Extreme heat is estimated to cause about half a million deaths a year and puts billions of workers at risk; most of these deaths are preventable with warnings, cooling, shade and outreach to people most exposed, but protection reaches only part of the people who need it while dangerous heat becomes more frequent.
Updated October 15, 2026. Written from the sources listed below, not independently peer-reviewed.
In 2025, Maricopa County in Arizona, which includes Phoenix, recorded 430 heat-related deaths. Out of every 100 of them, how many were people experiencing homelessness?
489,000
Heat-related deaths each year
Average per year between 2000 and 2019, from modelled estimates cited by WHO. About 45% of the deaths occurred in Asia and 36% in Europe.
Data year 2019. Source: World Health Organization, 2026
16
Extra days of dangerous heat per person caused by climate change
Days of dangerous heat the average person was exposed to in 2024 that would not have been expected without climate change (Lancet Countdown 2025 estimate).
Data year 2024. Source: World Health Organization, 2025
Over 1 billion
People without adequate cooling
UNEP estimate in its Global Cooling Watch 2025 report; without action the number could triple by 2050.
Data year 2025. Source: UN Environment Programme / Cool Coalition, 2025
Unit: % of the global workforce
Modelled ILO estimates; the ILO reports only these two points. Exposure means a worker is likely to face excessive heat at some point during their work, not that they are exposed every day.
There is no quantified international target for heat deaths. In July 2024 the UN Secretary-General issued a Call to Action on Extreme Heat that asks countries to care for the most vulnerable, protect workers, build resilience using data and science, and limit warming to 1.5 °C. It calls for heat-ready health systems and social protection and for wider heat-health warnings: WHO estimates that scaling up heat-health warning systems in 57 countries could save almost 100,000 lives a year.
Heat-related deaths among people over 65 rose by about 85% between 2000–2004 and 2017–2021. Older people living alone and people with heart, lung or kidney disease face the highest risk.
Source: World Health Organization, 2026Infants under one year and adults over 65 were exposed to more than 20 heatwave days per person, four times as many as twenty years earlier.
Source: World Health Organization, 2025More than 2.4 billion of the world's 3.4 billion workers are likely to be exposed to excessive heat at work. Heat is linked to an estimated 22.87 million work injuries and 18,970 deaths a year, and to 26.2 million people living with chronic kidney disease (2020 estimates).
Source: International Labour Organization, 2024In Maricopa County, Arizona, 208 of the 430 people who died from heat in 2025 were experiencing homelessness, and 313 of the deaths followed exposure outdoors. Among indoor deaths, air conditioning was present in 94% of cases but not working in 72% of those.
Source: Maricopa County Department of Public Health, Arizona, 2026Each country is shaded by its latest available value, so you can see where the gap is widest. Hover or tap a country, search for one, or open the full table.
Loading country values…
Values in days per year.
Number of days in a year on which the daily mean heat index, which combines temperature and humidity, rose above 35 °C, showing the latest year available per country (mostly 2024; many cooler countries show a value of 0 for 2020). It is a single national figure for one year, so hot regions within large countries are hidden and values change from year to year; it measures hazard, not deaths or how well people are protected.
Source: World Bank Climate Change Knowledge Portal, via World Bank Open Data. Latest year available per country; years differ.
Values load from World Bank Open Data. Simplified Natural Earth borders; they imply no position on disputed territories.
Instead of one “percent solved” score, DO GOOD rates three separate dimensions. A problem can have mature solutions that still reach too few people. These ratings are an editorial reading of the sources on this page, not a measured index.
Has the situation improved in the real world?
LowThe Lancet Countdown estimates an average of 546,000 heat-related deaths a year, with the death rate 23% higher than in the 1990s, and a loss of 640 billion potential working hours in 2024.
How mature and validated are the available solutions?
ModerateWHO describes heat's health effects as predictable and largely preventable, and heat-health action plans, warning systems and cooling are well understood. Evidence on which combinations of measures save most lives, especially for people outdoors, is thinner.
How widely are effective solutions actually used?
LowMore than 1 billion people lack adequate cooling, and WHO's estimate that wider heat-health warning systems could save almost 100,000 lives a year shows how much protection is still missing.
Global death figures are modelled and differ between studies (489,000 versus 546,000 a year), and few places investigate individual heat deaths. Exposure estimates from climate data are more robust.
Infants and older adults now face about four times as many heatwave days as twenty years ago, the heat-related death rate has risen 23% since the 1990s, and the average person faced 16 days of dangerous heat in 2024 that would not have been expected without climate change.
