India and Pakistan are currently experiencing an extremely strong heat wave: more than a billion people in South Asia experienced extreme temperatures in March and April, well above 40°C, even before the monsoon started. And this is not over. Between 2030 and 2050, the World Health Organization (WHO) expects climate change to cause nearly 250,000 additional deaths per year worldwide.
The Intergovernmental Panel on Climate Change (IPCC) predicts “more intense, longer and more frequent heat waves. Before human activities caused global warming, the heat wave that hit India would have occurred every 50 years,” notes Marian Zachariah of Imperial College London. “From now on, we can expect it to be ‘once every four years’.
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Heat waves are multiplying and getting more intense, to the point of becoming a real public health problem. Because beyond a certain temperature, the human body is put to the test. Humans keep their body temperature within narrow limits: around 37.5°C, with a variation of 1 to 2 degrees, despite large variations in the outside temperature. If this temperature rises too much, the body gradually becomes disrupted. Heat wave-related deaths in India have risen by more than 60% since 1980, according to Jitendra Singh, India’s Earth Sciences minister.
Wet bulb temperature, an important index
In recent years, heat waves have become commonplace around the world. We are no longer surprised that the thermometer in the heart of Paris has crossed 35 degrees, and the city of Nagpur, in India, has had days of 44 degrees for several weeks now. Temperatures even harder to tolerate at high humidity.
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This is called the wet bulb temperature or wet bulb temperature or Tw. This is a measure of heat that takes into account the humidity in the air. This is an especially important index because the human body’s ability to cool itself is based on the principle of evaporation: by sweating, we remove most of the heat from our body. But humidity reduces the evaporation capacity of sweat. Heat loss is therefore less in hot and humid climates, and this is exactly what makes it dangerous.
dying of heat
Can you die from the heat? Yes, and pretty quickly! The phenomenon of thermoregulation of the human body can only work if the wet temperature is lower than that of the body. Even under the best conditions (shade rest, optimal hydration, good general health) a human cannot survive above 35 degrees Tw for more than six hours. Since these ideal conditions are rarely met, the first heat-related illnesses can occur from 27 degrees Tw.
Imagine yourself in a sauna. You have to stay in it, you have no choice. Now imagine having physical activity there!
Diane Rainard, adventurer
In 2015, wet bulb temperatures in India and Pakistan reached 30 degrees Tw: 4,000 people died from the heat during this heat wave.
For Diane Rainard, an explorer from Réunion and used to expeditions in tropical environments, this kind of heat is comparable to a passage in a hammam or sauna. “You have to stay in it, you have no choice. Now imagine having physical activity there! Faced with these extreme temperatures, heat-related illnesses quickly manifest themselves: cramps from dehydration, heat exhaustion that can lead to fainting, and finally heat stroke, which can be fatal.
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Symptoms of heat stroke are easy to spot: dizziness, weakness, clumsiness and poor coordination, fatigue, headache, blurred vision, nausea, and vomiting. The person does not realize that their temperature is high and the sweating may disappear, a sign of intense dehydration. Without quick cooling measures (for example, taking a cool shower or drinking several liters of water), 80% of people with heat stroke will die, according to MSD Manual.
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Unequal battle against the heat
To protect yourself from such temperature spikes, there are of course a few good things to do. “I think everyone has these reflexes: look for drafts, go to the shade and drink as much water as possible…” explains Diane Rainard. “You can lose up to one or two liters of water per hour! So you have to hydrate as much as possible,” the adventurer insists.
Another important point: make sure you drink water that is sufficiently rich in electrolytes, minerals that help regulate nerve and muscle function and help maintain water balance. Overconsumption of water can lead to hyponatremia, a condition that occurs when the sodium level in the blood is too low. To counter this risk, you can drink sports drinks, or add a spoonful of salt to 3 liters of water before drinking it.
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Stay hydrated, stay cool… Advice that may seem basic, but isn’t yet for everyone. According to the WHO, “residents of low-income or disadvantaged countries and communities” are most affected by extreme heat due to climate change. In India, for example, while the middle class has become accustomed to air-conditioned life, slum dwellers are powerless against such heat waves.
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Difficult to stay cool in houses without windows, sometimes without electricity, or to stay hydrated when access to drinking water is not a given. An unequal battle against heat that will have serious consequences in the coming years: The World Bank’s Groundswell report estimates that by 2050, 216 million people in the developing world will be “climately displaced”, forced to migrate within their country to deal with the consequences escape from climate change.