Although more people die in winter than in summer, including from infectious diseases, heat also kills.
This is suggested by the horror reports from southern Europe, according to which thousands have already fallen victim to the extreme temperatures this summer.
However, such information is not unproblematic – figuring out exactly how many people died from “heat death” is a statistical challenge.
Johanna Kuroczik
Editor in the "Science" department of the Frankfurter Allgemeine Sunday newspaper.
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A direct effect of extreme heat would be dehydration.
The body regulates its temperature by sweating.
If it is very warm and not enough is drunk, the affected person loses a lot of fluid.
This leads to a shift in the concentration of blood salts, which puts a strain on the organs, and possibly not enough oxygen gets there.
Other ailments are aggravated indirectly by heat, such as respiratory or cardiovascular diseases.
For a study in The Lancet in September 2021, researchers used data from the Global Burden of Diseases study to evaluate 65 million deaths in nine countries.
They came up with 17 causes of death associated with extreme heat.
In Germany, the Federal Statistical Office analyzes the causes of death.
The number of deaths caused by dehydration, for example, has increased eightfold since 2000 and was almost 3,300 people in 2020, according to the data on the topic "summer".
However, the victims of "damage from heat and sunlight" are listed in the nationwide cause of death statistics.
"However, this only reflects a very small proportion of the deaths (partly) caused by heat," explains Werner Brachat-Schwarz from the State Statistical Office in Baden-Württemberg.
Doctors rarely write "heat damage" as the cause of death on the death certificate.
In the years 2018 to 2020, the number of deaths from heat increased
Scientists led by Claudia Winklmayr and Matthias an der Heiden from the Robert Koch Institute based their evaluation of heat-related mortality in Germany between 1992 and 2021, which was published in the "Deutsches Ärzteblatt" at the beginning of July, on data on excess mortality, correlated with the temperature.
Winklmayr emphasizes that these are always estimates.
In the summers of 2018 to 2020, the high temperatures led to a statistically significant number of deaths, 8,700 in 2018, 6,900 in 2019 and about half as many in 2020. In the past cool summer, however, there was no heat-related excess mortality.
For this information it is necessary to estimate how many people would have died in the periods without particular heat.
With the help of this hypothetical background mortality, it can be determined at what point heat becomes lethal.
"The temperature threshold above which an increase in the mortality rate can be observed differs in the age groups considered," says Claudia Winklmayr.
In the past three decades, the number of deaths from heat has increased, which is related to the aging population.
However, the scientists also saw a faint hint of declining heat mortality.
It is believed that this could be due to people's heightened risk awareness.
One problem, however, is that the number of heat-related deaths in Germany is not continuously and uniformly recorded.
In some federal states such as Baden-Württemberg, the state statistical offices take care of this.
Here, deaths from hot summers were compared to those from cooler ones.
In the pandemic years, however, no statement can be made due to the additional corona deaths that are independent of the heat, says Brachat-Schwarz.
Other federal states do not record the number of heat deaths at all.
It's different abroad: In Spain, for example, these estimates are updated daily online in the "Panel MoMo".
Since April, around 2,500 people have fallen victim to the heat, around 1,600 of them were older than 85 years.
The “Global Burden of Disease” study by the specialist magazine “The Lancet” calculated that 356,000 deaths in 2019 were due to the heat worldwide.
However, the methodology is not uniform internationally either.