Heat harms the body when it can no longer cool itself. It can cause heat exhaustion and heatstroke, a medical emergency that is often fatal, and it strains the heart, lungs and kidneys, worsening existing illness and mental health conditions. WHO cites estimates of about 489,000 heat-related deaths a year between 2000 and 2019, and the 2025 report of the Lancet Countdown puts the annual toll at an average of 546,000, with the heat-related death rate 23% higher than in the 1990s. Heat also causes accidents, disrupts health services when power or transport fail, and cost an estimated 640 billion potential working hours in 2024.
The people most at risk are those who cannot avoid the heat or cannot cope with it: older people, especially those living alone; people with heart, lung or kidney disease, diabetes, dementia or mental health conditions; outdoor and manual workers; and people in poor-quality housing or informal settlements without cooling. People experiencing homelessness combine several of these risks. In Maricopa County, Arizona, nearly half of the people who died from heat in 2025 were experiencing homelessness. WHO describes the health effects of heat as predictable and largely preventable, yet heatwaves are becoming more frequent, longer and more intense as the climate warms.
Heat rarely appears as the only cause on a death certificate, so most global figures are statistical estimates rather than counts. The main measures are:
Heat-related deaths
Researchers compare deaths on hot days with those expected at more comfortable temperatures and attribute the excess to heat. These modelled estimates depend on method and period: about 489,000 deaths a year for 2000–2019 in the estimate WHO cites, and an average of 546,000 a year in the Lancet Countdown's 2025 report.
Exposure to dangerous heat
The Lancet Countdown counts heatwave days and days of dangerous heat that people experience, and estimates how many of them would not have occurred without climate change. In 2024 the average person had 16 such additional days.
Heat at work
The ILO estimates how many workers are likely to be exposed to excessive heat at some point during their work and how many injuries, deaths and cases of disease can be attributed to it. The latest figures are modelled estimates for 2020.
Local death investigations
Some places review every suspected heat death. In Maricopa County, Arizona, the medical examiner refers suspected cases to public health staff, who record whether heat caused or contributed to the death, where the exposure happened and the person's living situation. Few places publish data this detailed.
What works
Plans led by the health sector link weather forecasts to warnings, public advice and emergency measures, and prioritise settings without cooling such as hospitals, care homes and schools. WHO works with the World Meteorological Organization on these warning systems and estimates that scaling them up in 57 countries could save almost 100,000 lives a year.
Limits
The life-saving figure is a modelled potential, not an observed result. Warnings help only if they reach people without phones, homes or a cool place to go and if they trigger concrete action on the ground.
What works
Checking on isolated older people, handing out water and helping people experiencing homelessness reach cooled spaces target the groups that die most often. Maricopa County in Arizona states that all heat-related deaths are preventable and publishes detailed death data so that partners can design interventions for these groups; its heat deaths fell from 608 in 2024 to 430 in 2025.
Limits
The county's report does not attribute the fall to any specific measure, and weather differs from year to year. 313 of the 430 deaths in 2025 followed exposure outdoors, so cooled spaces help only people who can reach them and are willing to stay.
What works
Water, shade, rest breaks, adjusted working hours, training to recognise heat illness and occupational heat rules reduce exposure for the more than 2.4 billion workers likely to face excessive heat.
Limits
Rules are hard to enforce for informal, agricultural and self-employed workers, who are often the most exposed, and lost hours mean lost income for people paid by the day or by the piece.
What works
Shading, insulation, reflective roofs and natural ventilation can lower indoor temperatures by 0.5 to 8 °C and often pay back within two to eight years; combining fans with air conditioning can cut energy use by 30%.
Limits
More than 1 billion people lack adequate cooling, many cannot afford air conditioning or electricity, and under business as usual emissions from cooling could rise from 4.1 to 7.2 gigatonnes a year by 2050.
What works
Nature-based measures such as trees, parks and water in cities reduce excess urban heat, and the UN Secretary-General's Call to Action names them as part of building resilience.
Limits
Greening takes years to have an effect and needs space and water. Informal settlements, which can be hotter than other parts of a city, often have the least room for it.
Promising, but not proven at scale. The stage shows how far each approach has come; it is not a recommendation.
The UN Call to Action asks countries to adapt social protection to heat, for example by paying support or activating services before or during a heatwave. How much such schemes reduce illness and deaths has not yet been widely evaluated.
UNEP's Global Cooling Watch 2025 sets out a pathway combining passive design, low-energy and hybrid options, high-efficiency equipment and a faster phase-down of climate-warming refrigerants that could cut cooling emissions by 64% by 2050 compared with business as usual while widening access. It is a modelled scenario that depends on national policies being adopted.
