Over the last half-century, the probability of heat extreme events has changed by orders of magnitude in almost every region of the world, with occurrences that are now up to a hundred times more in respect to a century ago. Of all-natural disasters, extreme high temperature events are the main cause of weather-related mortality and they are also expected to be the main factor responsible for additional deaths due to climate change in the coming years.
In cities, the heat island effect creates higher temperatures than in vegetated areas. But conditions within urban areas are not equal in all their parts - either due to their physical form or to the specific needs or vulnerabilities of inhabitants - therefore not all districts of a city are equally vulnerable to heatwaves. Thus, identifying those areas which are particularly vulnerable to heat stress is particularly important to implement interventions at local level aimed at improving the capacity to cope with the impacts of heat waves on citizens' health.
The literature review "The heat-health nexus in the urban context: A systematic literature review exploring the socio-economic vulnerabilities and built environment characteristics", published on the Journal "Urban Climate" and conducted by the CMCC Foundation in collaboration with Ca' Foscari University of Venice, aimed at exploring which vulnerability factors determine the nexus between the heat and the health outcome in a urban context. The analysis selected forty articles from the vast literature on the subject, extracted from two well-known databases of peer-reviewed literature (Scopus e PubMed).
"It was central to our research to consider interdisciplinary areas that rarely coexist together in the same analysis" explains Marta Ellena, CMCC researcher and lead author of the study. "There are many studies in literature that investigate which characteristics can influence the vulnerability of individuals to heat stress, considering physical and mental health, demographics, social and economic status. In this analysis, we have added to these factors also the built environment characteristics, because the temperature-mortality relationship does not occur in a territorial vacuum. Rather, it is 'embedded' within the urban fabric, according to the context-specific way natural, physical and socio-economic processes interact."
Through the concept of "enhanced exposure", the study notes how different aspects of the physical environment can exacerbate (or mitigate) climate impacts within different places across cities.
"The population exposure is certainly linked to the physical exposure of the district to heat. The built-up areas within the cities collect solar energy during the day and release it during the night. Therefore, the urban contexts heat up and stay warm much more than the surrounding green areas, even during the night. This happens to a more or less severe extent based on their shape and design" says Margaretha Breil, urban planner and researcher at CMCC. "But we cannot consider only the physical exposure: alongside this phenomenon, known as "heat island", there are other conditions that can make a context more difficult to live in, and even more deadly".
As emerges from the study, social disadvantage can further intensify the exposure to the heat risk. Research cited in the paper found that mortality associated with heatwaves is higher where there are high levels of crime and low social cohesion. On the other hand, it was found to be less for communities characterized by extended family ties that allow for mutual care rather than forcing isolation.
"The quality of life in cities is not only determined by the shape of the urban space, but also by its accessibility. If there is access to a green area, citizens' quality of life improves. And this is true both during a pandemic, as we are all seeing in this period, and during a heatwave" explains Breil. "Nevertheless, if the green area is place of crime or heavy traffic, it is as if it did not exist: those who are afraid to go out or do not find pleasure in going out stay at home during a heatwave. And at home, the most vulnerable can even die".
As the authors highlight, understanding of these aspects and aggregate them into heat vulnerability indices could be crucial to identify and implement efficient social and physical infrastructure measures using ad hoc spatial planning considerations and urban governance decisions.