It was a shock when Michael Mosley, a doctor and well-known TV presenter in the UK, was found dead earlier this month after hiking in scorching temperatures on the Greek island of Symi.

But it is now one of a series of tourist deaths and disappearances in Greece as the country endures a powerful, early summer heat wave with temperatures pushing above 40 degrees Celsius (104 degrees Fahrenheit).

On Saturday, a Dutch tourist was found dead on the island of Samos. The following day, the body of an American tourist was found on Mathraki, a small island west of Corfu. Albert Calibet, another American tourist, has been missing since he set out for a hike on June 11 on Amorgos. And two French women disappeared on Sikinos after going for a walk.

The bodies of those who died still need to be examined to establish the precise cause of death, but authorities are warning people not to underestimate the impacts of the searing temperatures.

“There is a common pattern,” Petros Vassilakis, the police spokesman for the Southern Aegean, told Reuters, “they all went for a hike amid high temperatures.”

Some scientists say what’s happening in Greece offers a warning sign about the impacts of extreme heat on the body, and in particular the brain, potentially causing confusion, affecting people’s decision-making abilities and even their perception of risk.

As climate change fuels longer and more severe heat waves, scientists are trying to unravel how our brains will cope.

The brain is ‘the master switch’

Research has traditionally focused on the impact of extreme heat on muscles, skin, the lungs and the heart, but “the brain, for me, is the key to it all,” said Damian Bailey, a physiology and biochemistry professor at the University of South Wales. It’s the “master switch” for the body, he told CNN.

It’s in the brain that body temperature is regulated. The hypothalamus, a small diamond-shape structure, acts as a thermostat. It performs a delicate dance to keep the body’s internal temperature at or very close to 37 degrees Celsius (98.6 Fahrenheit). When it’s hot, the hypothalamus activates the sweat glands and widens blood vessels to cool the body down.

But the brain functions well within a narrow range of temperatures and even small changes can affect it. Many people will be familiar with a feeling of slowness and laziness on a warm summer’s day.

But as heat increases, it can have serious effects, including lowering the fluids in the body and decreasing blood flow to the brain, Bailey said. He compares the brain to a Hummer — it needs vast resources to function.

Tests he has run on research participants in an environmental chamber, where he cranked temperatures up from 21 to 40 degrees Celsius (around 70 to 104 Fahrenheit), showed a drop in blood flow to the brain by about 9% to 10%.

“That is a big deal in terms of not getting enough fuel into an engine which is running at high end all of the time,” Bailey said.

And it has an impact. Extreme heat can disrupt typical brain activity, said Kim Meidenbauer, a neuroscientist at Washington State University. The brain networks that usually allow people to think clearly, to reason, to remember, and to construct and formulate ideas, can get “thrown out of whack”.

It gets harder to make complex decisions, such as which path to take on a hike — a decision that sounds simple but requires weighing multiple different factors.

There’s also evidence to suggest people are more likely to make risky decisions and engage in impulsive behavior when they’re exposed to heat, she added.

An altered perception of risk coupled with impaired cognitive function can have very serious consequences. “You’re not just talking about potentially getting a little bit too warm and maybe having a sunburn,” she said. “You’re talking about potentially life-threatening (situations), like making poor decisions, having your judgement clouded.”

Scientists are only just beginning to unravel the range of impacts heat has on the brain, not just in terms of decision-making but mood, emotions and mental health.

“Our understanding is really pretty minimal,” Meidenbauer said. “It’s a major unknown at this point.”

Who is vulnerable?

Some people are more vulnerable to heat than others. Older people, especially those over 65, are more at risk, because their bodies don’t always thermoregulate as well. The people who have gone missing in Greece were all in their mid-50s and older.

Very young children and pregnant women also face elevated risk, as do those with pre-existing conditions, including mental health conditions.

But heat can be dangerous for anyone.

In 2016, a team of scientists followed 44 college students during a heat wave in Boston and found those without air conditioning experienced significant declines in cognitive performance.

“No one is immune to the health effects of heat,” said Jose Guillermo Cedeño Laurent, one of the research authors and an assistant professor at the Rutgers School of Public Health. “Our brain is an exquisitely sensitive organ,” he said.

Someone who is very fit understands the dangers and carries plenty of water is still gambling if they decide to go on a hike in very high temperatures, Bailey said.

“You make wrong decisions and it can cost you your life.”

How to protect yourself

There are behavioral things people can do to protect themselves and lower risk, experts say.

These include not exercising during the hottest parts of the day, instead going in the coolest parts of the day and seeking shade when possible. Wearing loose clothing and applying ice packs to the head and neck can also help.

Drinking water is vital and not just when you feel very thirsty, Bailey said. It’s important not to get to a point where the body is losing fluids faster than it can take them on. Experts also recommend electrolyte drinks, which can help replace some of the fluids lost through sweating.

Use location-sharing apps, said Meidenbauer. “Make sure someone knows where you are.”

Over the long term, regular exercise is important — providing it’s not outside during the hottest parts of the day — as it can help the body thermoregulate. “The fitter you are the more resilient you are to these climatic environmental stresses,” Bailey said.

It will take time to unravel the exact causes of death of those who lost their lives in Greece but there is a lesson that can still be taken away from the tragedies, Bailey said.

“No matter how intelligent or how fit you might think you are … if you’re going out in 40 degrees celsius plus temperatures, even if you’re well prepared, you’re running the gauntlet.”