Monitoring of meteorological characteristics and the impact of heat waves, particularly on all-cause mortality, has been conducted in France since 2004, under an interministerial directive on healthcare management related to heat waves in metropolitan France (formerly the National Heat Wave Plan). We have also conducted a retrospective analysis of heat wave events dating back to the 1970s. This historical data has helped us identify rapidly changing trends in heat wave characteristics since 2015, which in turn informs decision-making and policy regarding prevention and warnings. In recent decades, heat waves have been observed earlier (early June) or later (early September) in the season, with exposure occurring in school and work environments. Every department now experiences heat waves, with some enduring them several times in the same summer; this can lead to a form of risk trivialization and a reduction in the adoption of prevention measures. For instance, 65% of the regional heat waves between 2003 and 2019 occurred after 2015. Finally, although no event can be compared to 2003 in terms of geographical extent and duration at the national level, temperatures higher than those seen in 2003 are increasingly being observed in some counties. Indeed, Météo-France—the national meteorological service—issued its highest meteorological warning level (“red”) for the first time in 2019 and again in 2020.

These trends are having an ever-increasing impact on health. Since the plan was activated in 2004, more than 8,000 excess deaths have been recorded during heat waves, with 5,700 deaths between 2015 and 2019.

All heat wave data are available in Geodes, Santé Publique France’s cartographic platform presenting epidemiological health data (heat wave determinants). Furthermore, Santé Publique France produces an indicator of population exposure to heat for the National Observatory of Global Warming Effects.

In this article, we wanted to place a strong emphasis on the day-to-day operation of the heat wave and health alert system.

The heat wave alert is unique because it uses environmental forecasts from Météo-France to anticipate and prevent adverse health impacts. Prevention is possible by urging numerous on-the-ground actors to act quickly before a heat wave arrives and its effects are felt. This requires a close working relationship with these actors, involving daily exchanges between Météo-France, Santé publique France, and the Directorate-General of the French Ministry of Health at the national level. At the regional level, it involves communication between the regional units of Santé publique France throughout the country and regional health agencies.

During the summer, heat wave alert monitoring, health surveillance, and prevention require collective multidisciplinary mobilization from Santé publique France’s employees and collaborators, including those working in environmental health, occupational health, prevention and health promotion, alert monitoring, communication, and information systems.

The regional units play a key role not only in analyzing health data at the local level, but also in relaying information to their Regional Health Agency. At the end of the summer, national and regional records of the health impact of heat (and heat waves, if they occur) are published on Santé publique France’s website: these data are used to make regular adjustments to the surveillance system. Examples of these adjustments include extending the surveillance period until September 15, changes to the red alert criteria, and the semi-automation of health data analysis.

The social marketing strategy has also frequently evolved to progressively reach not only all individuals vulnerable to heat, but the entire population. This is achieved through a variety of communication channels (TV, radio, digital platforms, posters, etc.). New communication tools have been developed for this summer, with the primary goal of raising awareness that everyone is affected by heat waves and their potential consequences. See Heat Waves and Extreme Heat: Preventing Risks – Santé publique France.

In 2020, the COVID-19 pandemic also had to be taken into account in the health surveillance risk analysis; more specifically, we assessed whether heat protection recommendations were compatible with COVID-19 management measures.

An abnormally high mortality rate was observed during the 2020 heat waves among those aged 45 and older; this was particularly true in Northern France, where a “red” alert status was triggered. It is possible that heat vulnerability was exacerbated by the pandemic¹.

The increase in the frequency and intensity of heat waves in Europe—as documented by the IPCC (Intergovernmental Panel on Climate Change) and Météo-France—is one of the most striking visible examples of climate change. Furthermore, climatologists now have tools that can determine whether a given heat wave is the result of climate change or not. These same tools show that heat waves since 2015 would have been less intense (for instance, 1.5 to 3°C less intense in 2019) and, in some cases, even impossible (for instance, the late episodes of 2016) had there been no global warming.

In addition to changes in heat wave characteristics, climate change also leads to a shift in temperature distribution throughout the year and a marked increase in summer temperatures. Santé publique France collaborates with the international MCC Collaborative Research Network on climate and health studies. The MCC recently evaluated for the first time the current influence of climate change on heat-related mortality. The analysis³ was conducted in 732 cities by combining climate and epidemiological models, and 18 French cities were included. Results show that approximately 37% of heat-related deaths between 1991 and 2018 were attributable to climate change. The same estimate was found for France. This confirms that climate change is already having measurable negative effects on health. Projections for the future indicate more frequent, longer, and more intense heat waves. Furthermore, an increase in heat-related mortality is expected, especially if global temperatures rise by more than +2°C.

All these findings underscore the need for a systemic transformation so that we can both learn how to live with our new climate (adaptation) and take the necessary measures to limit warming (mitigation). Such a transformation would have a positive impact on health. Moreover, since 2020, Santé publique France has been developing an approach to raise awareness of these issues among the general public and political decision-makers through targeted advocacy activities. The agency is also currently developing another strategy to promote heat adaptation to complement the dissemination of prevention messages during heat waves, and to encourage meaningful, long-lasting, and equitable actions, particularly in urban environments, in built-up areas, and regarding workplace health. These initiatives are also shared at the international level, with extensive feedback exchanged among international public health agencies, the International Association of National Public Health Institutes (IANPHI), and the World Health Organization (WHO).