Editor’s Note: As the world experiences record-breaking temperatures and worsening climate challenges, it’s easy to feel overwhelmed. Discover how to align your personal interests with effective steps and learn how to get involved at Climate Week NYC September 22-29.
Aug. 19, 2024 – As summer winds down, 2024 has burned through the record books.
For now, July 22 of this year stands as the hottest day ever recorded on Earth, according to a NASA analysis.
But from the earliest days of summer, tens of millions of people across the United States have been under heat advisories as heat domes have expanded and contracted from coast to coast over sweltering weeks. Globally, 130 monthly national temperature records have been shattered.
My conversations with colleagues, patients, and neighbors living in Atlanta inevitably turn to the heat. Most share that the summer months have become a season of dread.
As temperatures rose relentlessly, my 81-year-old neighbor told me he found himself tossing and turning through the night, unable to escape the oppressive heat that enveloped him in his modest home. Despite the fan whirring at his bedside, the air felt thick and warm, leaving him drenched in sweat and often gasping for relief. That is why he finally installed an air conditioning window unit in the room where he spends most of his time.
"I can't remember it ever being this bad," he told me. "Even when I was younger, the nights were cooler. We could open the windows and feel a breeze. Now, it’s like it just doesn’t stop."
My neighbor’s struggles with the heat are not just about comfort — they are about health. His age and medical conditions – high blood pressure and diabetes – make him more vulnerable to the dangers of persistently high temperatures in a hotter world.
The Health Dangers of Persistent Heat
Though you cannot see the risks of a heat wave the way you can with other climate change-fueled disasters like wildfires or hurricanes, intense heat is dangerous and, in some cases, deadly.
“Heat has always been the number one killer among extreme weather events historically,” says John Balbus, MD, MPH, director of the U.S. Office of Climate Change and Health Equity.
But, he said, the dangers of heat are rapidly rising – driven by record temperatures and heat waves of record length.
“Things are different, and we have to respond,” Balbus said. “We're seeing this huge uptick, where last year, Maricopa County (Arizona) had 645 deaths, just in one county, where the national average used to be 700.”
Most of us know that heat waves can lead to illnesses ranging from heat cramps to heat exhaustion, and that heatstroke can have devastating consequences. Heat-related illnesses include much more than these conditions. Heart and lung disease complications, preterm birth and pregnancy complications, kidney stones, and electrolyte problems are all part of a growing list of heat hazards.
In fact, death from a heart attack, and other types of heart conditions, or lung disease are much more common just after a heat wave than death from heatstroke itself. Heat-related cardiovascular deaths may triple in the next few decades if we continue to heat the Earth’s atmosphere at our current pace.
That is because heat stress is not just a consequence of overheating; it is a result of “over-revving.” Our bodies go into overdrive to keep us at our “Goldilocks”, or ideal, temperature. This puts a strain on organs throughout our body – our blood vessels, heart, kidneys, and brain. The longer we stay in this mode, the higher our risk of getting into dangerous territory.
How Our Bodies Respond to Heat
When our bodies heat up, blood vessels close to our skin widen to shunt more blood to the surface. To force large amounts of blood toward the skin, our hearts have to pump faster and harder, and less blood flows to our inner organs. The heat from our core dissipates into the air, as long as the air around us is cooler than our body’s normal temperature of around 98 F (37 C).
When air temperatures reach 95 F (35 C) or warmer, it becomes much harder to release body heat into the air. This is where our sweat response becomes critical. When sweat evaporates, our skin cools down.
High humidity puts a major dent in the cooling power of our sweat. Humidity saturates the air with moisture – so less sweat evaporates into the air.
This is why high heat combined with high humidity situations are so dangerous for us. Even if we sweat a lot – instead of evaporating, the sweat pools and drips off our bodies, dehydrating us without the benefit of any cooling. In a vicious cycle, dehydration puts an added strain on our heart and blood vessels, and impairs other organs like our kidneys and brain.
