Comment to article: Cuerdo-Vilches, J. Díaz, J.A. López-Bueno, M.Y. Luna, M.A. Navas, I.J. Mirón, C. Linares, Impact of urban heat islands on morbidity and mortality in heat waves: Observational time series analysis of Spain’s five cities, Science of The Total Environment, Volume 890, 2023, 164412, ISSN 0048-9697, (


What is known about the subject?

 The Urban Heat Island (UHI) is a phenomenon that occurs in urban areas as they experience higher daily minimum temperatures than their surrounding rural areas. This effect happens for various reasons. First, because of the construction materials used in the building of our cities, which generally have a higher energy storage capacity (thermal inertia) absorbing energy from the sun and from human activity and gradually releasing it and affecting particularly night-time temperatures. The colour of the building materials can usually influence the daily variation in the surface temperature. Dark colours prevail, reducing the capacity to reflect this solar radiation. Second, the anthropogenic activity of the population is a determining factor of urban warming. This is conditioned by the use of fossil fuels, road traffic and the use of air conditioning in summer. This accumulated heat does not dissipate properly and overheats the city, especially during heatwaves. The UHI effect is often linked to negative impacts on population health. Therefore, it is important to define in what way do UHIs affect mortality and heat-attributable emergency hospital admissions, or whether UHIs rather affect the well-being of people.


What does this study add to the existing literature?

The purpose of this analysis was twofold. First, to analyse how UHIs affect both daytime (maximum) and night-time (minimum) temperatures. Second, to identify which of these temperatures recorded had a greater impact on the health of the population. With this aim, data was collected from the meteorological observatories of the State Meteorological Agency (AEMET), for urban and non-urban areas of five densely populated Spanish cities, in the period 2013-2018. The occurrence of UHIs and heatwaves, and the duration and number of events were included as dependent variables. Meteorological, geographical, population-based, air pollution and control variables were also added to the analysis. GLM and mixed models were conducted using statistical software.

Different UHI intensity depending on the city

The UHI intensity is quite different depending on the city, much higher in coastal cities (with mean values of 4.1 ºC in Valencia, compared to 1.2 ºC in Murcia). This difference is also reflected in their disparate impact on health. In coastal cities, both the UHI effect and the thermal impact on health are associated with the daily minimum temperatures. Therefore, the UHI is associated with morbidity and mortality in coastal cities, more accentuated in Barcelona, Valencia and Malaga, and not in inland cities such as Murcia and Madrid, where temperature impacts on health are due to their daily maximum values.

What are the implications of the findings?

Each city is different and the UHI effects on health need to be addressed at the local level. The most appropriate indicators (daily maximum temperatures for inland cities and daily minimum temperatures for coastal cities) are used to define the epidemiological heat wave. There is no universal recipe for tackling the impacts of climate change. Vulnerability is defined locally. The definition of a heat wave must take these features into account, as well as adaptation measures tailored to local needs. Therefore, studies at the local level are needed to provide policy makers with tools to implement appropriate public policies and plans involving effective urban interventions towards the mitigation and social adaptation to climate change.





Teresa Cuerdo Vilches
PhD in Architecture, researcher
Eduardo Torroja Institute for Construction Sciences (IETcc), CSIC