Eric Mackres is senior manager of urban analytics for the WRI Ross Center for Sustainable Cities and attended London Climate Action Week during the June 2026 heatwave. Usama Bilal is an associate professor of epidemiology and co-director of the Urban Health Collaborative at Drexel University.
As thousands gathered in London for one of the year’s largest climate gatherings last week, Western Europe faced its most severe heatwave ever recorded. The irony was not lost.
Across Europe, over a dozen countries issued urgent heat warnings and the World Health Organization recorded over 1,300 excess deaths. In London, where air conditioning is rare in buildings and on trains and buses, temperatures soared past 36 degrees Celsius (97F) and schools closed early. The mayor announced the city’s first heat action plan – an important step.
Extreme heat is now a public health crisis for many of the world’s cities, as the urban heat island effect intensifies dangerous temperatures – and it’s growing worse. Around 500,000 people die from extreme heat every year. As global temperatures rise, and with a severe El Niño getting underway, even more people will die and be hospitalised unless cities act soon.
But most cities are still taking a far too one-sized-fits-all approach to tackling heat, looking only at temperatures and not its local effects on people and their health.
People experience heat differently
How extreme heat affects people’s health can vary widely across a country and city, depending on their environment and demographics. Cities can save far more lives and prevent more hospitalisations by taking a tailored approach, using data to understand who’s most vulnerable and directing solutions toward them.
The good news: better data now exists that enable cities to pinpoint who’s most at risk. And that data can inform customised adaptation strategies to save lives. Indeed, the future of cities will hinge on their ability to deliver solutions to extreme heat tailored to at-risk people and neighborhoods.
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First, cities should start by measuring heat’s risks to people’s health locally. Our work in Brazil and across Latin America shows big differences in what temperatures are dangerous and how quickly risks escalate at higher temperatures. These variations exist between cities, between demographic groups and between neighbourhoods.
But it’s not as simple as finding the hottest places. In temperate Porto Alegre, in southern Brazil, a person’s risk of death increases by 25% at temperatures of 27 degrees Celsius (81F). In tropical Teresina, in northern Brazil, which is hot year-round, the same temperature does not elevate the risk of death. At 32 degrees Celsius (90F), a person’s risk of death increases by a milder 10%.
These differences also exist within cities where the climate is the same. Elderly people, the very young, lower-income communities and those without air-conditioning and shaded green spaces are all more likely to get sick, be hospitalised, or die from heat. Areas with more trees and green spaces usually have lower temperatures, and therefore lower impacts of heat.
Targeted heat alerts
Second, cities can use this data to develop early warning systems and outreach campaigns that give people more targeted heat alerts. Research in the UK found that the elderly, despite being among the most at-risk, often were unable to heed warnings during the 2022 heatwave. Well-designed heat warning systems and city responses strengthen people’s trust in health services. They can change people’s behaviours and better prepare municipal services, helping reduce illness, hospital visits and deaths.
Rio de Janeiro adopted a heat alert system in 2024 with five alert levels based on past heatwaves’ impacts on health and forecasts of when temperature and humidity will hit those dangerous levels again. The alert levels activate services like cooling centres, extra public drinking water, and changes to outdoor events. When a heatwave struck during Carnival in 2025, the city was able to deploy resources to protect and warn people while still allowing events to go on.
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Finally, cities should use local heat data to target cooling solutions to where they can help people the most. Solutions like tree cover, shade structures and cool roofs lower temperatures and can provide targeted relief for the most vulnerable people, like outdoor workers and those who travel by foot, bike or public transit.
In Florianópolis, Brazil, we helped the local government use heat impact modeling to design a green corridor and urban forestry project that will reduce pedestrians’ heat stress up to 7 degrees C. In Hermosillo, Mexico, our researchers worked with the city and found that certain neighbourhoods could feel up to 14 degrees C hotter than the shaded city center. A park is now under construction that will bring better shade and heat relief to one of the city’s most at-risk areas.
Connecting health and climate planning
Momentum to address extreme heat in cities is growing, from both national and local governments. At last year’s UN climate summit in Brazil, the Belém Health Action Plan saw 30 national health ministries commit to build climate-resilient health systems based on local data and evidence-based policies.
And over 160 local governments joined the Beat the Heat initiative, committing to develop urban heat action plans and deliver passive cooling projects to reduce health risks.
But there’s still a disconnect between health, urban and climate officials. Only 23% of World Meteorological Organization member countries integrate weather information into health surveillance systems. Heat-health impact models, though increasingly easy to scale, are not yet built for every city. Some cities still need to collect local data for specific demographics and neighbourhoods – and many need support.
National and local governments will need to partner on this tailored approach. It will require integrating local heat and health data into public health systems, city planning, infrastructure, and disaster preparedness.
We have the data to know who will be most impacted by extreme heat when – and the solutions to keep people alive and out of the hospital. It’s time for governments to use them.
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