🌡️ In Paris, a link between vegetation and mortality during heatwaves

In Paris, the most vegetated districts show a lower risk of death during periods of intense heat, while highly mineralized areas, with few green spaces and strong urban heat island potential, are the most exposed.

This is the finding of an international study conducted by Inserm, the Barcelona Institute for Global Health (ISGlobal), and the London School of Hygiene & Tropical Medicine, published on January 27 in the journal npj Urban Sustainability, based on data collected over nearly ten years, from 2008 to 2017. The scientists emphasize the importance of developing urban vegetation and better distributing green spaces between neighborhoods to protect the health of Parisians in the face of climate change.


Due to the urban heat island effect, Paris is particularly vulnerable to heatwaves, whose intensification is expected due to climate disruption caused by human activities. The capital is even among the European cities with the highest risk of heat-related excess mortality: when temperatures are above normal, mortality is multiplied by 1.6 in people over 85, according to a study published in The Lancet in 2023[1].

To better understand risk factors and identify solutions to make the city more resilient, a research team compiled, district by district, data on heat-related excess mortality, cross-referencing it with built environment and urban characteristics (vegetation, heat islands) as well as socio-economic indicators.

The study, published on January 27 in the journal npj Urban Sustainability, is based on data collected over nearly ten years, from 2008 to 2017.

A first finding, already well-documented in scientific literature[2], is confirmed: green spaces have a protective effect against heat-related excess mortality. The presence of trees, parks, gardens, or green roofs is indeed associated with lower temperatures in the capital.

But not all green spaces offer the same protection. The scientists emphasize that trees are significantly more effective than lawns in mitigating the risk of death during intense heat episodes, thanks to the shade they provide and evapotranspiration.

Furthermore, scientists observed that the cooling effect of vegetation only partly explains its protective effect on the health of Parisians: "Although the exact mechanisms still need to be explored, key factors such as reduced pollution and improved physical and mental health could play a determining role," explains Hicham Achebak, a researcher at Inserm and the study's first author.


Above all, the study reveals strong inequalities within the capital. More than the socio-economic level of residents, it is age and building characteristics (those built before the 1970s being less well insulated), as well as the presence of green spaces, which varies from 1.4% in the 2^nd district to 20.4% in the 13^th, that determine the risk of recording more deaths during periods of intense heat.

The most vegetated districts are those where the risk of heat-related death is lowest. Conversely, the most mineralized areas, poor in green spaces, show increased vulnerability. "This statistical reality highlights a clear divide between the center of the capital, more subject to the urban heat island effect, and the peripheral districts, better protected by their green spaces," the researcher emphasizes.

The study also highlights a counter-intuitive result: "Contrary to common belief, the wealthiest districts of Paris are, on average, more vulnerable to heatwave-related excess mortality. This phenomenon is explained by a low presence of green spaces, combined with dense, old, and mineral-built environments in these neighborhoods," explains Hicham Achebak.

Using modeling, the scientists then attempted to estimate how many deaths could be avoided by continuing to develop green spaces in the capital.

"The greenest districts of Paris currently have about one-fifth of vegetated areas. According to our calculations, if all districts reached this level, heat-related mortality could decrease by about one-third, both during moderate heat periods (temperatures above 22 °C) and during the most intense heatwaves (above 25 °C), explains Grégoire Rey, former director of the Inserm Center for the Epidemiology of Causes of Death (CépiDc), and the study's last author. These estimates are based on modeling and should be interpreted with caution, but they reflect the crucial protective role of vegetation that we observe."

Beyond efforts to vegetate the capital, several complementary levers are identified by scientists to adapt Paris to intense heat: the use of reflective materials for roofs and roadways, improving the energy efficiency of buildings to limit heat emissions (particularly from air conditioning), and reducing automobile traffic, which would decrease both residual heat and air pollution.

"These results underscore the importance of developing urban vegetation and better distributing green spaces between neighborhoods. Strengthening the place of nature in the urban environment thus appears as a major lever to protect the health of Parisians in the face of climate warming," concludes Hicham Achebak.

Notes:

[1] Masselot, P., Mistry, M., Vanoli, J., Schneider, R., Iungman, T., Garcia‑Leon, D., et al. (2023). Excess mortality attributed to heat and cold: A health impact assessment study in 854 cities in Europe. The Lancet Planetary Health, 7(4), e172-e178. https://doi.org/10.1016/S2542-5196(23)00023-2

[2] Hartig T, Mitchell R, de Vries S, Frumkin H. Nature and health. Annu Rev Public Health. 2014;35:207-28. doi: 10.1146/annurev-publhealth-032013-182443. Epub 2014 Jan 2. PMID: 24387090.