Since Sunday, June 27, the northwestern United States and western Canada have been trapped under a heat dome, imposing extreme temperatures of up to 49.5 ° C, recorded Tuesday in Lytton (British Columbia) . From the start of the week, sudden deaths have multiplied in the Vancouver region, authorities announcing that at least 486 people have succumbed suddenly since Friday in the Canadian province, about three times more than the average over a period comparable.
These deaths are qualified as sudden because they occur “From the first day of the heat wave”, Explain Rémy Slama, environmental epidemiologist and director of the public health institute at the National Institute of Health and Medical Research (Inserm). While these heat waves are increasing as climate change intensifies, the epidemiologist believes that the global rise in temperatures will have a negative impact on our health and, ultimately, on mortality.
Do these “sudden deaths” in Vancouver, reported by local media, literally mean that people are dying of heat?
It has been known for decades that high temperatures increase mortality. This effect is essentially short-term, that is, from the first day of the heat wave, mortality increases. It is in this sense that it is correct to say that it is a “sudden” mortality. It will be more or less “visible”, without detailed scientific analysis, depending on the size of the heat wave, its duration and maximum daytime and nighttime temperatures.
All these excess deaths, attributable globally to a heat wave, however, are not necessarily attributable to the heat wave at the individual level. Indeed, it is not the mention “hot stroke” which appears on the death certificates, because in general the doctor identifies a physiological cause, responsible for the death but which would have remained benign without the additional blow caused by the death. heat.
To protect itself from high temperatures, the body has developed a certain number of mechanisms: sweating, dilation of heat by radiation with the blood capillaries or relaxation of the muscles, with the aim of promoting the release of energy and lowering the temperature. body temperature. But these mechanisms are very expensive in water and energy for the body. For the image, it is a little as if the body was constantly running long distance. The body gets very tired, especially when there are pre-existing pathologies.
So, the immediate cause of death can be a failure of the affected organs (kidneys, heart), respiratory problems, or even more indirect mechanisms that influence the risk of suicide, probably through effects on the nervous system. It is a situation similar to the presence of a pollutant which would have several targets in the organism.
The phenomenon of exhaustion and fatigue of the body may accumulate, as the heat wave spreads. The longer the heat wave lasts, the less opportunity people have to rest and cool off and the greater the impact will be at the population level.
Can bodies then get used to the higher temperatures?
This has been studied in some cities like New York where, at a given temperature, the heat was much more damaging at the beginning of the 20th century.e century than at the beginning of the XXIe. There is, however, very little chance that this observation is linked to a physiological adaptation of New Yorkers, but more likely to a societal adaptation to higher temperatures.
In the United States, the development of air conditioning has resulted in city dwellers spending much less time in the outdoors. Thus, they tolerate high temperatures better, but it is indeed a phenomenon of societal adaptation, which also poses a major ecological problem. In New York, this adaptation to climate change was made with the development of individual air conditioning, which is particularly polluting and has deleterious effects on the ozone layer and greenhouse gases, thus ultimately accentuating global warming. The adaptation model of a city like New York is therefore not at all transposable to the Earth scale.
In any case, it is not obvious that the organism adapts to higher temperatures, whether on the scale of a lifetime or that of a small number of generations, or even of several tens of years.
In the longer term, what effects will global warming have on our health?
It is obvious that heat waves have effects on our health, but in order to understand climate change, two difficulties must be considered. The first is that there is also an adverse effect of cold temperatures; it is even major on mortality. So the consequence of global warming will be, depending on the area, either to increase the number of very hot days, or to decrease the number of very cold days. And in the latter case, global warming would therefore ultimately have a beneficial effect since these areas would have fewer very cold days, without still having very hot days, thus mechanically reducing mortality. The second difficulty to take into account is that it is not easy to attribute with certainty this or that heat wave to climate change, even if it is now certain that some of these heat peaks are linked.
That said, predictions do exist and estimate that from the south of France – and more generally from the south of Europe – climate change and its mechanisms on temperature will cause excess mortality in summer, which will not be fully compensated for. by the decrease in mortality in winter. It is obviously not the same in all areas of the Earth. Other risks also exist, such as more frequent and violent fires, the phenomena of storms and hurricanes, the effects linked to changes in the living areas of vector-borne diseases – for example areas where the West Nile virus [virus du Nil occidental] or Lyme disease is endemic – or the consequences of dysfunctions in transport systems or energy supply, which would be disrupted by heat.
As for health in itself, the rise in temperatures does not create disease or dysfunction in the human organism, but its harmful effect already strongly affects people suffering from pre-existing pathologies.