Our brain works hard to coordinate and regulate our body’s temperature. But rising temperatures and humidity make us increasingly susceptible to heatstroke, especially during exercise.
It’s important to know the risks of heatstroke and take it seriously. Not only does heatstroke have a relatively high mortality rate, but those who recover still may face future health problems, said Orlando Laitano, assistant professor of applied physiology and kinesiology at the University of Florida. “We now believe that heatstroke is almost like concussion.”
In one recent study, Laitano and his colleagues found that heatstroke in mice could change the genome and create more susceptibility to future heat and immune diseases.
Currently, approximately 30 percent of the human population is exposed to dangerous environmental heat for at least 20 days every year, according to a recent study. This number may increase up to 74 percent of the global population by the year 2100 if anthropogenic climate change continues unabated.
“In the face of global warming and climate change, that becomes — no pun intended here — a hot topic,” said Laitano, who co-authored a recent review on heatstroke.
Understanding heat stroke and heat illness
Anyone can develop a heat-related illness, though older adults and young children are especially vulnerable. High humidity, strenuous physical activity and consecutive sweltering nights also increase our risk.
Heat illness can range in severity from mild heat exhaustion to life-threatening heatstroke. Heat exhaustion is more common, and may involve symptoms such as dizziness, fatigue, nausea or headache but not a dramatic increase in body temperature.
Heatstroke is typically defined as having a core body temperature that is above 104 or 105 Fahrenheit, which causes severe dysfunction of the central nervous system, including confusion, dizziness and unconsciousness, and can lead to multiorgan injury and more.
However, there can be significant differences in individual heat tolerance and there are people who collapse below this threshold, Laitano said.
Another potential challenge is that heat illness can distort judgment, and the person affected may be unaware that they are in trouble.
How our brain coordinates temperature regulation
The cells in our body function properly only in a relatively narrow band of temperatures, which is regulated to be between 98 and 99.5 degrees Fahrenheit (36.7 and 37.5 degrees Celsius) in a healthy human. Extreme heat damages our cells, degrades proteins and harms DNA.
“Whenever you have that, you’ll see the cascade of effects that causes things to shut down and causes damage to your body,” Stearns said.
Our central nervous system works hard to coordinate control over our body’s temperature to prevent harm. Temperature sensors in our skin and internal organs, called thermoreceptors, are specially tuned to different temperature ranges and send signals to our brain’s touch cortex, which allows us to perceive the heat and respond to it, such as getting out of the sun and heading indoors.
We also have an internal thermostat located in our brain’s hypothalamus called the preoptic area. By sensing our core body temperature, it can activate automatic autonomic systems to begin cooling the body when it reaches a certain temperature, such as through sweating and dilating our blood vessels.
The primary way we cool ourselves is through sweating through our sweat glands. As the sweat evaporates, it cools our skin. This evaporative cooling accounts for about 80 percent of our cooling capacity during exercise, Stearns said.
Sweating, however, is a “double-edged sword,” Laitano said. “It’s very important because it will help you thermoregulate, but also leads to dehydration.”
Our brain’s internal thermostat also activates the sympathetic nervous system causing blood vessels in our skin to dilate. Our heart more than doubles its output to meet the increased demand, and pumps warm blood from our body’s core to the surface, where it should cool off — if the outside air isn’t hotter.
Hot, humid days are particularly dangerous for heatstroke because they challenge our natural abilities to thermoregulate. The hot air raises our skin’s temperature instead of lowering it, and the higher the humidity, the harder it is for our sweat to evaporate and cool us.
The two types of heatstroke
There are two types of heatstroke: classic heatstroke and exertional heatstroke.
Classic heatstroke is caused by passive heating from the external environment, such as during heat waves. It typically affects children and older adults who are less able to regulate their body temperature. In young children, sweat glands may not be fully developed. And with age, we begin to lose our ability to both sense thirst and thermoregulate effectively.
Exertional heatstroke occurs when we physically exert ourselves with strenuous activity in hot weather. Physical activity heats up our skeletal muscles when it consumes energy. The more intensively the muscle contracts, the more heat is produced, which heats up our body from the inside in addition to the ambient heat of the environment.
Exertional heatstroke can affect anyone doing physical activity, but is more likely to affect younger adults, particularly athletes and military personnel.
“In everyday life, we don’t see a lot of it outside of competitive scenarios because people are smart, and they’ll stop if they’re not feeling good, before they reach the point of heatstroke,” Stearns said.
But people in sports and military contexts may be motivated to push past their thermoregulatory limits. Exertional heatstroke is the third leading cause of mortality for athletes during physical activity, behind only heart problems and head or neck trauma.
It is estimated that the mortality rate of exertional heatstroke could reach around 27 percent, while mortality in classic heatstroke could be much higher, due to its predominance in already vulnerable people.
How heatstroke can damage the brain
Early data show that 10 to 28 percent of patients who survive heatstroke may sustain long-term cognitive or neurological damage, particularly involving dysfunction of the cerebellum, a brain region important for coordinating our movement.
Brain imaging months or years following heatstroke has also found damage to cells in the cerebellum and other brain areas, including the hippocampus, midbrain and thalamus can also be damaged.
“This is going to sound really sad. It’s very rare to have somebody who has long-term damage from heatstroke and survives,” Stearns said. “Most of those cases, unfortunately, perish. But there are many cases that require lifelong care that are out there.”
How to prevent and treat heatstroke
Give yourself time to acclimate to the heat and stay hydrated. “The first 10 to 15 days of anyone doing any new activity in a warm environment are really important to progressively and gradually do that. Because you allow your body time to adapt,” Stearns said.
Don’t exercise outdoors alone, and learn to recognize the signs of heatstroke in yourself and others. Getting out of the heat, even for a few hours, is protective during heat waves. Air conditioning and electric fans can help, but may not always be available, such as during a power outage. Dousing the skin in water may be an effective alternative.
If you feel unwell, weak, confused or agitated, those are all “red flags,” Stearns said. Stumbling, collapsing or inability to coordinate movement are also signs to look out for in yourself and others around you.
Begin cooling immediately even before a full diagnosis if heatstroke is suspected, experts say. “There is a big association between how fast you cool someone and the prognosis, what the outcome is,” Laitano said.
You can apply ice wrapped in towels to the neck, groin or the extremities. Cold water immersion, where the person’s whole body is immersed in iced water, is the first-line treatment for heatstroke.
With better education, more public awareness and prompt treatment, “heatstroke doesn’t have to be deadly,” Stearns said.
Do you have a question about human behavior or neuroscience? Email BrainMatters@washpost.com and we may answer it in a future column.