Sleeping well at night is problematic in many cities with warm summer climates. Nighttime temperatures drop little compared to daytime ones, and breezes diminish or disappear. Several reports, such as those by Lancet Countdown, have already warned of the effects of high temperatures on human health. Recently, the Climate Change and Health Reference Unit of the Carlos III Health Institute (ISCIII) published a study revealing that the effects of the so-called urban heat islands, added to climate change and pollution generated by traffic and industry, increase the risk of hospitalizations and deaths in cities and their surroundings.

Chilean architect Cristina Huidobro, a member of Arsht-Rock, the Rockefeller Foundation’s Center for Resilience Studies, describes this heat as a “silent killer” due to the lack of medical protocols to identify deaths caused by it. The expert assures that this effect of climate change causes more deaths than hurricanes and floods and also stresses that it is one of the most avoidable disastrous factors if action is taken efficiently in large cities.

According to a study by Professor of Physical Geography Javier Martín Vide and the Climatology Group of the University of Barcelona, in the current context of climate change, the urban heat island has ceased to be a rare local phenomenon. It has become a new meteorological risk during heat waves.

Cities with high population density and larger sizes tend to form more intense heat islands than smaller or less densely populated ones. Some data from the most significant cities corroborate this: temperatures in the center of Paris and London can be up to 2-3°C higher than in the surrounding rural areas. At the same time, Los Angeles is known to have one of the most intense heat islands in the world: temperatures in the city center can be over 5°C higher than in adjacent areas.

Urban typology is a determining factor. North American city centers, with a higher concentration and height of buildings and fewer trees, generate more significant heat islands than European cities, which tend to have more dispersed buildings and more vegetation in the streets.

In addition to health, studies and research indicate that periods of extreme heat significantly impact water pollution. Runoff, caused by rain, washes pollutants accumulated in streets and buildings. Many of these come from the chemical decomposition of certain substances by heat, such as plastics, some paints, organic residues, and asphalt that can release volatile organic compounds.

If the asphalt in a city like Los Angeles can reach 75ºC and concrete 65ºC, the grass temperature does not exceed 42ºC. Architectural studies have shown that planting trees around a house can reduce the use of air conditioning by 50%. In tall buildings in cities, achieving such a reduction is impossible, but it has been shown that the taller and leafier the trees are, the more they influence energy savings for cooling. In addition to absorbing carbon dioxide (CO2), the vegetation cover mitigates the heat island effect by providing shade and cooling through plant evapotranspiration.

Three percent of the planet’s land surface is occupied by cities, which become vital players in adaptive processes. The more than 6.5 billion people living in cities in 2050 will have to cope with the effects of heat islands that can seriously affect their health and worsen environmental pollution.