Exposure to unusually hot or cold temperatures is associated with an increased risk of mortality. In light of climate change, the rapid shift in temperature patterns in France, and the implementation of policies to mitigate the effects of extreme temperatures, it is important to investigate whether the impact of temperature on mortality has changed over time.

In this context, Santé publique France has published an analysis of data dating back to 1970 on the evolution of the relationship between temperature and mortality in France. The results suggest that the population may be acclimating to a new climate, likely linked to improvements in socioeconomic and medical conditions. Although the risks associated with extreme heat have decreased, the days on which such temperatures are observed are more frequent, and thus lead to a greater impact on mortality. These results underscore the importance of continuing efforts to reduce the impact of cold and hot temperatures on mortality and of taking action to mitigate climate change and thereby avoid the most adverse scenarios.

Cold or hot: Unusual temperatures have a direct impact on our health

Unusual cold or hot temperatures have a direct effect on health, particularly in terms of mortality and healthcare utilization. In France, a “J”-shaped curve is observed between temperature and mortality: very cold temperatures are associated with a moderate increase in the risk of death, while very hot temperatures are associated with a very significant increase in this risk. As for moderate temperatures, they are associated with a low risk of death.

Climate change is resulting in a shift in temperature distribution toward higher values and greater variability. Adapting to this new climate has become a major public health issue.

While climate changes are already noticeable in France, are we seeing a parallel shift in the effect of temperature on mortality?

Santé publique France examined 45 years of daily temperature and mortality data in 18 mainland cities to answer this question. The results highlight contrasting trends in risk depending on temperature levels:

• an extremely cold average temperature (-7°C) is associated with a lower risk of death in the 1970s than in the 2010s,

• an extremely hot average temperature (+28°C) is associated with a higher risk of death in the 1970s than in the 2010s,

• for non-extreme temperatures, risks remain stable over the period.

These changes are gradual over time. They suggest a possible acclimatization of the population to a new climate and also reflect an improvement in socioeconomic and medical conditions.

However, changes in risk do not translate into impacts:

• the number of deaths attributable to the coldest temperatures has remained stable over time, accounting for approximately 0.6% of total mortality each year;

• the number of deaths attributable to the hottest temperatures has doubled since the 1970s, now accounting for 0.2% of total mortality in the 2010s.

In other words, although the risks associated with extreme heat have decreased, the days on which these temperatures are observed are more frequent, and thus lead to a higher mortality burden.

Reducing the impact of cold or hot temperatures in France: what are the proposals?

These findings underscore the importance of continuing efforts to reduce the impact of cold and hot temperatures on mortality. They also call for action to mitigate climate change and thus avoid the most adverse heat-related scenarios.

Prevention of extreme temperatures and climate adaptation must be based on three pillars:

• improving the overall health of the population,

• taking action on living environments and work organizations to reduce exposure,

• and continuing to organize a coordinated response to the most extreme events.