The heat index – a measure of relative humidity and air temperature – is one way to track how “hot it feels” beyond air temperature. At a heat index of 90 F or higher, the risk of heat exhaustion or heatstroke shoots up, especially for people who are outside for long periods of time or doing any type of physical activity.
S. Tony Wolf, PhD, a physiology researcher and assistant professor in the Department of Kinesiology at the University of Georgia, warns that heat and humidity alerts are not one-size-fits-all for heat risk. Heat strain risk varies based on age, health status, and activity level.
Wolf points to research by his team that shows older adults are more likely to have heat stress at lower temperatures and humidity levels than younger people. “If they go from resting, just sitting in a chair hanging out, to doing just very minimal physical activity, it reduces those combinations of temperature and humidity even further,” he said.
While anyone can experience heat strain, some people are more vulnerable, including pregnant women, people with chronic conditions, infants, young children, and older adults. Outdoor athletes, outdoor workers, and unhoused people are also at higher risk.
Hot New World
Unlike heat waves of the past, modern heat events have become more frequent, last longer, and are often more intense – with higher humidity and relentless heat lasting deep into the night in many parts of the world.
“What we've all observed over the last 2 years are increasing numbers of unprecedented heat waves,” says Kristie Ebi, PhD, MPH, a professor of global health at the University of Washington.
These are not the heat waves of our grandparents’ or parents’ generation.
“The temperatures are different,” Ebi said.
According to projections, temperatures are going to continue to rise and heat waves will continue to last longer and get more frequent and intense for at least the next 2 to 3 decades.
“And then after mid-century, it depends on what happens with our greenhouse gas emissions,” Ebi said.
Humid Days and Tropical Nights
Not only do hot and humid days wreak havoc on our health, warming temperatures at night also increase health risks.
As global temperatures skyrocket upward, nights are no longer the relief they once were. Historically, hot days were broken up by cooler nighttime temperatures – offering a brief respite. But recent trends indicate a worrying departure from this pattern. Since records began in 1895, nighttime lows during summers in the U.S. have warmed almost twice as fast as daytime highs.
"Tropical nights," where nighttime temperatures stay above 68 F (20 C), are growing more frequent. This is particularly concerning in parts of the country that are not used to these types of conditions. According to Climate Central, occurrences of hot summer days bleeding into the night have increased in 202 U.S. cities, or 84% of the 241 cities analyzed by Climate Central researchers since the 1970s.
Experts say there are more rapidly warming nights because of global warming and urbanization. The increase in greenhouse gases traps more heat in the atmosphere, while urban areas — often referred to as "heat islands" — amplify the effects. Concrete, asphalt, and a lack of trees and other vegetation mean cities absorb and retain heat, blanketing the city and causing temperatures to remain high even after sunset.
When long stretches of heat extend beyond daylight hours, there is less time for our body to recuperate overnight. Persistent exposure to heat keeps our bodies functioning in overdrive without enough time to rest, recover, and reset, especially for those living without reliable air conditioning. Heat also reduces the quality of our sleep, which also can harm our health.
“You're starting each day from a higher baseline, and then it takes less of an exposure to push people into heat strain,” Ebi said.
The new era of heat presents alarming health challenges, as intensifying heat extremes topple our body’s ability to cope.
Health professionals are already seeing the consequences in the health of their patients.
Renee Salas, MD, MPH, a climate change and health expert and emergency room doctor at Harvard’s Massachusetts General Hospital, fears that people may be lulled into a false sense of security around heat because they have tolerated high temperatures in the past.
She shared the story of a patient who ended up in a Boston ER with heat exhaustion.
“He was a healthy, fit, middle-aged male who went running – like he always does – but did it during a severe heat advisory day,” Salas said. Though he had run on hot days before, this was a particularly hot and humid day. Though his detour to the ER was a surprise to him, health professionals across the country are seeing the impacts on more and more patients who may not see themselves as vulnerable.
“The impact of heat is insidious,” Salas said. And, she said, we are only at the tip of the melting iceberg in our understanding of how this new era of heat will impact our health and the health care system.