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The exceptional heat that we’re currently experiencing shows no sign of abating. In fact, the Met Office expects that temperatures on July 27 could reach 37 degrees celsius across the south east of England. If everything comes together, there is a slim chance that the UK could record its hottest ever temperature, exceeding the 38.5 degrees celsius recorded in Faversham, Kent in 2003. And with high temperatures, comes danger.

There's plenty of scientific evidence that supports the fact that excess heat has deleterious effects on our health, causing everything from depression to heart attacks. For instance, the freak heatwave that hit Europe in the summer of 2003 — the hottest summer on record — claimed more than 70,000 lives, mostly older people, across the continent. "I remember visiting France in 2003 when a heat wave killed around 40,000 people," says Alexander Kumar, a clinical fellow at King's College London. "I was initially surprised but since have visited famines, lost crops and hot countries all over the world – excessive heat and heatwaves pose a real threat to global and planetary health.”


In the UK that summer it’s estimated that more than 2000 people died as a result. “In terms of global health security the US department of defence recognises heatwaves as the most deadly type of weather phenomenon, causing physical damage to infrastructure, wildfires and most concerning, human and animal health,” Kumar says.

How does heat affect our bodies?

Thermoregulation is one of our vital physiological functions. Humans are endotherms, which means that various mechanisms keep our core body temperature within a normal range of 37 degrees celsius, regardless of the temperature outside. This thermoregulation is controlled by the hypothalamus, a part of the brain that effectively acts as a biological thermostat via temperature-sensing nerve cells.

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As temperature increases, our body tries to cool down. One way to do that is by secreting water with dissolved ions — sweat. Water absorbs heat so as sweats evaporates it liberates heat by evaporation, that’s why sweat cools people down.



The problem with heatwaves is that excessive heat can put humans under a lot of physiological stress. Because sweating is the most effective mechanism, during hot weather the people who are most vulnerable are the ones whose sweating mechanism is, for various possible reasons, subpar — the elderly, the chronically ill and children. “Elderly, children and pets are most at risk of heatwaves,” Kumar says.

“Alongside having a large body surface area different to adults, children and in particular babies are unable to consider limiting their time in the sunshine and are unable to regulate their oral fluid intake during heatwaves leaving them at great risk of the ill health effects."


People with chronic health conditions, like diabetes and hypertension, are also found to suffer more during a heatwave. "They often have problems with fluid balance and their ability to dissipate heat, often on medications including diuretics, antihypertensive and with infections – requiring careful fluid balance," Kumar says.

If our body reaches temperatures exceeding 40 degrees celsius, then we reach a state where our physiological coping mechanisms are insufficient. Hyperthermia — more commonly known as heatstroke — a medical emergency that can be characterised by symptoms like lethargy, syncope, delirium, seizures. “If for example you are abandoned in a hot desert, this can all help but only for a limited time,” Kumar says. “The physiological compensatory clock starts ticking - if you don’t change your circumstances either by seeking cool shade, rehydration and rest, you risk your life". According to a study, more than 600 people die every year in the United States from heatstroke.

How Public Health England prepares for heatwaves

Public Health England (PHE) estimates that more that 2,000 people died as a possible consequence of the ten-day heatwave that affected England from August 4 to 13, 2003. A year later, PHE published its first Heatwave Plan for England, a nationwide initiative that was intended to prepare and mitigate for future heatwaves.

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Humans can adapt to the heat and throughout a normal summer they became less sensitive to hot temperatures. However, heatwaves are by definition intense and that’s why deaths can typically occur during its first few days. National contingency plans that anticipate heatwaves are therefore crucial. As part of its Heatwave Plan for England, PHE has introduced a Heat-Health Watch System, a colour-coded alarm system designed to implement different contingency plans throughout the country.

Level one (green) is automatically issued on June 1 until 15 September 15, kickstarting what is known as the “Heatwave and Summer preparedness progress”. In case the Met Office predicts a 60 per cent probability of a heatwave within two to three days, level two (amber) is triggered. When the heatwave hits, then we enter level three (orange). As part of this plan, various recommendations are issued. “To beat the heat, try to keep out the sun from 11:00 to 15:00, walk in the shade if you can, apply sunscreen and wear a hat if you have to go out in the heat. Also try to carry water with you when travelling,” Waite says.

Only when temperatures cool back to normal, will the Heat-Health system return to level one. However, if conditions worsen further, level four might be triggered. This constitutes a national emergency and as such can only be issued by the government.

Will heatwaves get worse with climate change?

If carbon emissions are not reduced, things will get a lot worse. In fact, the one degree temperature increase due to anthropogenic activity has already made heatwaves a lot more frequent then they were in the past. According to estimates, the extreme temperatures registered during the 2003 heatwave will, by 2040, constitute normal summer temperatures. By then, heatwaves will be longer, more frequent and more intense.


Humans can physiologically adapt to warmer temperatures, but only to an extent: “In general low 30 degrees is too hot to work (less when it is very humid) and low 40 degrees is too hot to live (without air conditioning),” Says Corinne Le Quéré, director of the Tyndall Centre for Climate Change Research at the University of East Anglia.

Urban areas are usually a few degrees hotter than rural parts — an effect called the urban heat island. This is due to the reduced air flow and absorption of heat by buildings, among other factors. That's why, in the future, rethinking urban design will be crucial, particularly retrofitting building with cool roofs and proper insulation. “We need to regulate construction so that houses don’t trap heat and have proper ventilation," Le Quéré says. “Green spaces are the key because they keep water in the city where it can evaporate and provide some cooling.”

This summer, as the UK continues to bake under unrelenting heat, such radical thinking feels a long way off.