Heat can have an immediate impact. A person’s health can deteriorate rapidly and require urgent medical attention.

Every year, during heat waves, there is an increase in healthcare visits for conditions specifically or largely caused by heat exposure: heatstroke or hyperthermia, dehydration, and hyponatremia.

The most severe cases lead to death or significant long-term complications. During the 2003 heat wave—a severity unmatched since—an excess of nearly 15,000 deaths from all causes was recorded over a 15-day period. Subsequent heat waves were less severe but nevertheless resulted in a very high number of excess deaths from all causes: nearly 2,000 deaths in 2006, 1,739 in 2015, and 1,480 in 2018, 1,924 in 2019, and 2,816 in 2022, with a relative lull in 2017, 2021, 2023, and 2024, despite the implementation of the National Heat Wave Plan as early as 2004 (which became an interministerial directive on the management of heatwave episodes and the ORSEC guide on health management during heatwaves).

In fact, management and prevention measures, regardless of their effectiveness, cannot prevent all heat-related excess mortality.