Extreme heat and health: what the Lancet Countdown counts

The 2025 Lancet Countdown on health and climate change reported an average of 546,000 heat-related deaths a year between 2012 and 2021, and a 23% rise in heat-related mortality per 100,000 people since the 1990s. A 2026 Europe-specific report found heat-attributable deaths rising across nearly all European regions. These are modelled estimates of population burden, and it helps to be clear about what they can and cannot say.

What did the Lancet Countdown report about heat?

The ninth annual global Lancet Countdown report, produced with the World Health Organization and 128 experts from 71 institutions, was published in October 2025. As UCL reports, it found:

• Heat-related deaths averaging 546,000 a year between 2012 and 2021.
• Heat-related mortality per 100,000 people up 23% since the 1990s.
• About one heat-related death every minute worldwide, in the words of Prof Ollie Jay.

The 2026 Europe report, covered by Euronews, found heat-attributable deaths increased across nearly all European regions between 2015 and 2024 compared with 1991 to 2000. It also reports that daily extreme heat warnings rose by 318% over that period, and that heat is linked with illness, sleep disruption, worsening chronic disease and adverse birth outcomes.

What do these figures measure, and what are the limits?

Two points of care. First, the 318% is a rise in warnings, a measure of how often authorities flagged extreme heat, not a rise in deaths. Second, “heat-related” and “heat-attributable” deaths are estimates derived from models linking temperature to mortality; they describe populations and do not identify who is affected in a given heatwave.

The global report also tracks the response: 135 of 196 WHO member states (69%) reported high to very high health emergency management capacity in 2024, and it calls for stronger early warning systems and urban planning.

What population reports cannot show is individual variation in heat response, which depends on age, health conditions and circumstances. The Lancet Countdown does not turn a national total into personal guidance, and this post does not either. It is a measure of scale, and it is worth reading alongside the air quality evidence covered previously on the blog, since both are environmental exposures with measurable population effects.

The bottom line

• The 2025 Lancet Countdown reported an average 546,000 heat-related deaths a year (2012 to 2021), with mortality per 100,000 up 23% since the 1990s.
• The 2026 Europe report found heat-attributable deaths rising across nearly all regions; the 318% figure refers to extreme heat warnings, not deaths.
• These are modelled population estimates, and the reports call for stronger early warning systems and urban planning.