It’s hot, very hot. In France, the population is experiencing the third most intense heat wave observed since 1947. “Each heat peak has an impact on the health of the population”underlines Catherine Le Gall, head of emergencies at the Argenteuil hospital center (Val-d’Oise), who has welcomed several people suffering from severe hyperthermia in recent days and expects to receive more in the next two weeks. “There is always a latency, because organisms decompensate after several days”specifies the doctor, who is worried to see real heatstroke, sometimes adding to Covid-19 infections.
This heat wave, which reached its peak on Monday July 18, will however probably be less than that of 2003, which caused the death of 15,000 people. The causes of death were then directly related to the heat: heat stroke, hyperthermia and dehydration. How can you die of heat? A brief tour of the mechanisms at work.
It is between 36°C and 38°C that human cells and organs function normally. To be able to maintain this internal temperature, the body is equipped with a thermostat. Temperature-sensitive neurons located at the base of the brain, in the region of the hypothalamus, and a network of receptors located in the skin and muscles constantly monitor thermal variations. When the internal thermometer begins to rise, three mechanisms come into play: intense dilation of the blood vessels located in the skin – where most heat loss takes place –, sweating and a decrease in the production of heat.
Cascading reactions
“To evacuate heat, the most effective mechanism is sweating, which allows water to be evacuated which, by evaporating, will carry thermal calories and thus lower the temperature”, explains Pierre Hausfater, head of the emergency department at the Pitié-Salpêtrière hospital in Paris. Indeed, 1.7 ml of evaporated sweat can evacuate 1 kilocalorie, as the doctor recalled in a summary on heat stroke in 2012. In extreme working conditions, studies show that you can lose up to 12 liters of water a day. Even under more normal conditions, this water reservoir must be filled to maintain the ability to thermoregulate.
But, first difficulty, the sweating mechanism “requires increased blood flow to the skin and therefore to have a very high cardiac output, which cannot be done by the elderly or those having treatments that voluntarily block the acceleration of the heart”, adds the professor of emergency medicine. Another problem is that older people feel less thirsty, even if they are dehydrated. This is why the authorities recommend ritualizing the consumption of water during the day.
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