It begins when you stop sweating. Perspiration usually cools your body down by releasing heat into the air as it evaporates; given enough sweating, your body runs out of water to push through your pores. You flush all over as blood moves toward your skin—your body’s attempt at shuttling warmth away from your core. Organs cramp up as they’re deprived of oxygen. Your thinking gets fuzzy. You might start hallucinating. You vomit so your stomach can stop wasting energy on digestion. Your heart pounds and your head aches. You may begin to have seizures.
When death finally comes mere hours later, it’s in the form of a heart attack or a stroke. Your internal temperature may spike above 105 degrees Fahrenheit, but by the time you’re found, your body will have gone cold. And no one will know that the true cause of death was heat.
The human physique is capable of many feats, but when it gets too hot, it falls apart. “We have to maintain a very specific range of body temperatures,” says Shane Campbell-Staton, a UCLA evolutionary biologist who studies the impact of heat on humans and other animals. Most of us are comfortable when the air around us hovers at about 70 degrees Fahrenheit, which allows our bodies to maintain an internal thermostat of around 98 degrees. When the environment pushes us past those limits, the delicate balance of chemical reactions that keep us alive starts to wobble, leading to cascades of negative effects that can very quickly become fatal.
Exposure to extreme heat could be the culprit behind thousands of deaths in the US each year and many more around the globe, according to those researching the phenomenon, though it’s hard to say how many for certain, given that most of them go unrecorded. But whatever that grim tally is, we know one thing for sure: We can expect more deaths in the future.
Climate models suggest temperatures will rise dramatically across the country by midcentury, exposing a greater percentage of the population to dangerously high heat (according to the National Weather Service that means triple digits, or anything in the 90s with high humidity). We can protect ourselves by changing our lifestyles to suit these climes, but it will take a concerted effort both locally and nationally to keep the swelter from causing mass casualties.
[Related: These beautiful, terrifying maps show how hot we’ll get in 2090]
Some people are more vulnerable than others. The elderly, babies, and folks on certain medications aren’t as able to regulate their internal temperatures; people without homes or access to air conditioning don’t have safe spaces in which to cool down; construction workers and other laborers have no choice but to be outdoors, often during the hottest parts of the day.
But anyone can succumb. The National Weather Service’s heat index identifies that temperatures as low as the 80s come with the risk of heat illness if your exposure is prolonged or you’re engaged in strenuous outdoor activity. The risk is heightened by higher humidity and higher temperatures. It’s important that the whole country understands the threat, says Marium Husain, a doctor at Ohio State University’s cancer center who advocates for more climate-change aware health policy.
That’s especially true as folks across North America take to the great outdoors to enjoy summer activities. A hike or even just a day in the garden can take a bad turn when it’s blistering. “Sometimes, the effects of hyperthermia can be so subtle that an individual doesn’t even realize that they’re moving into a state of physiological stress,” Campbell-Staton says.
Officially, only about 700 people die of exposure to extreme heat per year. Most of those casualties come from vulnerable populations, including the unhoused and elderly. But researchers believe the actual number is much higher, says Scott Greene, a University of Oklahoma geographer who has been researching the subject since the ’90s. Rather than looking at deaths coded as hyperthermia or hyperthermia-involved, Greene and others in his field examine how many people died in a given area during an unusually hot period. They search for what are known as “excess deaths”—in other words, fatalities that spike above the number that would be typical for an area with the same demographics during that time of year. (The rate of excess death was also an early indicator of COVID-19’s deadliness.)
Similar analysis by other researchers suggests that heat is either a direct or indirect cause of thousands of deaths in the US each year—far higher than the official count. The circumstances are right for that number to keep going up. Recent data from the Union of Concerned Scientists predicts that by midcentury, more than one-third of American cities and more than 90 million people will experience 30 days or more of 105 degree Fahrenheit temperatures per year. Those locales are predominantly in the Sunbelt and the Southern Great Plains. Temperatures in the Northeast will crest 90 degrees Fahrenheit far more often than they do now, and the Midwest can expect a spike in 100-plus-degree Fahrenheit forecasts.
These future predictions are frightening, but the crisis is already at our door. Heat is already the leading weather-related killer in the US, ahead of winter storms, hurricanes. and flooding among others.
There’s still time, however, to prevent gruesome heat stroke deaths. When Greene started researching this field in the ’90s, the outlook for the 2000s was dire. But a stretch of fatally hot weather during that period led cities across the country to start planning ahead, and those precautions have already saved thousands of lives. The most important innovation, Greene says, was the widespread adoption of warning systems that make residents aware of extreme temperatures and their health risks. Cooling centers that allowed people to get out of the heat regardless of their socioeconomic status also played an important role.
In places like Phoenix or Las Vegas that now regularly experience extreme heat waves, many people have learned to live with the effects, Greene says. Simply being aware of the dangers can go a long way toward saving lives, he adds. “We’re way below where we thought we would be in terms of the number of deaths in 2020.” In 1997, he and his colleagues suggested that the excess deaths associated with hot days in 44 large cities could be greater than 2,000 every summer by 2020.
But Greene is still concerned about hot spells in unexpected places—ones that take locals by surprise, especially in cities. A phenomenon called the “urban heat island effect” can raise temperatures in the concrete jungle by several degrees compared to the surrounding countryside, meaning that densely packed metropolises can fall into the danger zone even while folks in the suburbs feel just fine.
And even though new monitoring networks and infrastructure has helped, we have lots more work to do. Everywhere in the country, vulnerable people continue to die of heat exposure—and whether it’s 700 or many more, they’re deaths that could be prevented. “The main thing that separates us from the rest of the tree of life is our unique ability to buffer ourselves against extremes,” says Campbell-Staton.
But only some of us have access to the resources that can keep us safe from heat. To keep dropping the number of heat deaths, even as temperatures go up, coordination between cities, states, and even the federal government is needed, Greene says. He wants to see a more robust centralized forecasting effort to predict temperature spikes and a stronger response system to dispatch resources to the area that will be hit. These efforts could help raise the profile of heat as an issue, he says, and save lives while they do. But for now, it’s important to realize just how many people are at risk of dying due to extreme heat—and just how few of them know it.