It’s 5 a.m. and still dark at the Carlton Complex fire camp in central Washington, except for the fire’s orange glow on a distant ridgeline. Wildlands firefighter Bre Orcasitas, two colleagues, and three volunteers suit up: heavy duty fire-resistant pants, shirt, jacket, and helmet. Their boots weigh 2 kilograms; the backpacks they will haul to the fire—loaded with 6 liters of water, food for a 16-hour shift, safety gear, and hand tools—can weigh 30 kilograms. Sometimes the burden includes a 12-kilogram chain saw.
On this day in August 2014, the crew is not just fighting flames, but also taking part in research. Orcasitas outfits each person with a chest harness and sensors that will record their heart rate, elevation gain, distance traveled, carbon monoxide intake, and skin temperature. Each swallows an ingestible radio thermometer that relays deep body temperature to the chest monitor every 15 seconds via Bluetooth. Orcasitas and her two colleagues will record each firefighter’s activities, be it cutting down trees, digging a fire break, or burning vegetation to keep a larger fire away. It’s all part of a study to assess heat exposure in wildlands firefighters—the biggest ever to do so. From 2013 through 2016, more than 300 firefighters participated.
High body temperatures are inevitable in firefighting: A study in 2013 uncovered about 50 heat-related injuries across the United States during that fire season. But other data from their project have surprised Orcasitas and her colleagues. Warmth from the firefighters’ physical exertion, not heat from the fires, was the greatest danger, the researchers found. Another surprise: “The assumption across the fire community was that if somebody went down, it was because they just didn’t drink enough water,” Orcasitas says. But the team found otherwise. “You can’t drink yourself out of a heat-related injury,” explains project leader Joseph Domitrovich, an exercise physiologist at the U.S. Forest Service’s National Technology and Development Program. “It’s not the magic bullet that people thought.”
Across the globe, researchers like Domitrovich are working to pin down how heat affects workers and vulnerable populations, such as the elderly. They are studying low-tech measures—sometimes nothing more than a splash of cold water on the skin—to make people safer and more comfortable in hot conditions. And they are exploring the body’s ability to adapt to the heat. “Deaths and illnesses caused by heat are largely preventable,” says June Spector, an occupational and environmental health physician-scientist at the University of Washington, Seattle.
The work has taken on urgency as global temperatures rise, heat waves become more frequent and intense, and casualties mount. Between 1999 and 2010, the U.S. Centers for Disease Control and Prevention logged 8081 heat-related deaths in the United States, one-third of them in people age 65 or older. Already, about one-third of the world’s population experiences conditions that create heat stress, says Nathan Bradley Morris, a human thermal physiologist at the University of Copenhagen. At high risk are construction workers and farm hands, respectively 13 times and 35 times as likely to die from heat as other workers. Farm hands in the United States already face an average of 21 days in summer when humidity and temperature combine to exceed recommended limits, and the risks are growing. In the United States, climate change will double the number of unsafe days by 2050 and triple it by 2100, Spector and colleagues reported on 25 August in Environmental Research Letters.
Amplifying the trends, the world’s population is moving to cities, which tend to be hotter than the countryside, says Lisa Leon, a research physiologist at the U.S. Army Research Institute of Environmental Medicine. In Paris, about 12,000 people died in 1 week during a heat wave in 2003. Even those who don’t work in the heat are vulnerable, especially people who are older and overweight or whose hearts may not be strong enough to cope with the physiological stress of heat, says Christopher Gordon, a retired physiologist from the Environmental Protection Agency. And much of the global population is aging and getting heavier.
By 2100, heat exposure will threaten the health of an estimated 4 billion people, according to an International Labour Organization report. The report forecasts that the world’s productive working hours will decline 2.2% because of increasing heat, resulting in $2.4 trillion in economic losses, concentrated in Southern Asia and Western Africa. “There are inequities in who is exposed to heat,” Spector says.
For centuries, physiologists have studied how the human body reacts to heat stress, tracing its remarkable ability to keep its core temperature at about 37°C even when the outside air is many degrees hotter. Sensory nerves on the skin respond to the environment’s temperature, and internal sensors keep tabs on heat gain, some of which is generated by the body itself. Intense physical activity can turn the body into a furnace, raising heat production 15-fold. Whether the source is environmental or internal, if heat is not dissipated, it can strain and ultimately shut the body down.
When internal sensors tell the brain the body is warming, the hypothalamus sends signals that dilate blood vessels close to the skin, causing more blood to circulate there and lose heat—provided the air is cooler than the body. When it’s not, or if heat transfer to the air isn’t enough, the sweat glands get to work. Another part of the brain, the medulla oblongata, gets in touch with the heart, which increases its rate and the amount of blood pumped per beat. The body’s fluids shift, redirecting blood to the skin and helping fill sweat glands.
