When it comes to heat, the human body is remarkably resilient — it’s the humidity that makes it harder to cool down. And humidity, driven in part by climate change, is increasing.
A measurement of the combination of heat and humidity is called a “wet-bulb temperature,” which is determined by wrapping a completely wet wick around the bulb of a thermometer. Scientists are using this metric to figure out which regions of the world may become too dangerous for humans.
A term we rarely hear about, the wet-bulb temperature reflects not only heat, but also how much water is in the air. The higher that number is, the harder it is for sweat to evaporate and for bodies to cool down.
At a certain threshold of heat and humidity, “it’s no longer possible to be able to sweat fast enough to prevent overheating,” said Radley Horton, a professor at Columbia University’s Lamont-Doherty Earth Observatory.
Scientists have found that Mexico and Central America, the Persian Gulf, India, Pakistan and Southeast Asia are all careening toward this threshold before the end of the century.
“Humid heat risks are grossly underestimated today and will increase dramatically in the future,” Horton said. “As locations around the world experience previously rare or unprecedented extremes with increasing frequency, we run the risk that our previous messaging about extreme heat risk — already woefully inadequate — will fall further short of the mark.”
You might think that being closer to the beach would be a great way to catch that ocean breeze and cool off. But Horton said proximity to water in extreme conditions could make things worse. As warming temperatures cause the water to evaporate, it adds humidity to the air.
“If you’re sitting in a city along the Persian Gulf, the sea breeze could be a deadly breeze,” he said.
To better understand why these places are becoming too hot and humid for humans to endure, you have to first understand how the body cools itself.
As the sun heats up the air, the ground, objects and people, the human body will react in an effort to cool itself.
The skin sweats. Evaporation of this water cools the body — as long as the surrounding humidity levels allow the evaporation to take place.
If the hot air is too humid, that heat exchange is blocked and the body loses its primary means of cooling itself.
The wet-bulb temperature that marks the upper limit of what the human body can handle is 95 degrees Fahrenheit (35 Celsius). But any temperatures above 86 degrees Fahrenheit (30 Celsius) can be dangerous and deadly. Horton and other scientists noted in a 2020 paper that these temperatures are occurring with increasing frequency in parts of the world. To put things in perspective, the highest wet-bulb temperature ever recorded in the Washington region, known for its muggy, unbearable summers, was 87.2 degrees (30.7 Celsius).
“Extreme humid heat overall has more than doubled in frequency since 1979,” the study’s authors wrote.
These conditions are reaching that deadly threshold in places like South Asia and the Middle East and could regularly cross it by 2075, scientists say.
Horton and his colleagues found that parts of the United Arab Emirates and Pakistan have each passed the 95 degree mark for one or two hours more than three times since 1987.
On the coast of the Gulf of California, in the Mexican state of Sonora, scientists are also seeing a “very significant” increase in wet-bulb and air temperatures, said Tereza Cavazos, a senior researcher in the department of physical oceanography at the Ensenada Center for Scientific Research and Higher Education.
During the summer, parts of the gulf can reach temperatures of 86 to 87.8 degrees Fahrenheit (30 to 31 degrees Celsius), which causes the water to evaporate more quickly. The combination of warmer waters and increasing heat trends in Sonora are causing the wet-bulb temperatures to reach dangerous levels.
“Just increasing 1 or 2 degrees Celsius can be the tipping point for changing the impact,” Cavazos said.
The blistering heat is resulting in difficult living conditions, especially for communities that lack resources to provide relief.
Why some will survive while others die
Even below these thresholds, cooling down is hard work on the body. The efforts to fight the effects of heat puts pressure on your heart and kidneys. With extreme heat, people’s organs can start to fail. If you have preexisting conditions, it’s even more likely.
As your body works to cool down, the heart works harder in an effort to pump blood up just below the surface of the skin, where it can get cooler.
The kidneys work harder to conserve your body’s water.
When your body temperature gets too high, it will ultimately cause your body’s proteins to break down, its enzymes to stop regulating your organs’ functions and your organs to start shutting down.
This is a heat stroke: Your body essentially cooks to the point where you have multi-organ failure.
In heat waves, many deaths are due to health problems exacerbated by the extreme conditions.
“It’s very clear during a heat wave, more people do die of heat stroke,” said Zachary Schlader, an associate professor at Indiana University Bloomington who focuses on thermal stress and the human body. But even more die of heart-related conditions. “The body responds [to heat] in such a way it could make the organ vulnerable.”
During heat waves there are some simple ways to take care of your body.
If you have air conditioning, the solution is simple: Go inside.
If you don’t have those resources, hydrate. Drinking water can ease the load on the heart, kidneys and other organs.
Take a break: Even moderate physical exertion such as walking greatly increases the heat your body’s muscles will generate.
Protecting yourself from such stress is inextricably tied to socioeconomic status and resources.
“The poorest people are the most vulnerable, and they are already suffering,” Cavazos said, noting that Sonora depends on farming, meaning a lot of people have to engage in physical labor in the dangerous heat.
In regions like the Persian Gulf, extreme heat is the new normal: Qatar has adapted so extensively to the blistering climate that it air-conditions the outdoors. But not everyone has access to outdoor air conditioning, including those building the facilities that have them. When the wealthy country began construction on venues to host the 2022 World Cup, it faced an uproar over its treatment of workers building the stadiums.
In 2019, the United Nations warned that during the four hottest months of the year, outdoor laborers in Qatar were working under “significant occupational heat stress conditions.”
Qatar in May imposed regulations expanding the time frame that prohibits outdoor labor from taking place to between 10 a.m. and 3:30 p.m. during the hotter months of the year, while also outlawing any work if the wet-bulb temperature is more than approximately 89 degrees Fahrenheit.
Merely surviving in those conditions depends on your place in society and what that affords: access to air conditioning, insulated homes, jobs that don’t require extreme physical exertion under the sun and policies in place to protect you from dangerous conditions.
“As humans, we have learned to adapt,” Cavazos said. “The problem is the cost. Some will not survive.”