Extreme heat is worsening other climate impacts on health

Europe’s recent record-breaking heatwaves are a prescient reminder of how dangerous heat can be. During one seven-day stretch in late June, more than 10,000 excess deaths were recorded in the region by the European Mortality Monitoring Network. This is a spike that scientists have so far only been able to attribute to higher temperatures. But the health problems during these events are increasingly not due to heat alone. This is a CATCH story This story is part of Dialogue Earth’s work on the Community Adaptations to City Heat (CATCH) project, in partnership with Boston University. The project is funded by Wellcome. All Dialogue Earth content is editorially independent. Read more stories from CATCH. In recent years, scientists have started to build a clearer picture of how extreme heat combines with other environmental hazards, such as cyclones, wildfires and ozone pollution. Their work suggests these compound events are becoming more common due to climate change and, in turn, carry greater health risks than each event would on its own. Damage to infrastructure and disruption to everyday life from a hurricane undermines people’s ability to cope with heat, which itself can then hamper disaster recovery efforts. Wildfires destroy landscapes while the smoke they generate adds to the adverse health impacts of heat, across many kilometres. “Heat is insidious,” says Kurt Shickman, senior fellow at the World Resources Institute’s Ross Center for Sustainable Cities. “It reduces people’s ability to weather other disasters.” Hurricanes and heat: Resilience as a solution In August 2021, Hurricane Ida made landfall in the United States. The category four tropical cyclone brought fast winds, intense rain and storm surges to the states of Louisiana and Mississippi, as well as painful memories of the devastation wrought by Hurricane Katrina 16 years before. Six people died as a direct result of the Ida’s winds and floodwaters. But the cyclone caused more deaths – via an indirect, compound route. Hurricane Ida produced the most extensive power outage in Louisiana’s history. The utilities company Entergy, which supplies electricity to the southern US states of Arkansas, Mississippi, Texas and Louisiana, reported that around 30,000 utility poles were downed. Such impacts left at least 1.2 million households across eight US states without electricity for hours, if not days. For some people in Louisiana, days stretched into weeks. In those weeks a blistering heatwave arrived. Heat index temperatures, which combine air temperature and relative humidity to give a “feels like” value, surpassed 37.8C. With millions of homes lacking electricity and unable to run air conditioning, the heat proved more deadly than the initial cyclone. According to the official tally, 13 people died from “post-Ida heat exhaustion”. Ida is one in a string of recent tropical hurricanes – Irma in 2017, Laura in 2020 and Beryl in 2024 – that have made landfall on the US gulf coast before causing heat-related deaths during the massive power outages that followed. As well as taking down infrastructure directly, heat piles the pressure on city power grids: electricity demand for air conditioning shoots up, while cables overheat and carry less power. One analysis of the impact of compound weather events and heatwaves on power outages across the US observed that the combined stress often led to “cascading failures and prolonged recovery times”. These problems are compounded by defoliation, caused by hurricane winds blowing leaves off trees or bringing them down entirely. The resulting loss of shade and evapotranspiration, by which trees naturally cool the surrounding air, has been shown to leave neighbourhoods with higher exposure to dangerous heat. A 2023 study estimated that after hurricane Laura, residents of Cameron, Louisiana experienced heat index temperatures above 30C for 24 additional hours compared to a simulated “normal foliage” scenario. This effect was found to be particularly strong at night. Disaster relief workers are especially vulnerable to the cyclone-heatwave cocktail: hours of physical work outside in restrictive protective gear, with little rest or shade. Conditions for rescue workers after hurricane Beryl were found to have frequently exceeded workplace heat safety thresholds. Patricia Fabian, a heat expert at Boston University in the US, says post-cyclone power cuts put people at risk of more than just heat stress. Food and water supplies become erratic; medication that needs to stay refrigerated soon becomes unusable; during days of stifling heat, people are more likely to go swimming to cool down in lakes and rivers, which are more likely to be contaminated with sewage because heavy rain has overwhelmed the stormwater drains. “That results in spikes in gastrointestinal disease,” she explains. Combined cyclone-heatwaves have been relatively rare in the past, but they are expected to occur more as the climate warms. In 2019, a study published by Nature Climate Change observed that the arrival of a major tropical cyclone, followed within 30 days by a heat index temperature above 40C, has historically happened three times every 30 years. If global heatwaves become more frequent and intense in line with projections, this could rise to seven times every 30 years at 1.5C of average global warming. That grows to 11 times at 2C. If global warming reaches an average of 4C, the study says such events “could occur at least annually”. Keeping the power on could be vital for saving lives when this happens. “Better maintenance” of electrical infrastructure can make cities less vulnerable to outages during hurricanes, explains Ning Lin, a civil and environmental engineering expert at Princeton University in the US. Trees and branches falling onto power lines cause the most problems. Therefore, city governments and grid operators can lower this risk by burying cables underground, regularly checking for old or damaged poles and trimming trees, she says. Some city residents are already bringing in their own heat solutions. The city of New Orleans in Louisiana was hit hard by both Katrina and Ida. So, as