Over 80% of India’s elderly could face dangerous heat stress at 3°C global warming, says Lancet study

“Importantly, these hours are not spread evenly throughout the year. They are concentrated between May and September,” Prof Qinqin Kong, lead author of the study, told The Indian Express via email. At more than 3°C of warming, older adults could experience intolerable heat repeatedly during both day and night for several consecutive months, leaving little opportunity for the body to recover. Such exposure would put enormous pressure on access to cooling, electricity, health services and heat-response programmes, Prof Kong said.

Why older people are more vulnerable

Prof Kong said older people face serious health risks from heat at lower temperatures than previously estimated. Previous studies relied heavily on measurements from healthy young adults to determine when temperature and humidity would become challenging for the human body.

Older people are generally less able to regulate body temperature: they sweat less, their blood vessels respond more slowly and their hearts work harder in the heat. As a result, earlier studies may have underestimated the number of people at risk from extreme heat by at least two-fold, she said.

Dr Mahaveer Golechha, professor at IIPH-Gandhinagar and a senior heat action plan expert, said the research is a timely reminder that there is no single temperature threshold at which heat becomes dangerous for everyone.

“Ageing diminishes the body’s ability to control heat, and older people are likely to experience greater physiological strain at much lower temperature-humidity levels than healthy young people,” he said.

The study’s finding that South Asia is among the most vulnerable regions should be treated as a grave public health warning, Dr Golechha added. Heat action plans should not rely on single maximum-temperature thresholds. Heat-health early warning systems should account for age-related vulnerability, temperature and humidity, duration of exposure, night temperatures and local demographics.

What the study found

The study accounts for age-related differences in heat tolerance on a global scale. Researchers drew on laboratory experiments measuring heat tolerance in three age groups: younger adults (15–39), middle-aged adults (40–59) and older adults (60 and above). They then mapped where heat would become dangerous for each group and how many people would be affected.

Using a high-resolution heat-stress metric, the researchers modelled scenarios ranging from 1°C to 4°C of global warming above pre-industrial levels, in 0.5°C increments, alongside population projections.

At 1.5°C of warming, an estimated 22% of the world’s older adults—about 290 million people—would face unsafe heat for at least a month each year. The study defines this as at least 180 hours of dangerous heat annually, assuming roughly six hours of such exposure per day. Young adults would not face a similar level of risk until 4°C of warming, when 17%—about 490 million people—would be affected. Each additional degree of warming exposes roughly one billion more people to temperature-humidity combinations beyond which the body can no longer maintain its heat balance.

What “intolerable heat” means

Prof Kong stressed that the study’s “heat limit” is not a line between life and death. Crossing it does not mean someone will die within a short period. Rather, it marks the point at which the body can no longer keep its core temperature stable, even during very light activity such as desk work.

Beyond this point, the body continuously gains heat, and prolonged exposure can eventually become life-threatening.

The burden is also highly unequal. The greatest exposure is concentrated in tropical and subtropical regions, particularly South Asia, the Persian Gulf, West Africa, Southeast Asia and eastern China. Many of these areas are densely populated and have fewer resources for adaptation.

“India, Pakistan, Bangladesh, Myanmar and Niger stand out because they combine large population exposure and high poverty rates, which we use as an indicator of limited capacity to adapt,” Prof Kong said. The combination of intense heat, large vulnerable populations and limited resources is particularly concerning, she added.

Implications for outdoor work

The findings also have implications for outdoor work in India. In the Indo-Gangetic Plain, the period of greatest heat stress overlaps with the monsoon and Kharif agricultural season. “If extreme heat persists through both the day and night, simply shifting outdoor work to cooler hours becomes much harder,” Prof Kong said. This could affect health and livelihoods as well as agricultural productivity and food security.

Why heat warnings alone are not enough

Dr Golechha said heat-health strategies must go beyond issuing warnings and identify those most vulnerable to heat. “Provide special alerts and assistance for the elderly, people living alone, people with chronic illnesses, people living in poorly ventilated homes, and people without reliable cooling,” he said.

The authors also called on governments to develop heat-emergency plans and improve access to cooling and heat-warning systems. Cutting emissions, they said, remains essential to reducing older people’s risk of prolonged, round-the-clock heat and protecting future generations from widespread heat exposure.