Maricopa County records the living situation and exposure setting of every heat death, which showed that 208 of 430 people who died in 2025 were experiencing homelessness and that broken air conditioners were common in indoor deaths. Few places collect data this detailed.
Knowledge gap
Global figures are modelled and differ between studies, and few places investigate individual heat deaths, so local authorities often do not know who is dying or where.
Implementation gap
More than 1 billion people lack adequate cooling, and without action the number could triple by 2050. People without stable housing, power or money for electricity are left out.
Implementation gap
More than 2.4 billion workers are likely to be exposed to excessive heat, and heat is linked to an estimated 18,970 work-related deaths a year. Labour rules rarely reach informal and agricultural work.
Coordination gap
Protecting people from heat involves health, labour, housing, energy, social protection and urban planning, and the UN Call to Action spans all of these. No single agency usually owns the problem, and responses depend on coordination that is often missing.
Technology gap
Under business as usual, emissions from cooling could rise from 4.1 to 7.2 gigatonnes a year by 2050, so expanding air conditioning without efficient equipment and clean power would add to the problem it addresses.
Implementation gap
In Maricopa County, air conditioning was present in 94% of indoor heat deaths in 2025 but not working in 72% of those cases. Repairs, power bills and poorly insulated housing put people at risk even where cooling exists.
Unresolved questions, not vetted breakthroughs. Answering any of them would change what is possible.
How can countries without detailed death investigations estimate who dies from heat, where and in what circumstances, so that warnings and outreach can be targeted?
Which combinations of outreach, water, shaded and cooled spaces and housing reduce heat deaths among people experiencing homelessness, and how much does substance use or mental illness change the risk and the right response?
At what combinations of heat and humidity do older people, infants and people with chronic disease reach dangerous levels of strain, and how should warning thresholds account for them?
How can access to cooling be widened fastest for people on low incomes using passive design, fans and efficient equipment, and what does it cost compared with the health losses avoided?
Call or visit older neighbours, relatives who live alone and people with chronic illness during hot spells, and make sure they drink water and have a cool place to spend the hottest hours.
Learn to recognise heat exhaustion and heatstroke, such as confusion, a very high body temperature or collapse. Heatstroke is a medical emergency: call emergency services and start cooling the person while you wait.
Donate water, electrolyte drinks, hats and sunscreen to, or volunteer with, organisations that do street outreach to people experiencing homelessness, and share where local cooling centres are.
Support shade trees, cooling centres and heat-health plans in your local area, and protections such as water, shade and rest breaks for people who work outdoors.
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World Health Organization (31 July 2026), published 2026
WHO's current fact sheet on heat. Source of the estimate of 489,000 heat-related deaths a year (2000–2019) and its regional split, the 85% rise in heat deaths among people over 65, the groups at highest risk, the health effects of heat and WHO's work on heat-health action plans and warning systems.
World Health Organization (news release, 29 October 2025), published 2025
WHO's summary of the 2025 report of the Lancet Countdown on health and climate change, the latest global edition (the 2026 report had not been published by October 2026). Source of the 546,000 deaths a year, the 23% rise in the death rate since the 1990s, 16 extra days of dangerous heat in 2024, heatwave days among infants and older adults, and 640 billion lost labour hours.
International Labour Organization (news release, 22 April 2024), published 2024
Release of the ILO report Ensuring safety and health at work in a changing climate. Source of the 2.4 billion workers exposed to excessive heat, the rise from 65.5% to 70.9% of the workforce since 2000, and heat-attributable injuries, deaths and chronic kidney disease (2020 estimates).
World Health Organization (news release, 25 July 2024), published 2024
Release of the UN Secretary-General's Call to Action on Extreme Heat. Source of its four areas of action, the call for heat-ready health systems and social protection and nature-based urban cooling, and WHO's estimate that scaling up heat-health warning systems in 57 countries could save almost 100,000 lives a year.
UN Environment Programme / Cool Coalition (Global Cooling Watch 2025, 11 November 2025), published 2025
UNEP's summary of Global Cooling Watch 2025, launched at COP30. Source of the more than 1 billion people lacking adequate cooling, projected cooling emissions, the 64% emissions cut in the sustainable cooling pathway and the effect and payback of passive cooling.
Maricopa County Department of Public Health, Arizona (April 2026), published 2026
Annual review of every heat-related death in Maricopa County (Phoenix area), one of the most detailed local surveillance systems. Source of the 430 deaths in 2025, the 208 people experiencing homelessness, outdoor and indoor exposure, air conditioning failures and the fall from 608 deaths in 2024.