This number can measure how dangerous a heat wave is for you

Deadly temperatures have already claimed dozens of lives around the world this year. In India, extreme heat has killed at least 60 people so far. Mexico has seen at least 61 deaths. And summer hasn’t even officially begun. 

Heat has proven to be dangerous in the United States as well. Looking at data from the Centers for Disease Control and Prevention, the Associated Press found that roughly 2,300 people died in part due to excessive heat in 2023, the hottest year on record. Some of the deadliest heat was in the southwestern US, a warning sign that there’s an upper limit to heat tolerance even in a region otherwise accustomed and adapted to hot weather. 

Amid these extraordinary, sweltering late spring temperatures, an old measure of heat risk is getting some renewed attention: the wet-bulb globe temperature (WBGT). It measures temperature, humidity, and sunlight, and it’s shown that it can better warn against the dangers of hot weather than temperature alone. The upper limit wet-bulb temperature for human survival is considered to be 95 degrees Fahrenheit for young, healthy people, but in the recent heat wave in India, the WBGT reached 100 degrees. 

Yet in the US, WBGT hasn’t caught on, despite how much danger the country faces from high temperatures. According to the National Weather Service, heat has been the deadliest weather phenomenon in the US over the past decade. The threat is growing due to climate change: The likelihood and severity of heat waves is increasing. Yet in the US, there’s no official tracking of heat-related deaths nor a federal standard to protect workers from extreme heat.  

Health authorities across states and the federal government are now racing to come up with ways to save lives as the temperatures in 2024 climb to even higher peaks. Last month, the US Department of Health and Human Services launched its Health and Heat Index to help communities plan for heat-related dangers. It uses temperature records and historical emergency room visits to measure the potential heat threats in every ZIP code, with the goal of helping communities prepare for scorching weather. 

But the dangers aren’t spread evenly. Unlike disasters like tornadoes or torrential downpours, which can kill indiscriminately, the specific harms from extreme heat vary from person to person. 

While people in a given region can feel the same warmth, whether that leads to just damp clothes or a trip to the ER depends on someone’s underlying health conditions, age, humidity, how long they’ve been outside, and even how strong a breeze is blowing. 

That’s why the scientists who study the health risks from heat warn that thermometers are not enough. If the goal is to reduce the number of deaths from extreme heat, we need more sophisticated measurements of the weather and a better grasp of our individual vulnerabilities. 

And critically, the people baking under this extreme heat need the education to grasp these hazards and the tools to cope. 

How heat affects the body

To understand the dangers of extreme heat, it helps to know a little about physiology. Most human bodies operate within a narrow temperature band around 98.6 degrees Fahrenheit. 

“All your biochemistry and physiology, the function, is optimized for that temperature,” explained W. Jon Williams, a research physiologist at the National Institute for Occupational Safety and Health. 

Everything from the organ level (how well your heart pumps, how efficiently blood moves oxygen, how well kidneys filter waste) down to the molecular level (how proteins fold, how DNA replicates) works best within these thermal confines. As a consequence, the human body devotes a lot of resources to holding steady at this temperature. In fact, only about 20 percent of the calories from the food we eat fuel the work done by the body like moving muscles. “The rest is given off as heat, a byproduct of energy that’s not used for work,” Williams said. “That’s the source of our body temperature.” But as muscles start to do work, they use up energy, which generates even more heat. 

When ambient temperatures — the kind you read on most thermometers or weather apps — are below the body temperature, any extra warmth can radiate away. The surrounding air can also carry away heat. If these mechanisms don’t cool the body off enough, it starts to produce sweat, which cools the skin as it evaporates. 

“If it’s dry enough, if the humidity is low enough, it’s an extremely efficient way of taking body heat away to the environment,” Williams said. 

However, as humidity increases, the cooling effectiveness of sweating decreases since sweat doesn’t evaporate as readily. If heat and humidity rise in tandem, that increases the chances of the body’s temperature rising too. And if surrounding temperatures are hotter than 98.6 degrees Fahrenheit, the body ends up absorbing heat from the environment. 

Too much heat in the body can quickly lead to complications like heat exhaustion. This presents with symptoms like dizziness, fatigue, muscle cramps, clammy skin, and nausea. If the core body temperature rises past 104 degrees Fahrenheit, it can cause heat stroke, a life-threatening condition where organs shut down. 

The warning signs of heat stroke are confusion, delirium, seizures, or passing out. It’s often accompanied by rapid breathing and a racing pulse. But there are some confusing symptoms as well: A victim’s skin can turn hot to the touch, flushed, and dry, but the skin can also be extremely sweaty and pale. Some people’s blood pressure will tank, while in others, it can surge. Nonetheless, if you suddenly experience these symptoms or notice them in someone else, call 911 immediately. 

The added challenge is that the specific threshold where these complications set in can vary from person to person. 

