Around 30 years ago, cold weather was associated with increased mortality from respiratory diseases. Winter was essentially a period of risk and summer could be a relief. But now the circumstances have changed and that’s not necessarily true, according to a new study.

A group of researchers looked at the deaths linked to respiratory diseases in Spain from 1980 to 2016 and found that the seasonality of temperature-attributable mortality from respiratory diseases has shifted from the coldest to the hottest months of the year.

The proportion of deaths from respiratory diseases in the colder months has decreased on average by 16.5% per decade, while that of hot weather has remained stable, the findings showed. In the 1980s, deaths from respiratory diseases were more frequent in January and December, but now the peak is in July and August.

According to the findings, the decrease in temperature-attributable mortality during the winter months is not due to the growing temperatures because of climate change. Instead, this can be explained due to the adaptation of the population to the lower temperatures in Spain.

“This is a complete reversal of the seasonality of mortality,” said Hicham Achebak, researcher at Barcelona’s Global Health Institute and lead author. “The use of heating is more widespread than that of air conditioning, in addition to clothing you can fight the cold and it is difficult to do the opposite in summer.”

In the study, the authors linked adaptation to the cold to the social transformations experienced in Spain since 1980. GDP per capita has grown from 8,789 euros in 1991 to 22,813 and spending on the health system has increased from 605 euros per capita to 2,182.

This economic growth has also been reflected in the percentage of homes with air conditioning, which has gone from 4.16% in 1991 to 35.5% in 2008, and for central heating, from 15.83% in 1991 to 56.86%.

The study’s results, based on data from more than 1.3 million deaths, show that “the main problem is no longer winter and measures to adapt to hot temperatures will have to be promoted,” said Achebak. In addition, data suggest that high-temperature respiratory problems affect women more.

Although the authors of the work have not investigated the mechanisms that can explain this difference, Achebak highlighted “physiological studies that point to differences such as that women sweat less than men and sweat is a way to evacuate heat.” He also mentioned that there’s an older female population in Spain that could explain the higher female mortality.

The study is also important in the context of the coronavirus in the coming months, with Achebak pointing out that “if next winter there is higher mortality, it will be due to the specific effect of coronavirus infection, not because of the cold, which will not be an additional risk factor”.

“Deaths attributable to hot or cold temperatures are caused by a combination of exposure to extreme temperatures and the vulnerability of the population,” explained Joan Ballester, co-author of the study. “Reducing this vulnerability may require policies associated with socioeconomic development, such as those aimed at improving health services.”

The study was published in the journal Nature Communications.