An active person can easily sweat out 2 liters of water per hour, which cools the body as it evaporates. It’s not the head, as often believed, but the hands (with their high concentration of sweat glands) and the torso (with its large surface area) that are key sites for sweat-aided cooling. The fluid loss stimulates the release of hormones that enhance thirst and alter kidney function to reduce urine production. These fluid-conserving measures help shore up blood volume so the heart can maintain blood pressure at safe levels.
Dehydration greatly adds to heat strain and the risk of serious injury. Staying well-hydrated can be protective, but it may not keep body temperature from rising if internal or external heating is too great, Domitrovich and others have found. First comes heat exhaustion, characterized by cramps, fatigue, headache, nausea, or dizziness—signals that prompt most people to move out of the heat or stop exercising. As long as sweating continues, the body can still cool itself somewhat. But if a person gets too hot (this tipping point varies, but hovers around 42°C for exertional heat stress), sweating can stop and body temperature can skyrocket, sometimes even rising above 44°C. The brain falters; confusion, agitation, slurred speech, even coma can result.
Even if people recover, heat can cause problems. Some 15% of people exposed to chronic heat stress at work develop kidney problems, possibly making it one of the first epidemics due to global warming, a European consortium called Heat Shield reported in 2018 in The Lancet Planetary Health. Moreover, studies have documented an increase in other injuries on hot days, possibly because heat and dehydration can impair thinking.
At their worst, life-threatening temperatures can damage the brain and cause organs to fail. Deprived of normal blood flow, the gut can leak, causing widespread inflammation. Blood vessels can get damaged and blood can coagulate. Cells can even fall apart as their proteins break down.
“Heat stroke is one of the three leading causes of death for athletes, soldiers in training, and laborers,” says Douglas Casa, an exercise physiologist at the University of Connecticut (UConn), Storrs.
That’s what happened to Korey Stringer, an offensive lineman for the Minnesota Vikings. He collapsed on day two of spring training in 2001, but his heat stroke was not treated aggressively, and he died the next day. A professional athlete, he was fit—but not fit enough for such a strenuous workout early in the season. Motivation to exercise may override the body’s signals to stop, Leon says.
Gatorade and the National Football League launched the Korey Stringer Institute at UConn in 2010, and now grants and private support keep it going. As its CEO, Casa has worked extensively with athletes and with the U.S. military to protect recruits from Stringer’s fate. For the most part, the military and sports trainers have “a good handle on treating heat stroke, but I don’t think people are doing enough to prevent it from happening in the first place,” Casa says.
At the institute, suited up with heavy backpacks, rifles, and full military attire, volunteers march around a lab warmed to the temperature of a hot desert. Casa studies how bodies react to the stress of working in the heat and tests protective measures, including new fabrics for hot weather clothing, new safety and recovery procedures, and wearable sensors that can sound the alarm if conditions reach dangerous extremes.
In a study last year, for example, some of the volunteers got thirsty during a bout of fast walking on a treadmill in a room at 35°C with 30% humidity. Afterward, experimenters allowed the subjects to drink enough to replace 25% of their lost fluids and asked whether they were still thirsty. They were not. Thirst was quenched well before they had enough water, Casa and his colleagues reported in November 2019 in Nutrients. “The absence of thirst does not mean the absence of dehydration,” he warns. He recommends that elite athletes and others who exert themselves in hot conditions figure out their sweat rate and adjust their drinking accordingly.
Casa has also field tested a strategy for runners suffering from exertional heat stroke. For decades, his team has provided medical care for a 12-kilometer race on Cape Cod in August. The race is short enough that runners keep up a punishing pace but long enough that their bodies can overheat; whereas about one in 10,000 marathon racers develops heat stroke, far more—one in about 650—racers in this event do so. At the Cape Cod race, his team treats up to 45 heat stroke victims a year by immersing them in cold water. Getting their body temperature below 40°C within 30 minutes is enough to ensure a full recovery, the team has found. “Most people would be surprised to know we can treat heat stroke,” Casa says. In fact, he says, the treatment guarantees recovery if used promptly and properly.
Far better to avoid heat strain entirely, says Nigel Taylor, a thermal physiologist retired from the University of Wollongong. That means adjusting to heat by avoiding air conditioning, which Taylor says “prevents us from adapting to our climate.”
Heat tolerance varies from person to person, not just because of age and health, but also because of genetic factors. One study of 42,000 Indigenous miners in South Africa when they were first sent to work in the hot mines found that about 15% could not handle the heat, whereas 25% coped just fine.
