions where Hispanic workers and other minorities are disproportionately affected.

But extreme heat isn’t just a problem for the American South. Missouri, in the very heart of the country, has historically been home to a higher percentage of adults with chronic diseases than the U.S. — and it’s trending upward.

In Kansas City, where officials are on the brink of adopting a detailed Climate Protection and Resiliency Plan, there is up to a 16-year life expectancy gap between majority-white and majority-Black neighborhoods, a marker of vulnerability. Impoverished areas have acres and acres without tree canopy, making those neighborhoods hotter and harder to live in.

In Florida, the state with the highest average chronic disease prevalence among Medicare patients, Miami appointed a Chief Heat Officer. Low-wage hospitality, tourism and service workers in the state are among the most likely to suffer under oppressive heat in coming decades.

Some, like agricultural workers, already endure sweltering temperatures. And working under persistent heat, coupled with dehydration and exposure to pesticides, has been shown in some studies to lead to kidney injury and an increased risk of developing chronic kidney disease and kidney failure. Experts suggest employers alter work schedules to avoid peak heat hours, and provide drinking water and shade to workers.

While the National Weather Service uses historical and regional data to identify aberrations in temperature, those reports don’t take into account how the most susceptible are harmed at lower temperatures than might merit a weather alert. The heat index considers temperature and humidity to figure out how hot it is in areas with shade. WetBulb Globe Temperature, on the other hand, uses temperature, humidity, wind speed, sun angle and cloud cover to better calculate heat stress when a person is in direct sunlight. It’s becoming a more well-known metric. The Arsht-Rock Resilience Center’s Extreme Heat initiative aims to get cities around the world to go a step further: to name heat waves like hurricanes, and stratify people by risk. Seville, Spain, and Athens, Greece, are piloting programs this summer, and several U.S. cities, including Los Angeles, are planning to do the same.

“We need to move toward a more refined approach to protecting people,” said Bernstein, a pediatrician at Boston Children’s Hospital, who is part of the Arsht-Rock project.

There is a significant fiscal impact, too. Dollars spent on chronic disease already make up almost 75 percent of aggregate health care spending, and a massive share of Medicare and Medicaid spending.

As the population of the U.S. ages and more people develop chronic conditions, productivity will dip, and treatment costs will strain the health care system even more, a one-two-punch costing the economy more than $1 trillion each year, a 2018 analysis by the Milken Institute found. And a stressed economy means basic necessities — everything from healthy foods, to heating and cooling, and health care — are out of reach for more people.

“Climate shocks are not going away, which also makes addressing all these other health challenges more difficult,” Bernstein said.

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