Washington DC - History professor Ethan Borden Krizman was in high school when he collapsed during a race. The terrible heatstroke incident occurred 20 years ago now, but he still remembers it well.

The current heatwaves across Europe and much of the US have brought it all swirling back for him.

"I started getting tunnel vision, and I could feel my body weakening, and I collapsed, like just I completely blacked out," he told the AFP, recounting how he was evacuated just short of the finish line on a 95 degree day in California.

Krizman says he can feel himself "getting on edge" when he sees news alerts about extreme heat across Europe, where scores of deaths attributed to the heat have been reported.

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"I can feel my heart [rate] elevating," he said.

Krizman says that ever since his brush with heat stroke, his tolerance for extreme heat has significantly diminished.

While children under the age of five and the elderly are most at risk in heatwaves, teens and healthy adults can also suffer serious symptoms that can ultimately be fatal, especially when engaging in physical activity.

At the time, Krizman was a competitive runner, logging about 70 miles a week.

"I wasn't an Olympic-level athlete, but I was probably more physically fit than 90-plus percent of the population," he said. "I had been hydrating all day long."

The number of deaths linked to extreme temperatures increased in the US from 1999 to 2023, with an acceleration seen in the last seven years of that period, according to a study published in the American Medical Association's JAMA in 2024.

Heat waves now cause more deaths in the country than hurricanes or floods.

In 2018, University of Maryland football player Jordan McNair died at the age of 19 from complications related to extreme exhaustion and heatstroke.

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Medical resident Evan Dishion died while on a hike with friends in Arizona in 2022.

"Even a doctor who runs marathons and is 32 years old can die of a heatstroke," his widow Amy Dishion told AFP.

Since her husband died, when their daughter was just three months old, Amy has fought to raise awareness about the dangers of extreme heat.

"As human beings, our bodies have a threshold of what we can survive in," she said, lamenting that decision makers routinely ignore this fact.

Krizman shares that view, noting that many athletic competitions take place even when temperatures soar.

"Is it really worth risking a teenage kid's life to go out there and play a sport when it's 100-plus degrees out?" he said.

He urged everyone to "take it seriously. Don't assume that 'Oh, it's just hot and I'll be okay.'"

Krizman offered easy-to-follow advice: stay hydrated, make sure you have eaten, and limit physical activity.

"Do your best to stay out of the heat during the peak hours of the day, and know where you can go to potentially cool off," he added.