GoKawiilA paramedic moves a person to an ambulance in Spain during a 2022 heatwave that killed more than 300 people in five days. Credit: Angel Garcia/Bloomberg/Getty
Extreme heat is killing hundreds of thousands of people every year — and the annual toll is growing.
An estimated 210,000 more people died of heat-related causes each year from 2012 to 2021 than from 1990 to 19991, on average (see ‘Heat’s toll’) . Some 75,000 deaths per hot season have been linked to heatwaves in Asia from 1990 to 20192, and nearly 14,000 people are estimated to have died in June this year in Europe during a record-breaking heatwave3. This week, extreme heat has once again blistered parts of the continent and East Asia.
Source: The 2025 Global Report of the Lancet Countdown
So how do scientists know how many people are killed by heat? Creating an accurate tally is not straightforward. Heat increases the risk of heart attack and exacerbates many serious health conditions, but death certificates rarely cite heat as the cause of death.
To avoid relying on death certificates for information, researchers have developed methods to isolate heat-related deaths from total deaths. The methods can’t give a precise death count, but researchers say that estimates of a heatwave’s death toll are necessary to develop measures to prevent future deaths. “Nobody needs to die in a heatwave,” says Kristie Ebi, a climate epidemiologist at the University of Washington in Seattle. “If you don't know how many people died in a heatwave, you're not going to take appropriate preventive action.”
How are heat deaths tallied?
Most public-health agencies estimate heat-related mortality using the ‘excess deaths’ method, which compares the total number of deaths during a heatwave with the figure for a baseline period without extreme heat. Subtracting the baseline death count from the heatwave death count yields the number of people assumed to have died from heat.
This method is straightforward and fast, says Antonio Gasparrini, a biostatistician and epidemiologist at the London School of Hygiene and Tropical Medicine, but it can’t distinguish between excess deaths caused by the heatwave and those caused by other factors. The method also can’t be used until after a heatwave has passed, limiting its effectiveness in forecasting the death toll for an upcoming heatwave.
A person with heat stroke is treated in a hospital in India.Credit: R. Satish Babu/AFP/Getty
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