The heat wave that has been hitting the southern part of France for the past few days has affected nearly 30 million French people. And some more than others. “I no longer sleep at night, I can’t take it anymore,” breathes Christophe, a neo-Bordeaux in his sixties. “I have been living in the South West for two years and I am not getting used to it”.
The just retired explains that he cannot bear these heats. Unlike his son who is doing like a charm, except on the very rare days at over 40°C. How to explain that organisms react so differently?
“We are homeostatic beings”, immediately underlines Dr. Richard Handschuh, member of the MG France union. In concrete terms, humans must maintain a constant temperature, around 37° C. When the outside temperature rises, the body must evacuate excess heat to avoid overheating. Our body has for this “a central regulation system of the internal temperature”, specifies Bruno Megarbane, head of the medical and toxicological resuscitation department at the Lariboisière hospital, in Paris. This is the role of evaporative sweating or vasodilation of vessels. “These phenomena can malfunction in some individuals and this translates into a certain predisposition to heat stroke in the event of a heat wave”, explains Doctor Megarbane.
Extreme ages, i.e. infants or the elderly, correspond to periods of life when the body has difficulty adapting to high temperatures, says the head of department. How to explain it? “The elderly feel very little heat and therefore sweat little, which raises the overall temperature of their body,” explains Dr. Richard Handschuh. The latter are thus more victims of hyperthermia. “Infants, for their part, evacuate heat only through the face” continues the member of the union of general practitioners. Their body temperature rises much faster.
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Another source of inequality comes this time from genetic factors. “Populations that have lived in hot areas for a long time are certainly more resistant,” as in tropical areas, says Dr. Megarbane. This heat resistance is the product of a very long time as it has developed over several generations. “Those who survived when there was no medicine as we know it had natural abilities that allowed them to be better adapted to the environments in which they found themselves,” explains Dr Handschuh.
However, this genetic dimension is not insurmountable. Being regularly exposed to heat can also play a role. “The body develops a greater capacity to withstand” if it is regularly exposed to high temperatures, explains Richard Handschuh. The human body can thus, over a shorter time, adapt physiologically. We can see “an increase in sweating capacity, activation of hormones that adapt water and sun retention in relation to losses or even cardiovascular performance”, explains Dr. Bruno Megarbane.
A final vulnerability factor concerns the medical condition of individuals. “People with comorbidities or even chronic diseases are more vulnerable because their physiological adaptation system is less able to make the changes necessary for their body to lose heat,” explains Dr. Bruno Megarbane. The same goes for people who use drugs or alcohol.
Taking certain medications can also penalize certain organisms in the face of high heat. “Some drugs block sweating like neuroleptics, antidepressants, antiparkinsonians,” says Dr. Megarbane. And they are not alone. Medications for heart patients, which slow down the heart, or even diuretics endanger organisms in the event of high heat because they modify adaptation to hyperthermia.
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Drugs that slow the heart can “decrease the mechanism that allows sweating and dilation of vessels in the skin and sweating,” explains Richard Handschuh. Because to evacuate the heat, “the body increases the heart rate to vasodilate the vessels”. Diuretics, on the other hand, have dehydrating effects on the body. “People on diuretics can find themselves dehydrated because they evacuate more liquid through the urine,” he says. In hot weather, it may therefore be “advised” to those concerned to reduce the intake of these drugs, under the supervision of a doctor.
For scientists, “adapting the way of life” thus becomes the watchword to follow. Concretely, it is a question of avoiding physical activities in full sun, of preventing as far as possible the isolation of the most vulnerable, of building more ventilated housing or, if possible, of equipping oneself with a air conditioner. “In the early 2000s, people didn’t have all these practices in mind,” recalls Dr. Megarbane. Before adding: “Since this period, we have very clearly strengthened individual defenses against heat stroke”. More generally, in the face of this type of temperature, everyone is invited to be cautious.
