Extreme heat is a leading cause of death in America – and as the UK swelters through its own increasingly frequent and intense heatwaves, the same warning applies closer to home. While summer officially began in the United States with record temperatures, the pattern is mirrored across the Atlantic: the incidence of high summer temperatures in Britain is already 10 to 25 per cent higher than it would be without climate change, and the trend is accelerating. In summer 2022, the UK recorded its warmest year on record, with temperatures exceeding 40°C for the first time. May 2026 saw a record-breaking heatwave with 35.1°C in London. The danger is not simply discomfort – it is a matter of life and death.
How heat endangers health
When temperatures rise, the human body is forced to work harder to maintain its core temperature. Blood vessels widen to allow heat to escape, sending extra blood to the skin’s surface. “These important changes allow more blood to flow through arteries and veins close to your skin’s surface,” explained Michael Crawford, an exercise physiologist at the Cleveland Clinic. “When blood travels there, it’s able to lose a bit of warmth to the air around you.” But for those with underlying conditions, this natural cooling mechanism can backfire. “In the emergency department, when the temperature rises, we see increases in both people experiencing heat-related illness as well as people with worsening of their chronic illnesses,” said Dr Paul Biddinger, chief preparedness and continuity officer at Mass General Brigham. “People at especially increased risk of illness include those with chronic medical conditions, like heart disease, that are made worse by the heat, older adults, young children and some people living in underserved communities.”
The UK is no exception. Here, older people (65 and over), babies and young children, and those with pre-existing conditions such as heart disease, lung problems, diabetes and kidney disease are particularly vulnerable. The country’s urban centres, especially London, face added risk from the urban heat island effect, while much of the infrastructure – homes, workplaces and transport – was built to retain heat, making it harder to cool down during a heatwave.
Heart disease: a lethal mismatch
Heart disease remains the leading cause of death in the United States, and extreme heat forces the heart to pump harder than normal. In people with clogged arteries, the body’s attempt to redirect blood to the skin can create a dangerous imbalance. “You can develop a mismatch between what the heart needs when it’s working harder to face the heat and what the body is able to deliver,” said Lauren Siewny, medical director of the Duke University Hospital Emergency Department. This increased workload can trigger irregular heart rhythms, heart attacks and exacerbation of heart failure.
The same strain is evident in the UK. Studies show that hot weather places significant stress on the heart, lungs and kidneys. The UK Health Security Agency and the Met Office have warned that cardiovascular disease deaths related to extreme heat are projected to more than double over the next two decades. In summer 2022, approximately 3,000 excess deaths were recorded in England and Wales during five heat periods. In summer 2025, an estimated 1,504 heat-associated deaths occurred in England, with older age groups worst affected. Projections suggest heat-related deaths could triple by 2050.
Kidney stones and kidney damage
Kidney disease – the eighth-leading cause of death in the US – is also made worse by high temperatures. The kidneys rely on fluids to filter waste and toxins from the blood and to keep blood vessels clear so nutrients can reach them, according to the National Kidney Foundation. Dehydration impairs this process, potentially causing permanent kidney damage and raising the risk of kidney stones. “Over time, kidney stones that don’t pass can lead to serious complications, which increase the risk for kidney disease,” said Dr Ira Kohn, a urologist at Geisinger Health. “Having one kidney stone increases the risk of developing another one.” The Mayo Clinic notes that kidney stones are pebble-like deposits of minerals that form when urine contains less water.
In the UK, the link between heat and kidney problems is well documented. A seven-day heatwave in July 2021 saw a 29 per cent increase in acute kidney injury (AKI). The risk of heat-related AKI is higher in older individuals, males, and certain ethnic groups. Chronic kidney disease patients in hotter climates experience a more rapid decline in kidney function. With the UK experiencing four declared heatwaves in summer 2025 – the warmest summer on record – the strain on kidneys is becoming a persistent public health concern.
Arthritis, gout and inflammation
More than 91 million U.S. adults are estimated to suffer from arthritis, including 12 million who have gout, the most common form of inflammatory arthritis. Gout can be triggered by kidney problems and excess uric acid in the blood. “Dehydration related to heat can raise levels of uric acid, which causes gout,” said Dr N. Lawrence Edwards, a rheumatologist and chairman of the Gout Education Society. Both heat and humidity can also increase inflammation. Research from the American Heart Association in 2024 suggests that heat exposure may impair immune cells, leading to inflammation. “With rising global temperatures, the association between heat exposure and a temporarily weakened response from the immune system is a concern,” said Daniel W. Riggs, an assistant professor of medicine at the University of Louisville. “During the hottest days of summer people may be at higher risk of heat exposure, they may also be more vulnerable to disease or inflammation.”
In the UK, the picture is similar. While warmer weather can sometimes ease arthritis symptoms by improving circulation, extreme heat and high humidity can exacerbate inflammation and swelling, particularly in autoimmune conditions such as rheumatoid arthritis. Humidity can contribute to dehydration, worsening gout by reducing the body’s ability to flush out uric acid.
Migraines triggered by heat and dehydration
Changes in pressure and temperature can lead to head pain, and so can dehydration, according to the American Migraine Foundation. “About 1/3 of people with migraine say dehydration is a trigger, and for some, even the slightest hint of dehydration can be the fast track to debilitating head pain,” it said. Bright sunlight and glare associated with hot weather can also strain the eyes and activate light-sensitive migraine pathways.
In the UK, the same triggers apply, with the added factor of high humidity. The UK Health Security Agency’s Adverse Weather and Health Plan includes guidance on staying safe in hot weather, with advice on hydration, keeping cool, and protecting oneself from the sun. Heatwaves also place additional pressure on NHS infrastructure, ambulance services and healthcare providers, with general practices advised to develop heatwave preparedness plans. Between 2020 and 2024, approximately 10,000 people in the UK died due to heat-related causes – a stark reminder that extreme heat is not just an American problem, but a growing threat to public health on both sides of the Atlantic.