part of the Community Lighthouse Project, local community buildings are being equipped with solar panels and batteries to enable them to assist local people if power networks go down. For Shickman, the high chance of power cuts during a compound heat event makes the case for rolling out passive cooling measures (creating shade, planting street trees, using heat-reflective materials for buildings) even stronger. He wants passive cooling features to become a key part of what makes a building safe legally, via health and safety codes. This would place them on a par with fire extinguishers and smoke alarms. “It’s critically important that it’s the focus – it works regardless of whether there are electrical inputs,” Shickman points out. “We need to keep our communities and our buildings as cool and as safe as possible. [Even with passive cooling] they’re not going to be cool, but they are going to be safer than a building that has been absorbing heat the whole time.” Heat and wildfires: Warning signals Extreme heat and wildfires are another dangerous pairing. When combined with unusually long periods of low rainfall or drought, hot weather has contributed significantly to increased fire activity in recent years. On their own, wildfires can be deadly. In 2025, a series of catastrophic blazes across the US, Europe and South Korea resulted in over 300,000 evacuations and over 90 deaths. But there is also evidence of “synergistic” negative effects on health from heat and smoke. Breathing in fine particulate matter (that with a diameter of less than 2.5 micrometres), is a major secondary cause of wildfire-linked health impacts. Both heat and smoke inhalation contribute to cardiovascular and respiratory problems. Experiencing both at once raises the chance of complications, and wildfire smoke plumes can choke cities hundreds of miles away. In 2024, researchers in the US published an analysis of satellite, environmental and public health data collected in California between 2006 and 2019. They found that in areas exposed to extreme heat (when the heat index was higher than 85% of historical summer days) and dangerous levels of wildfire smoke on the same day, hospitalisations for cardiorespiratory disease were higher than would be expected from the sum of the risk of both hazards separately. The effect was more pronounced in certain neighbourhoods, says study author Miriam Marlier, who works on the overlap of environmental change and public health at UCLA Fielding School of Public Health in the US. The at-risk neighbourhoods had factors that are all too familiar to those who study urban heat: “[Lower levels of] local tree canopy coverage, local income and health insurance coverage were related to higher synergistic effects,” she says. Upgrading buildings to better resist heat can help here, but key to saving lives is warning people about incoming risks. In the US, two separate government agencies are responsible for sending out warnings on heatwaves and air pollution. Marlier observes that public alerts issued ahead of heatwaves and air pollution from wildfire smoke are often “at odds with each other”. She explains: “If it’s hot, you’re told to open your windows to be outside to try and cool down. What are you told when it’s smoky? You’re told to close everything up, stay inside, try to keep inside as clean as possible to protect your health. What are you supposed to do when it’s hot and smoky?” Marlier and her colleagues propose issuing joint alerts for when both heat and wildfire smoke are forecast, with a lower threshold to compensate for the greater risk to people’s health. Shickman agrees: “Access to early warning systems is absolutely critical.” These warnings should focus not on the physical attributes of the weather, he argues, but on potential health outcomes: “The way we assess the danger of a hurricane has nothing to do with what it means for humans. We don’t talk about storm surge of hurricanes, we talk about wind speed [although the former does more damage]. It’s even worse with heat. It’s just temperature, but that’s not really an indication of what the danger is for people.” He points to the city of Seville in Spain, which has piloted a new approach to heatwave warnings along these lines. In 2022, the city council worked with researchers to examine decades of local weather data to identify which conditions – rather than simply which temperatures – were linked with the most heat deaths. This was used to recalibrate the public alert system, which by the end of summer had identified two potentially dangerous heatwaves that were not flagged by Spain’s weather agency AEMET. Last year, a non-profit healthcare provider in New York experimented with using an AI agent to give personalised heat warnings. “Rachel” contacted 17,000 customers living in high-heat-risk neighbourhoods by text chat or phone call to offer heat safety advice, such as asking if they had access to air conditioning and cool water. The company reported 47% fewer in-patient admissions compared to a control group. Hidden cascades As Europe is learning, extreme events linked to climate change are making overlapping risks more common. But Shickman warns there is more insidious compounding going on that must not be overlooked: persistent, chronic heat. “There’s a hidden cascade that happens when heat hits a city. It’s the grinding intensity of the heat, day and night, that causes knock-on effects that aren’t always attributed to heat but are really related to heat. “If someone is having their educational potential stunted because of heat, that compounds over their whole life. Is it a big splashy heatwave along with a hurricane? No. But it’s a disaster nonetheless.” As the world continues to overheat, more people will feel negative health effects of compound disasters. This points to the growing need for city governments and companies to recognise heat as a serious health threat worth dedicating resources to. While low-tech measures to adapt cities to extreme heat are readily available, says Shickman, the real challenge is getting investment. “It’s the guys in the bank and the boardroom. That’s where the innovation and mindset change needs to happen.”