On a hot summer afternoon, the centre of a large city can be several degrees warmer than the countryside around it. This difference, known as the urban heat island effect, occurs because asphalt, concrete and dark roofs absorb sunlight during the day and release the heat slowly at night, while traffic, air-conditioning units and industry add heat of their own. As a result, city nights often remain uncomfortably warm, and the body never gets the chance to recover.
That lack of recovery matters for health. During heatwaves, hospital admissions for heart and kidney disease rise, and deaths increase most sharply among people over 75, those who live alone and those with long-term illnesses. Studies of past European heatwaves have found that the risk of dying was higher in the warmest neighbourhoods of a city, which were often also its poorest, with fewer trees and older, poorly insulated housing.
City planners have several tools to reduce the problem. Trees are among the most effective: a street lined with mature trees can be noticeably cooler at ground level than a similar street without them, because their leaves block sunlight and release water vapour. Light-coloured roofs and pavements reflect more sunlight, and parks act as cool islands whose benefits extend into the surrounding streets.
However, these solutions are not free of difficulties. Trees take decades to reach full size, need water during droughts and can be damaged by the very heat they are meant to reduce. Reflective surfaces, if badly placed, may bounce heat onto pedestrians or neighbouring buildings. Researchers therefore recommend combining measures and, above all, directing them first to the areas where vulnerable people live.
Some cities have also begun to treat heat as a public-health emergency rather than just an environmental issue. Early-warning systems, telephone calls to elderly residents and air-conditioned shelters in libraries and schools are now part of summer planning in several Spanish cities. Whether these programmes reduce deaths has not yet been clearly shown, but they reflect a change in thinking: heat is no longer seen as a mere inconvenience, but as a risk that can be measured and managed.
