In the summer of 2026, an estimated 2,700 people lost their lives prematurely in England and Wales as two record-breaking heatwaves swept across the country. The figure, from scientists at Imperial College London, the Met Office and the London School of Hygiene and Tropical Medicine, shows that extreme heat is not just uncomfortable – it can be lethal.
A heatwave is a prolonged period of unusually hot weather. In the UK, a heatwave is officially declared when a location records at least three consecutive days with daily maximum temperatures reaching or exceeding a specific threshold (which varies by region). But as the 2026 events showed, heatwaves can be far more intense, with temperatures soaring to 35.1°C in May and 37.7°C in June – the latter smashing a June record that had stood since 1957.
“An explainer on why heatwaves kill, the role of climate change, and what UK readers should know.”
These extreme temperatures are being made worse by climate change. Human activity, mainly burning fossil fuels, has warmed the planet by about 1.4°C since pre-industrial times. This extra heat added an estimated 3–4°C to the maximum temperatures in the May and June heatwaves, the researchers said. More than 42% of the deaths in the two heatwaves were attributed to the extra heat caused by climate change – around 1,152 people.
For the UK, a country not traditionally built for heat, the consequences are stark. Many homes are poorly insulated against heat, leaving people vulnerable during prolonged hot spells. The heat puts immense strain on the body: the heart pumps harder to cool the skin, and dehydration can worsen the effect. This can trigger heart attacks, strokes and other fatal emergencies. The very young, the elderly and those with existing health conditions are most at risk, but even fit and healthy people are not safe – the heat has been called a 'silent killer' because early symptoms are easily overlooked. In June, high humidity made it even harder for the body to cool down through sweating.
During the peak of the June heatwave, an estimated 440 people died each day over three days. That compares with about four road traffic deaths per day and 35 from alcohol or drug use, according to government statistics. The June heatwave also prompted an unprecedented three consecutive days of red warnings from the UK Health Security Agency and the Met Office – the highest alert level, warning of danger to life for everyone.
The problem is not going away. The Climate Change Committee has warned for more than a decade that the UK's plans to protect people from extreme weather are inadequate, and the UK Health Security Agency previously found that more than 10,000 people died in Britain because of summer heatwaves between 2020 and 2024. Without cuts to emissions and better adaptation, the death toll is likely to rise.
Q: What is a heat dome? A heat dome is a stalled area of high pressure that traps hot air over a region, causing temperatures to build up. Both the May and June 2026 heatwaves were caused by heat domes, which were made more intense by climate change.
Q: How does heat kill? Heat puts physical strain on the body, making the heart pump harder to cool it down. It can cause heat exhaustion, heat stroke, and stress on the heart and kidneys, and worsen chronic conditions like heart disease, lung disease and diabetes. Even healthy people can be affected, especially if humidity prevents sweating from working effectively.
Q: What can I do to stay safe in a heatwave? Stay hydrated, keep out of the sun during the hottest part of the day (11am–3pm), cool your home by closing curtains and opening windows at night, and check on vulnerable neighbours and relatives. If you feel unwell, seek medical help. The NHS and UK Health Security Agency issue specific guidance during heatwaves.
What happens next is uncertain, but scientists are clear that as long as fossil fuels continue to be burned, extreme heat will become more frequent and more deadly. The researchers involved in the study urged the UK to adapt – for example, by fitting air conditioning in hospitals and care homes, introducing maximum temperature rules in workplaces, and improving home insulation. Meanwhile, global efforts to reach net-zero emissions are essential to stop the problem worsening. The next heatwave could come at any time; being prepared could save lives.
