Heat ledgers: What happens to the body, exactly, in extreme heat? - Hindustan Times

Heat ledgers: What happens to the body, exactly, in extreme heat?

ByNatasha Rego
May 24, 2024 06:56 PM IST

Find out. Plus, meet the climatonauts trekking through deserts to answer the question: What will survival look like, in a climate we have not evolved for?

The human body can take a fair amount of cold. It is heat it cannot withstand.

Climatonauts in the Nafud desert in Saudi Arabia, where temperatures reached 45 degrees Celsius and water was limited. (Human Adaptation Institute / Lucas Santucci) PREMIUM
Climatonauts in the Nafud desert in Saudi Arabia, where temperatures reached 45 degrees Celsius and water was limited. (Human Adaptation Institute / Lucas Santucci)

For millennia, humans have lived in a narrow climatic zone. The human body itself has a narrow temperature range of 36 to 38 degrees Celsius (the ideal of 98.6 degrees Fahrenheit being the equivalent of 37).

The brain itself is so susceptible to heat that the head retained all its original hair, even as we lost this protective cladding across the rest of our bodies.

What exactly unfolds within the body as ambient temperatures rise? Think of it as the equivalent of shivering, but more internally drastic.

In extreme cold, our muscles expand and contract rapidly in an effort to generate heat. This is what we call shivering. In high temperatures (typically over 30 degrees Celsius for Indians), the body begins to produce perspiration, to cool itself. Particularly in the presence of a cooling wind, equilibrium can thus be maintained, though it is costing vital fluids and salts that must be replenished.

In extreme heat, or extreme humidity, we cannot produce enough perspiration, or the perspiration cannot evaporate, causing the cooling system to fail.

This is why wet bulb temperatures are becoming the danger mark to watch out for.

A wet bulb temperature of more than 35 degrees Celsius — meaning that mercury will not drop below this point, even with a damp cloth on it — is more likely to cause heat collapse in a human that an ambient temperature of 50 degrees Celsius with lower humidity levels.

When the cooling system fails, the body starts to compensate in other ways. And this is where things get dangerous.

As the body weakens from the loss of fluids and salts to perspiration, the heart begins to beat faster, pushing blood to the extremities to initiate sweating. The increased strain registers as a change in breathing patterns. The person begins to pant, and their pulse rises further.

The body is now building towards a point of heart fatigue and exhaustion, says Vidhya Venugopal, who is researching climate change and health at the Sri Ramachandra Institute of Higher Education and Research (SRIHER), Chennai. It is overheating in much the same way a cheetah does; except, a cheetah can stop running.

If the ambient temperature doesn’t fall — as, for instance, with people who work outdoors and collapse or die as a result of extreme heat — the body begins to lose the battle with the sun.

Blood pressure levels start to fluctuate. Internal organs begin to feel the effects of lowered oxygen levels and weakened blood flow. Communication between nerve cells in the brain is disrupted, causing anxiety, headaches, disorientation and impaired judgement. Kidneys undergo tissue damage. Other organs will follow. This is heat stroke.

A person will first lose consciousness, unable to keep the systems running amid the strain. “The collapse of heat stroke is the body’s way of giving in,” Venugopal says. “As ambient temperatures rise around the world, high humidity levels will have a negative impact, especially in the coastal zones, of which India has many.”


What will mitigation look like, for the human body and a climate it has not evolved for? Scientists are racing to answer this question. The answers will differ for different regions, based on the kind of heat and the heat tolerance of the population.

Some solutions could be universal, if they can be made affordable, effective and scalable. Smart apparel makers, for instance, are working on garments that sense the body’s strain and respond in ways that help it, even using solar energy to power the sensors, microprocessors and other electronic components at work within.

Scientists are at work, building action plans. Urban planners are rethinking the role of public infrastructure.

In India, National Disaster Management Authority and India Meteorological Department are framing Heat Action Plans (HAPs), advisory documents on how to raise preparedness and lower the impacts of heat waves.

