What happens to our body when temperature rises above 50C?

A severe heatwave is scorching northern India and Pakistan, raising temperatures above an unprecedented 50C and leaving hundreds of people dead.

India’s weather department is investigating a reading of 52.9C recorded in the outskirts of Delhi on Thursday. If confirmed, it will be the hottest temperature ever recorded in the country.

M Mohapatra, the weather department’s director, said average temperature in the capital was in the 45-50C range, several notches above normal for this time of the year. It was the same story in much of northern India.

Such extreme heat could be fatal. Indeed, at least 18 people were reported to have died from heatstroke and related illnesses on Thursday alone. They included a 40-year-old man in Delhi who was staying in a room without a cooler or a fan. His body temperature, doctors said, had crossed 107F, nearly 10 degrees above normal.

In all, India has recorded several hundred deaths from extreme heat this summer, but experts said the actual number could be far higher.

How hot is too hot for human body?

The human body has a very narrow range of temperature it finds ideal. It works best when the temperature is between 36C to 37.5C, or 96.8F to 99.5F. Anything over this is dangerous.

Ambient air temperature of 18-24C is good for most people, although people who are healthy and more well adapted to heat can sustain slightly higher temperatures.

When its temperature rises the body seeks to remove heat, mainly through sweating, though breathing and increased heart rate also help.

For most people but especially the vulnerable, prolonged exposure to temperatures above 40C causes heat stress as the body starts to struggle to cool itself down.

Heat stress is recognised by “fever above 104F, dry skin and loss of consciousness”, said Dr Arun Sharma, a professor of community medicine.

“High temperatures may also cause sunburn, irritation in the eyes and dehydration.”

For protection, Dr Sharma advised avoiding sun exposure, keeping the body hydrated with frequent intake of water, fruit juice, coconut water or lemon drinks, and avoiding sudden changes in temperature like walking out of air conditioned rooms straight into sunlight and vice versa.

The more extreme the temperature gets the more the body must sweat, increasing the risk of dehydration and even heatstroke.

“When the individual is dehydrated, extreme heat exposure will thicken their blood and cause organs to shut down, resulting in death within hours, popularly called heatstroke,” said professor Vidhya Venugopal, country director at the Sri Ram Institute of Higher Education and Research.

When the temperature rises above 46C, cells, the tiny building blocks of our body, start getting damaged or destroyed.

The body’s inability to regulate its temperature can also worsen existing chronic conditions like cardiovascular, respiratory and cerebrovascular diseases and ailments related to diabetes, according to the National Institute of Environmental Health Science in the US.

A recent analysis suggested that increasing heat can also worsen certain brain conditions.

Role of humidity

Air temperature alone doesn’t tell the full story. Humidity is an important factor in how we feel the heat, especially when it is as hot as 50C.

High humidity prevents the body from cooling itself by sweating, raising the risk of heatstroke and other potentially fatal conditions.

Humidity in the air is measured as “wet bulb temperature”, so named as it is measured by wrapping a wet cloth around a thermometer and seeing how much cooler it gets.

A wet bulb temperature of 32C is usually the maximum a human body can endure. This is equivalent to a dry temperature of 55C. The theoretical maximum wet bulb temperature is 35C, which means most humans are likely to suffer heatstroke at this level.

When air temperature is 46.1C and relative humidity is 30 per cent, the wet bulb temperature is 30.5C. But when air temperature is 38.9C and relative humidity is 77 per cent, the wet bulb temperature is about 35C.

Heat in India, as in much of South Asia, is extremely humid. The average relative humidity of Delhi is 67 per cent, which makes higher temperatures particularly dangerous.

Studies have shown that when air temperature reaches 35C and is accompanied by high humidity, it becomes risky for our health. Once 40C is reached, the heat can be dangerous even with low humidity levels.

At 50C, suffice to say, the risk is far higher.

Why cities are hotter

This risk is further increased in cities due to concrete infrastructure and lack of green spaces, which means they are often warmer than rural areas.

While some Indians can afford to use air conditioners and water coolers to beat the heat, half of the country’s workforce is compelled to work outside with little to not respite throughout the day.

“The unequitable impacts of heat waves, especially with temperatures beyond 50C in dry environments like Delhi, will affect the most vulnerable, such as the elderly, young children, workers exerting outdoors, people with comorbidities, and the poor with minimal cooling interventions,” said Dr Venugopal.

Night temperatures

It’s not just daytime heat that is scorching South Asia. Night temperatures have been as high as 36C in some places in northern India, which is particularly dangerous since it means people cannot cool off at night to fall asleep.

“Increasingly, overnight temperatures offer little relief during periods of extreme heat, which means people struggle to escape the health risks that it brings,” Dr Andrew Pershing, vice president for science at Climate Central.

Is 50C the new normal?

India’s heatwaves have been getting increasingly brutal over the last few years. Scientific assessments have found that the intensity, frequency and duration of this heat has been made worse due to the climate crisis.

“This devastating heat is not a natural disaster,” said Dr Friederike Otto, professor at Imperial College London and director of World Weather Attribution, a coalition which studies reasons behind extreme weather.

“The suffering India is facing this week is worse because of climate change, caused by burning coal, oil and gas and deforestation.”

“What we are seeing in India is exactly what scientists said would happen if we didn’t stop heating the planet.”

India isn’t alone in facing this weather.

Pakistan recorded temperatures above 52C in the southern Sindh province this week, the highest this summer and close to the country’s record high. Several other Asian nations have broken records for high temperatures this summer.

The past 12 months have been the planet’s hottest ever recorded. The confirmation came well after the United Nations warned in 2023 that we were living in an era of not just global warming but “global boiling”.

According to the UN’s top science body, IPCC, heatwaves that arrived once in 10 years without human-caused warming were now likely to occur 2.8 times as often. And if emissions weren’t cut much faster than planned, it said, they would become even more common.