How extreme heat can lead to death

When temperatures and humidity rise rapidly in the summer months, just a few degrees Celsius can make a difference to what happens in the human body and become a struggle for life and death.

The critical danger level for illness and death from unrelenting outdoor heat is several degrees lower than experts once thought, say researchers who lock people in heat boxes to see what happens to them.

Large parts of the United States, Mexico, India and the Middle East are suffering from extreme heat waves, which are made worse by man-made climate change. Several doctors, physiologists and other experts have now explained what happens to the human body in such heat.

Important body temperature

The core body temperature at rest is normally around 37 °C (98.6 °F).

That means it is only 4 degrees Celsius away from a catastrophe in the form of heat stroke, said Ollie Jay, professor of heat and health at the University of Sydney in Australia, where he heads the thermoergonomics laboratory.

Dr. Neil Gandhi, chief of emergency medicine at Houston Methodist Hospital in Texas, said that during a heat wave, anyone admitted with a body temperature of 102º Fahrenheit (38.9ºC) or higher and without a clear source of infection will be diagnosed with heat stroke or heat exhaustion.

“During some heat waves, we regularly see core body temperatures above 40 to 41 degrees,” Dr. Gandhi said. Another one or three degrees and such a patient is at high risk of death, he said.

How heat kills

Heat can cause death in three ways, Prof Jay said. The usual first suspect is heat stroke – a critical rise in body temperature that leads to organ failure.

When internal body temperature gets too high, the body diverts blood flow to the skin to cool down, he said. But doing so diverts blood and oxygen from the stomach and intestines, and toxins that normally stay in the intestinal area can enter the bloodstream.

“This triggers a cascade of effects,” said Prof. Jay. “Blood clots in the body and multiple organ failure and ultimately death.”

But the greater cause of death in heat is strain on the heart, especially in people with cardiovascular disease, he said.

This starts with blood rushing to the skin to remove body heat, causing a drop in blood pressure. The heart responds by trying to pump more blood to prevent you from passing out.

“They’re making the heart work much harder than it normally has to,” said Prof. Jay. For someone with heart disease, “it’s like running after a bus with a broken thigh muscle. Something is going to give way.”

The third leading cause of death is dehydration. Sweating causes people to lose fluids to a point that can put a lot of strain on their kidneys, he said.

Many people may not be aware of the danger, said Dr. Gandhi.

Dehydration can lead to shock and cause organ failure due to 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 physician at 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 existing health conditions and if they are taking certain medications,” said Dr. Salas.

Dehydration also reduces blood flow and worsens heart problems, said Professor Jay.

Attack on the brain

Heat also affects the brain. Several doctors said it can cause a person to become confused or have difficulty thinking.

“One of the first signs that you’re having problems with the heat is if you’re confused,” says Kris Ebi, a professor of public health and climate at the University of Washington. But as a symptom, that’s not very helpful because the person suffering from the heat probably doesn’t recognize it, she says. And as people get older, it becomes a bigger problem.

One of the classic definitions of heat stroke is a core body temperature of 104 degrees Fahrenheit “coupled with cognitive dysfunction,” said W. Larry Kenney, professor of physiology at Pennsylvania State University.

Humidity is what matters

Some scientists use a complicated method of measuring outside temperature called wet-bulb temperature, which takes into account humidity, sunlight and wind. It was previously thought that a wet-bulb reading of 35ºC was the point at which the body began to have problems, says Professor Kenney, who also runs a hot-box laboratory and has carried out almost 600 tests on volunteers.

His tests showed that the wet bulb danger limit was closer to 30.5ºC – a value that was gradually appearing in the Middle East, he said.

And that only applies to young, healthy people; for older people, the danger limit is a wet bulb temperature of 28ºC, he said.

“Wet heatwaves kill many more people than dry heatwaves,” said Professor Kenney.

When he tested young and old people in dry heat, young volunteers could perform up to 52ºC, while the older ones had to stop at 43ºC. In high or moderate humidity, the test subjects were not nearly able to perform at such a high temperature, he said.

“Humidity affects the ability of sweat to evaporate,” said Prof. Jay.

Cooling of heat patients

Heat stroke is an emergency and medical personnel try to cool the victim within 30 minutes, Dr. Salas said.

The best way is to immerse them in cold water. Essentially, “you throw them in a bucket of water,” she says.

Alternatively, emergency departments pump cold fluids intravenously into patients’ veins, spray them with nebulizers, place ice packs in their armpits and groin, and lay them on a cooling pad with cold water running through it.

Such measures do not always work.

“We call it the silent killer because it is not such a visually dramatic event,” said Prof. Jay. “It is insidious. It is hidden.”

Updated: June 22, 2024, 08:43