India among top countries with highest heatwave exposure for infants, older adults: Report

Heatwave exposure measures how much heatwave time reaches a population, not simply how many heatwaves occur. Researchers combine the number of people in a vulnerable age group with the number of heatwave days they experience. This produces “person-days,” a measure of total population exposure.

For example, if 100 infants each experience five heatwave days, their exposure equals 500 infant person-days. The report also gives a population-normalised measure. In 2025, older adults experienced an average of 12.8 heatwave days per individual, while infants averaged 8.7 days.

This matters because total exposure can rise when a population grows, even if each person experiences fewer days. The report found that global exposure fell from 2024’s record, but 2025 was still the second highest on record for both groups combined. It helps show who faces the greatest heat burden and how that burden changes over time.

India was among the countries with the highest heatwave exposure for both infants and older adults. China, the United States and Russia were also listed among the countries facing the highest exposure. These comparisons focus on two age groups the report identifies as especially vulnerable.

Across the world in 2025, older adults experienced 10.8 billion person-days of heatwaves. Infants experienced 1.1 billion person-days. A person-day represents one person experiencing one heatwave day, so the totals capture the combined burden across entire populations.

The figures were lower than 2024’s historical high of 15.3 billion person-days for older adults and 2.3 billion for infants. Even so, 2025 was the second highest year on record for combined exposure to both groups. The scale shows why heat protection must reach large populations, not only individuals in isolated hot spots.

Infants and older adults are especially vulnerable because their bodies may regulate temperature less effectively than those of healthy working-age adults. Infants have small bodies and depend on caregivers for drinking fluids, staying cool and moving away from heat. Older adults may have reduced thirst responses, limited mobility or existing health conditions.

Heat can increase strain on the body, leading to dehydration, heat exhaustion and potentially life-threatening heatstroke. It can also worsen heart, lung and kidney problems. Medicines or difficulty reaching a cool place may further limit an older person’s ability to respond, while infants cannot make independent decisions about protection.

The report therefore measures exposure separately for these groups rather than treating the population as equally affected. In 2025, older adults averaged 12.8 heatwave days and infants 8.7. Identifying these groups helps direct warnings, care and adaptation toward people facing greater risk.

Heatwaves can cause dehydration, heat exhaustion and heatstroke. They can also place severe stress on the heart, lungs and kidneys, worsening existing illnesses and increasing the risk of death. The danger is greatest when heat is prolonged, widespread or affects people who cannot cool themselves safely.

The report links these harms to a striking 2023 figure. Heat was associated with an estimated 710,000 deaths that year. That total exceeded deaths from malaria or HIV, according to the report. The year 2023 was also the hottest year during the 2016–2025 period covered in the article.

Climate change was estimated to have driven 71.5% of those 2023 heat-related deaths. Altogether, the authors estimate that climate change caused almost 3 million heat deaths between 2016 and 2023. These figures show that heat is not merely uncomfortable weather; it is a major and growing public-health threat.

A person-day of heatwave exposure is counted whenever one person experiences one heatwave day. Researchers add those units across everyone in an age group. Thus, 1,000 people exposed for three days produce 3,000 person-days, even if the exposure happens in different places or at different times.

The report also divides total person-days by the size of the relevant population. That creates an average number of heatwave days per individual. In 2025, older adults averaged 12.8 days and infants averaged 8.7 days. This calculation makes the large totals more understandable.

Both measures answer different questions. Total person-days show the overall burden placed on a population and its services. The average shows how intensely the typical individual was exposed. Using both avoids confusing a large population with greater individual risk and allows fairer comparisons between groups or years.

Human-caused climate change warms the planet’s baseline conditions. When the starting temperature is higher, unusually hot days become easier to reach. Heatwaves can therefore occur more often, last longer or reach higher temperatures than they would in a cooler climate. The article links this broader warming trend to growing health harms.

The report’s figures illustrate the mechanism at population scale. In 2023, the hottest year during 2016–2025, heat was linked to an estimated 710,000 deaths. Climate change drove 71.5% of those deaths. Warmer conditions increase the chance that vulnerable people face dangerous heat and reduce the time available for recovery.

Exposure is also rising faster than the world’s response, according to the report’s authors. Global heatwave exposure in 2025 was the second highest on record for infants and older adults combined. Cutting greenhouse gas emissions can limit further warming, while stronger health systems can reduce harm from the heat already occurring.

The Paris Agreement is an international climate agreement adopted in 2015, with countries committing to limit global warming and strengthen climate action. The Lancet Countdown compares progress during the decade since the agreement with the decade before it. Its authors say climate-related health harms grew faster after 2016.

The report points to record heat exposure and deaths as evidence of the widening challenge. In 2025, older adults experienced 10.8 billion heatwave person-days and infants 1.1 billion. In 2023, heat was linked to an estimated 710,000 deaths, with climate change driving 71.5% of them.

The authors acknowledge progress in clean energy, health-system readiness and health-protective regulation. However, they say it remains too slow, too small and unfairly distributed. They call for climate change to be declared a global health emergency, a new WHO instrument and health-centred climate decisions, including fair emissions cuts and grant-based health finance.