Older adults and young children tend to be more vulnerable to extreme heat. People with high blood pressure can experience adverse symptoms at lower temperatures than those with normal blood pressure. Medications like diuretics and psychotropic drugs can increase vulnerability to extremely hot weather. And the effects of heat can be cumulative: People who live and work under high temperatures day after day, and increasingly, night after night, face rising risks of heat-related illnesses. 

That’s why a simple temperature reading doesn’t provide enough information to measure when it’s too dangerous to be outside. Truly figuring out when it’s too risky requires accounting for more variables in the environment and in the individual. 

The wet-bulb globe temperature, explained 

The search for a better way to gauge risks from heat isn’t new, and there are some metrics that go beyond just the thermometer. One of the more common measures is the heat index. It accounts for both temperature and relative humidity in a single number to convey how the conditions feel.

But the heat index still doesn’t tell the whole story. José Arturo González Mendoza, a 30-year-old farmworker, died last September harvesting sweet potatoes in North Carolina. On the day of his death, the heat index was 96 degrees Fahrenheit, which is considered to be below the danger threshold. “The National Weather Service doesn’t even issue their alerts until you’re over a hundred degrees into the heat index,” said Ashley Ward, who leads the Heat Policy Innovation Hub at Duke University.

Ward noted that the heat index is measured in the shade, so it doesn’t account for the added stress of sun exposure. And it doesn’t account for whether still, muggy air is pinning humidity in place or if wind is helping people stay cool. 

A better metric, according to Ward, is the wet-bulb globe temperature, which was developed by the US military after World War II in response to a rash of heat-related deaths during training. According to the US Marine Corps, WBGT is “the most effective means of assessing the effect of heat stress on the human body.”

It’s often described as the reading on a thermometer with a damp sock around it, though the devices are a bit more sophisticated than that. WBGT integrates three distinct measurements: It measures open-air temperatures with a conventional thermometer. It measures temperatures inside a black globe to indicate what it’s like under direct sunlight. And yes, it measures temperatures with a thermometer wrapped in a wet cloth.  

The point of the damp thermometer is to simulate how well sweat can evaporate under the present conditions. The longer the thermometer stays wet as temperatures rise, the more humid the surrounding air, and thus the more difficult it is to cool off by sweating. 

These measurements are then plugged into a formula: The WBGT equals 10 percent of the air temperature plus 70 plus of the natural wet bulb temperature plus 20 percent of the black globe temperature. There aren’t as many WBGT monitoring stations as there are conventional thermometers, and coverage can be spotty. The National Weather Service has a prototype tool that can estimate WBGT near you. 

Ward noted that when Mendoza died, the WBGT was above 90 degrees Fahrenheit, well into dangerous territory. Institutions like the US military and even North Carolina high schools consider a WBGT above 90°F to be “black flag” conditions, where all unnecessary outdoor activity must stop. 

Ward said this could be a far more effective way to convey the threats from high temperatures, but it has yet to catch on. Though the WBGT was developed in the US, it has gained more traction in other parts of the world. “When I was in Hong Kong, they reported the wet bulb globe on their nightly news like we report the heat index,” Ward said. “There needs to be broad scale public education around wet bulb globe in order for us to be able to use it effectively as a public health metric.”  

Heat doesn’t have to be deadly or dangerous

A better heat risk measurement can’t do much unless people can act on it.  

For one thing, a lot of people are compelled to work in the heat. Almost one-third of US jobs require workers to be outside, though people who work inside in places like factories, foundries, and kitchens can face extreme heat too. The Occupational Safety and Health Administration reported that 121 workers died from heat between 2017 and 2022. 

Again, in the US, there is no federal workplace heat safety standard, though agencies are in the process of drafting a regulation. Five states — California, Oregon, Colorado, Minnesota, and Washington — do have their own workplace heat standards. Some cities are also enforcing their own heat rules and implementing policies to reduce ambient temperatures like cool roofs and green spaces to curb urban heat islands. But states like Florida and Texas are blocking local governments from setting stronger protections for outdoor workers, citing harms to businesses. 

Ward said that one of her frustrations is that it’s actually not that difficult to reduce the risks from extreme heat. “It’s not as if we don’t know,” she said. Much of the conventional wisdom still holds: Stay hydrated. Stay in the shade. Take frequent breaks. There is in fact a whole suite of non-air-conditioning interventions that can keep people cool and save lives. 

“Did you know that if you immerse your arms over your elbows or your feet up to your ankles [in water] that you can reduce your core body temperature by up to 3 degrees and save your life?” Ward said. 

But increasing access to cooling and air conditioning also needs to be part of the solution. People who are experiencing homelessness or can’t afford cooling are at much greater risk of illnesses during heat waves. 

Over the long term, humanity will also have to zero out its emissions of heat-trapping gasses that are warming the planet. But in the meantime, we can limit the suffering from extreme heat by giving people better information about when conditions are turning perilous and taking precautions. Many more hot days are ahead, but more people don’t have to die.