The Ahmedabad municipal corporation’s HAP, put in place in 2013, is being used as a template for other regions. As of this year, 120 districts across 17 states have a Heat Action Plan, which includes a warning system to ensure alerts reach vulnerable populations, norms for the rescheduling of working hours for outdoor workers, and recommendations for the creation of drinking water kiosks, special shelters and green corridors in urban settings. Oral rehydration solution or ORS counters are already showing up at primary health centres and anganwadis.

A March 2023 assessment of 37 HAPs at the city, district and state levels, by the think tank Centre for Policy Research, however, found that the plans were often not adapted for local context, were poor at identifying and targeting vulnerable groups, and were underfunded and insufficiently transparent, suggesting that current HAPs need to be strengthened over time.

“One can say that there are initiatives being taken. The government has started documenting heat related illnesses, installing ORS counters at public health centres and increasing bed capacity,” says Venugopal. “But how effective is this going to be in a country of 1.4 billion, of which about 700 million live in extreme poverty? Can any country be prepared for this?”


One man, and his Human Adaptation Institute in France, is seeking to answer that question.

Franco-Swiss adaptation researcher Christian Clot is conducting experiments to test the human body at extreme temperatures, with the help of explorers he refers to as “climatonauts”.

Where astronauts endure simulations, test their bodies in special environments and then undertake unique journeys in a search for ways to new worlds, climatonauts seek to do the same, as our Earth changes, he says.

So far, 10 men and 10 women have participated in a series of experiments called the Deep Climate Expeditions, conducted between December 2022 and June 2023. The teams were led by Clot through an equatorial forest in French Guyana (with temperatures of 35 degrees Celsius and humidity at 98%); the polar lands of northern Lapland, where temperatures with wind chill reached -50 degrees Celsius; and a Mad Max: Fury Road-like trek through the Nafud desert in Saudi Arabia, a dry region where temperatures reached 45 degrees Celsius and water was limited.

The teams featured people aged 22 to 51, from a range of professions and socio-economic backgrounds. The explorers have deliberately been picked from the general population, because natural disasters don’t differentiate, Clot says. Among the 20, then, was a neuroscientist, a software developer, a nurse, a jeweller, a security guard and a sailor.

As they made their way through the terrains, sensors monitored heart rate, blood pressure and brain activity, among other parameters. The group also underwent cognitive tests for memory, attention, metacognition and sensory perception.

As Clot, 51, studies the effects of extreme heat, humidity and cold on the body and brain, he says really cold conditions “are not something we will have to face in the future. But it’s important to compare how the body reacts to all extremes.”

Over 40 days, on each of the three expeditions, the group travelled 150 km to 250 km, with no motorised assistance and no communication with the outside world, dragging trolleys and sleds weighing between 150 and 200 kg each, carrying their food, water and equipment.

“The desert was by far the toughest of the journeys,” Clot says. The group had the most difficulty collaborating, cooperating and making joint decisions here. “As the brain deactivated some functions to remain optimal in these high ambient temperatures, it cut some ‘non-vital’ systems like memory and social function, and we had more difficulties working together,” Clot says.

He is now preparing to publish his findings by the end of the year. He is also preparing for his next phase of expeditions. Climatonauts will be vital to understanding our future in a hotter world, Clot says. “All that we know about the body and brain’s response to extreme heat is drawn from controlled test environments. But it is vital to assess human response to high temperatures and extreme humidity or lack of humidity, where exposure is continuous and fluctuating.”

In a note of hope, he adds, he did see social cohesion improve over time, in his groups. In Lapland, members of the group were in tears because of how painful the journey was, but they were also helping each other. “I do really think that at the end of the day, we are not made to do things alone.”

Get World Cup ready with Crick-it! From live scores to match stats, catch all the action here. Explore now!.

Catch your daily dose of Fashion, Taylor Swift, Health, Festivals, Travel, Relationship, Recipe and all the other Latest Lifestyle News on Hindustan Times Website and APPs.
Story Saved
Live Score
Saved Articles
My Reads
Sign out
New Delhi 0C
Thursday, June 20, 2024
Start 14 Days Free Trial Subscribe Now
Follow Us On