But Taylor and others have found that frequent exposure to heat also makes a big difference to heat tolerance. If an individual has a chance to get used to being hot—by spending time in hot weather or exercising strenuously—the body modifies its physiology and becomes less vulnerable to heat stress. Athletes and workers can then work harder and longer despite the heat.
He and others have studied adaptation in detail. They find that after just 1 week spending 2 hours a day working outdoors in a hot environment, the body begins to adjust. Normal deep body temperature drops. The body sweats at a lower temperature, and there is less strain on the heart. That’s because, even in the heat, heart rate doesn’t rise as much, and the heart pumps more blood per stroke. The body retains more fluids and blood volume rises, increasing water reserves for sweating and cooling. “The body has a really good capacity to acclimate,” says Daniel Gagnon, a human physiologist at the University of Montreal. But “as long as you keep inserting air conditioning in that process, you delay acclimatization,” says Elizabeth Repasky, an immunologist at Roswell Park Comprehensive Cancer Center.
In the heat of summer, there may be other ways to cool down, says Ollie Jay, a thermal physiologist at the University of Sydney. He has put octogenarians, children, people with heart conditions, even pregnant women into a heat chamber and has found that fans can often be as effective as air conditioning, at much lower environmental and financial costs. Jay and his team reported in November 2019 in the Annals of Internal Medicine that fans can be effective at up to 40°C, particularly in humid conditions, where they help evaporate sweat that would otherwise sit on the skin. (In dry environments, sweat evaporates regardless of fan use.)
Workers can be as productive and comfortable when cooled by fans instead of air conditioners, he and his colleagues reported last year in Energy and Buildings. “In terms of increases in work productivity, fan use in a simulated Vietnamese working environment of 30˚C with 70% humidity is the equivalent of 7˚C of cooling with air conditioning,” Jay says.
Splashing cold water on the skin can also work well, his studies show. “External dousing does the job of sweat without having to sweat,” which can lead to dehydration and strain the heart, Jay says. His team evaluated dousing and fans as alternatives to air conditioning during this summer’s U.S. heat wave. In 80 of 105 cities—the exceptions being in the Southwest—those alternative measures would have been 100% effective, he and his colleagues reported on 25 July in Science of the Total Environment. “It’s getting hotter, and heat waves are getting worse,” he says. “What’s important is using these findings to make changes in public policy.”
For outdoor workers, one simple—but not always easy—fix is to move to the shade. In a rural part of Indonesia, Spector, Yuta Masuda and Nicholas Wolff of the Nature Conservancy, and colleagues randomly assigned 363 laborers to work in a forest or a deforested area nearby. (Deforestation can increase local temperatures by up to 8°C.) The workers wore heart rate monitors and had their oral temperatures taken regularly to calculate core body temperature. During a 90-minute task in hot, humid, sunny conditions, people in open areas had temperatures above 38.5°C for 3 minutes longer than those in the forested area, Spector and her colleagues reported last year in Environmental Research Letters. The differences may seem small, but over the course of a day, body temperatures would likely keep climbing, putting these workers at great risk of heat-related illness. Those who worked in deforested areas also scored worse on cognition and memory tests, perhaps because of dehydration or discomfort, the team reports in a paper in press in Environmental Research Letters.
Wildlands firefighters also work in an exposed, hot environment. Yet it is not sunshine or fire they need to worry about most. “The heat from the fire almost never has any effect on our core body temperatures,” says William Knudsen, a wildlands firefighter based in Helena-Lewis and Clark National Forest. Nor is it dehydration, once considered the biggest risk for these workers. Orcasitas found, for example, that after a particularly strenuous day’s work, a well-hydrated firefighter came back feeling “like crap” and had a higher body temperature than a colleague who started and ended the day more dehydrated. And fellow study coordinator Knudsen saw firsthand how trouble can arise even in temperate weather. His team recorded its highest body temperatures on a high-elevation site with an air temperature of only about 21°C.
Instead, Domitrovich says, “It’s the internal heat production that’s most critical.” Big packs, heavy clothing, and a fast pace cause body temperatures to spike. “And if you are less fit, you are going to generate more heat,” Orcasitas says. So now, when Knudsen takes out his crew, he’s mindful of the weight they carry and how fast they hike, frequently taking a 2-minute break after 8 minutes of walking.
Orcasitas thinks the study will make a difference for her and her firefighting colleagues. “These are all seemingly simple, but [the findings] are a pretty big deal,” she says, because they’re easy to act on.
They could also help others stay cooler as global temperatures climb. “Heat stress is a very complex topic, so the more we can educate not just firefighters but everyone, the better,” Domitrovich says. “It will lead to increased safety for all humans.”
Read more from our special issue on keeping cool in a warming world.