Low temperatures are more deadly than heat waves. It’s the moderately cool temperatures that kill.
The fatal heatwave of 2003 and its 15,000 deaths left indelible traces in French minds. But it is not the hot weather that kills the most. On the contrary, moderate temperatures, whether hot or cold, are associated with more deaths, concludes a study in The Lancet. Rather, mortality flies when the mercury drops.
“It is often assumed that extreme temperatures are the cause of the majority of deaths, and most studies have focused on the effects of heat waves,” observes Dr. Antonio Gasparrini, lead author of this study.
To verify this, his team analyzed the causes of 74 million deaths that occurred between 1985 and 2012. In order to take into account the variety of climates. They included countries as varied as Sweden, Thailand and Brazil, without forgetting the middle latitudes (United Kingdom, United States, Italy, Japan…).
Constricted blood vessels
Temperature was identified as the cause of death in 7.71% of cases. The proportion varies greatly depending on the country: it represents only 3% of deaths in Sweden, Thailand and Brazil, against 11% in China, Japan and Italy.
But in all cases, it is clearly the cold which is the most responsible for the mortality (7.29%) compared to the heat (0.42%). And extreme temperatures have only a minor role, underlines Dr Gasparrini: “In fact, the majority of these deaths occur on moderately hot or cold days; most deaths are caused by cool temperatures. “
This conclusion was also obtained by a Belgian study. Presented in 2013 at the European Congress of Cardiology (Amsterdam, the Netherlands), it showed that moderate drops in temperature favor myocardial infarctions. Each drop of 10 ° C is therefore associated with a 7% increase in hospitalizations. It is the sudden drops in temperature that are involved, according to the researchers.
During their presentation, they questioned the body’s defense mechanism against the cold. To maintain body temperature, the skin constricts the diameter of peripheral blood vessels. This increases blood pressure, heart rate and blood viscosity, causing a clot to form in a person at risk.
Seasonal expression of the genome
The other explanation may come from a more recent study. She was interested in the activity of genes according to the seasons. The genome of 16,000 people living in the United Kingdom, the United States, Iceland, Australia and The Gambia have been combed through. The results, published in Nature Communications, show that a quarter of genes (5,000 out of 23,000) do not have the same activity in summer or in winter.
One gene particularly caught the attention of the study’s authors: ARNTL. It is much more active in summer than in winter. However, studies in mice have shown that it helps inhibit inflammation. “This is a truly surprising discovery,” said Professor John Todd, the study’s last author. In a way, it’s obvious: it helps to understand why so many diseases (…) get worse during the winter months; but no one has evaluated how much this actually happens. “
Redefining public health measures
The authors of the study Lancet therefore call for a redefinition of public health policies which “focus almost exclusively on minimizing the health consequences of heat waves. The data available in France confirm this. If an Extreme Cold plan exists, just like a Heat wave plan, the figures only mention deaths linked to heat peaks, which vary between 30 and 50 depending on the average temperature of the summer months.
A comment associated with the work, however, calls for caution. “Factors such as susceptibility and resilience were not included in the analysis, including socioeconomic status, age, and air pollutants. Before attempting to reduce the effects associated with temperature, further studies should be carried out on the interaction between temperature-attributable mortality and socio-economic factors, ”write Keith Dear and Zhan Wang of Duke University. Kunshan, China.
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