Evolving heat waves characteristics challenge heat warning systems and prevention plans

Surveillance of meteorological characteristics and the impact of heat waves, particularly on the all-cause mortality, has been performed 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 since the 1970s. This historical reference has helped us identify rapidly changing trends of heat wave characteristics since 2015, which in turn informs decision making and policy on prevention and warnings. In recent decades, heat waves have been observed earlier (early June) or later (early September) in the season, with exposure in scholastic and work environments. Every county (département in French) now experiences heat waves, with some enduring them several times in the same summer; this can lead to a form of risk trivialization and to a reduction in the adoption of prevention measures. For instance, 65% of the regional heat waves between 2003 and 2019 happened after 2015. Finally, although no event can be compared to 2003 in terms of geographical extension and duration at the national level, higher temperatures than those seen in 2003 are increasingly observed in some counties. Indeed, Météo-France - the national meteorological service - triggered its highest meteorological warning level (‘red’)  for the first time in 2019 and again in 2020. 

These evolutions are having an ever-growing impact on health. Since the plan was activated back in 2004, more than 8 000 excess deaths have been registered 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 the article, we wanted to place a great deal of emphasis on the daily functioning of the heat wave and health alert system. 

The heat wave alert is very specific because it uses environmental projections by Météo-France to anticipate and prevent adverse health impacts. Prevention is possible by urging numerous field-based actors to act quickly before the arrival of a heat wave and its effects. This necessitates a close working relationship with these actors, with 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 requires collective multidisciplinary mobilization from Santé publique France’s employees and collaborators, including persons working in environmental health, work health, prevention and health promotion, alert monitoring, communication and information systems.

The regional units play a key role not only in the analysis of health data at the local level, but also in transmitting messages 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 the lengthening of the surveillance period until the 15th of September, changes to the red alert criteria, and the semi-automatization of health data analysis. 

The social marketing strategy used has also frequently evolved to progressively reach not only all persons who are heat vulnerable, but the whole population. This is done by varying communication media (TV, radio, digital devices, posters, etc.). New communication tools have been developed for this summer, with the primary goal of raising awareness about the fact that everyone is concerned by heat waves and their potential consequences. See Canicule et fortes chaleurs : prévenir les risques – Santé publique France.

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

An abnormally high mortality rate was observed during 2020’s heat waves for those aged 45 years old and over; this was particularly true in Northern France where a ‘red’ alert status was triggered. It is possible that heat vulnerability was aggravated by the pandemic¹. 

The rise in the frequency and the intensity of heat waves in Europe - as documented by the Giec (Group of intergovernmental experts on climate change) and Météo-France - is one of the most emblematic visible examples of climate change. Furthermore, climatologists now have tools which can determine whether a given heat wave is the result of climate change or not². These same tools show that heat waves since 2015 would have been less intense (for instance, from 1.5 to 3°C less intense in 2019) and, in some cases, even impossible (for instance, the late episodes from 2016) had there been no global warming.

In addition to changes in heat wave characteristics, climate change also leads to a modification of temperature distribution throughout the year and a marked increase in summer temperatures. Santé publique France cooperates works 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 carried out 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 the result of 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 highlight more numerous, longer and more intense heat waves. Furthermore, an increase in heat-related mortality is to be expected, especially if global temperatures rise by more than +2°C.

All these findings highlight the necessity of a systemic transformation in order for us to both learn how we live with our new climate (adaptation) and to take necessary measures to limit warming (attenuation). Such a transformation would have a positive impact on health. Moreover, since 2020, Santé publique France has been developing an approach to increase the general population’s and political decision makers’ awareness of these issues, by targeting specific advocacy activities. The agency is also currently developing another strategy to promote heat adaptation to complement the diffusion of prevention messages during heat waves, and to encourage meaningful, long-lasting and equitable actions, particularly in urban environments, in built-up areas, and in terms of health in the workplace. These actions are also shared at the international level with a great deal of feedback between international public health agencies, the International Association of National Public Health Institutes (IANPHI), and the World Health Organization (WHO).