Extreme heat is a top killer in America, and the same silent threat is exacting a rising toll in the UK, where experts warn that soaring temperatures are exacerbating chronic illnesses that affect millions. While many people welcome warmer weather, the physiological stress of a heatwave can turn manageable conditions into life-threatening emergencies. In England alone, an estimated 10,781 heat-associated deaths occurred between 2020 and 2024, according to UK Health Security Agency data, and projections suggest that figure could rise nearly sixfold by the 2050s without adaptation measures. The strain falls hardest on those with pre-existing health problems, and the most vulnerable are often unaware of the danger they face.
How heat turns chronic conditions critical
Cardiovascular disease sits at the top of the list of conditions that become dangerous in extreme heat. The body’s natural cooling mechanism relies on widening blood vessels close to the skin and pumping extra blood to them, allowing heat to escape. “When blood travels there, it’s able to lose a bit of warmth to the air around you,” explained exercise physiologist Michael Crawford of the Cleveland Clinic. But for people with clogged arteries, this process is compromised. Lauren Siewny, medical director of the Duke University Hospital Emergency Department, told the Washington Post that a dangerous mismatch develops: “between what the heart needs when it’s working harder to face the heat and what the body is able to deliver.” Research cited by the UK Health Security Agency shows that extreme heat can cause a 17% increase in the risk of death due to cardiovascular disease, and respiratory admissions also spike significantly for every 1°C rise above a threshold temperature.
Kidney disease, the eighth-leading cause of death in America and a growing concern in the UK, is also severely aggravated by high temperatures. The National Kidney Foundation explains that kidneys need fluids to filter waste and toxins from the blood and to keep blood vessels clear for nutrient delivery. Dehydration impairs this function, raising the risk of permanent kidney damage and kidney stones. The Mayo Clinic describes kidney stones as pebble-like deposits of minerals that form when urine is too concentrated. “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 UK data echoes this: renal disease admissions rise on hot days, and the NHS has reported an average of 1,780 yearly hospital admissions associated with warm days in England between 2010 and 2018.
Diabetes is another condition worsened by heat. Extreme temperatures could cause a 14% increase in the risk of death for people with diabetes, according to UK Health Security Agency projections. Dr Paul Biddinger, chief preparedness and continuity officer at Mass General Brigham, noted that 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.” Those at particular risk include older adults, young children, people in underserved communities, and anyone whose medication affects thermoregulation and hydration—a point reinforced by UK guidance highlighting the vulnerability of those taking certain drugs for mental health or other conditions.
Pain, inflammation and the heat
Beyond chronic disease, extreme heat takes a toll on the aches and pains that affect tens of millions of people. More than 91 million US adults are estimated to suffer from arthritis, including 12 million who have gout. Dehydration related to heat can raise levels of uric acid in the blood, which triggers gout, explained Dr N. Lawrence Edwards, a rheumatologist and chairman of the Gout Education Society. Heat and humidity can also increase inflammation in the body more broadly. Research from the American Heart Association in 2024 suggests that heat exposure may impair immune cells, leading to a weakened inflammatory response. “With rising global temperatures, the association between heat exposure and a temporarily weakened response from the immune system is a concern because temperature and humidity are known to be important environmental drivers of infectious, airborne disease transmission,” said Daniel W. Riggs, an assistant professor of medicine at the University of Louisville. “Thus, 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 impact of heat on joint pain is complex; some individuals with arthritis report relief in warmer weather, but the combination of dehydration and inflammation can worsen symptoms for others. The same mechanisms that trigger gout also affect people with other forms of inflammatory arthritis, and the UK’s ageing population faces increasing exposure as heatwaves become more frequent and intense.
Migraines: the heat-triggered head pain
Finally, anyone who suffers from migraines may notice they become more frequent and severe during heatwaves. Changes in atmospheric pressure and temperature can directly cause head pain, and dehydration—common when the body loses fluids through sweating—is a powerful trigger. 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.” The UK data supports this: the productivity of two-in-five people has been negatively affected by hot weather, and sleep disturbances linked to heat can also lower the threshold for migraine attacks. While the link is well established, many people do not connect their symptoms to the heat until it is too late to prevent an episode.
