As temperatures and humidity rise outside, what happens inside the human body can become a life-and-death struggle decided by just a few degrees.
The outdoor critical danger point for illness and death from brutal heat is several degrees lower than experts once thought, say researchers who put people in hot boxes to see what happens to them.
With much of the United States, Mexico, India and the Middle East suffering sweltering heat waves exacerbated by climate change, several doctors, physiologists and other experts explained to The Associated Press what happens to the human body when such heat.
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Heat waves can increase your body temperature
The body's core temperature at rest is typically about 98.6 degrees Fahrenheit.
That's just 7 degrees away from heatstroke catastrophe, says Ollie Jay, professor of heat and health at the University of Sydney in Australia, where he heads the thermoergonomics laboratory.
Dr. Neil Gandhi, director of emergency medicine at Houston Methodist Hospital, said that during heat waves, anyone who comes in with a fever of 102 or higher and no obvious source of infection will be evaluated for heat exhaustion or the more serious heatstroke.
A man pours cold water over his head to cool down on a sweltering day in the Mediterranean Sea in Beirut, Lebanon, on July 16, 2023. As temperatures and humidity rise, what happens inside the human body could become a life-threatening event . death throes decided by just a few degrees. (AP Photo/Hassan Ammar, File)
“We will routinely see core temperatures higher than 104, 105 degrees during some heat episodes,” Gandhi said. Another degree or three and such a patient is at high risk of death, he said.
How heat kills
Heat kills in three ways, Jay said. The usual first suspect is heat stroke: a critical increase in body temperature that causes organs to fail.
When body temperature gets too high, the body directs blood flow to the skin to cool down, Jay said. But that diverts blood and oxygen away from the stomach and intestines, and can allow toxins that normally reside in the intestines to enter the bloodstream.
“That creates a cascade of effects,” Jay said. “Clot formation around the body and multiple organ failure and ultimately death.”
But the biggest killer of heat is the strain on the heart, especially in people with cardiovascular disease, Jay said.
It starts again with blood flowing to the skin to help dissipate core heat. This causes blood pressure to drop. The heart responds by trying to pump more blood to keep you from passing out.
“You're asking the heart to do a lot more work than it normally needs to do,” Jay said. For someone with heart disease, “it's like running for a bus with a dodgy hamstring. Something's going to happen.”
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The third main way is dangerous dehydration. As people sweat, they lose fluids to a point that can severely strain the kidneys, Jay said.
Many people may not realize their danger, said Gandhi from Houston.
Dehydration can progress to shock, causing organs to shut down from a lack of blood, oxygen and nutrients, which can lead to seizures and death, said Dr. Renee Salas, a professor of public health at Harvard University and an emergency room physician at the Massachusetts General Hospital.
“Dehydration can be very dangerous and even fatal for anyone if it gets bad enough – but it is especially dangerous for people with medical conditions and who are taking certain medications,” Salas said.
Dehydration also reduces blood flow and increases heart problems, Jay said.
Intense heat can 'fry' your brain
Heat also affects the brain. It can cause a person to become confused or have trouble thinking, several doctors said.
“One of the first symptoms you get when you get heat is feeling confused,” says Kris Ebi, a professor of public health and climate at the University of Washington. That's of little help as a symptom because the person suffering from the heat is unlikely to recognize it, she said. And it becomes a bigger problem as people get older.
One of the classic definitions of heat stroke is a body temperature of 104 degrees “in combination with cognitive impairment,” says Pennsylvania State University physiology professor W. Larry Kenney.
Heat and humidity are a dangerous duo
Some scientists use a complicated measurement of outdoor temperature called wet-bulb temperature, which takes into account humidity, solar radiation and wind. In the past, a wet-bulb reading of 95 Fahrenheit was thought to be the point at which the body started to have problems, said Kenney, who also runs a hot box lab and has done nearly 600 tests with volunteers.
His tests show the wet bulb hazard is closer to 87. That's a figure that's starting to emerge in the Middle East, he said.
And that's only for young, healthy people. For older people, the danger point is a wet bulb temperature of 82, he said.
“Moist heat waves kill many more people than dry heat waves,” Kenney said.
When Kenney tested young and old people in dry heat, young volunteers could function up to 125.6 degrees, while older people had to stop at 109.4 degrees. With high or moderate humidity, people could not function at temperatures nearly as high, he said.
“Humidity affects the ability of sweat to evaporate,” Jay said.
How to cool down patients with heat exhaustion
Heat stroke is an emergency and medical responders try to cool a victim within 30 minutes, Salas said.
The best way: immersion in cold water. Basically, “you drop them in a water bucket,” Salas said.
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But they are not always there. That's why emergency rooms pump patients intravenously with cool fluids, spray them with nebulizers, place ice packs in the armpits and groin, and place them on a cooling mat with cold water in it.
Sometimes it doesn't work.
“We call it the silent killer because it's not such a visually dramatic event,” Jay said. 'It's treacherous. It's hidden.'
