New Delhi: A rise in global temperatures of three degrees Celsius above pre-industrial levels could expose older adults in India, China, Pakistan and Bangladesh to dangerous heat, with the four countries accounting for 1.5 billion people, or 73 per cent of the total affected population worldwide, a new study published in The Lancet Planetary Health journal has projected.
Older people could face serious health risks from heat at lower temperatures than previously estimated, it found.
South Asia and the Persian Gulf regions are shown to be the most exposed, where older adults could face dangerous heat day and night for up to three months once warming reaches three degrees Celsius or above, researchers, including those from Stanford and The Pennsylvania State universities, said.
The greatest challenge is projected to fall on countries that combine a high heat exposure with high poverty rates and a limited access to cooling, including India, Pakistan, Bangladesh, Myanmar, and Niger.
Older adults are less able to regulate their body temperature under high temperatures and humidity as they tend to sweat less, their blood vessels respond more slowly, and their hearts work harder in the heat.
The researchers said previous research relied heavily on measurements from healthy young adults to estimate when temperatures and associated humidity would become challenging for the human body.
Previous studies may have underestimated the number of older adults at risk from extreme heat by at least twice, they said.
The study accounts for age-related differences in heat tolerance on a global scale, the team said.
They drew on previously published laboratory experiments that measured heat tolerance separately for three age groups -- younger adults (15-39), middle-aged adults (40-59), and older adults (60 and over).
The researchers then mapped where heat would become dangerous for each group, and how many people would be affected, using a high-resolution heat stress metric across different scenarios ranging from one to four degrees Celsius of global warming above pre-industrial levels, combined with population projections.
"India, China, Pakistan, and Bangladesh together account for 1.48 billion exposed individuals, representing 72.6 per cent of the 2.03 billion global total exposed population," the authors wrote.
The study provides new empirical heat limits for middle-aged and older adults beyond which the body cannot compensate for heat stress, they said.
Accounting specifically for the age vulnerability was found to substantially increase the magnitude and spatial extent of projected risk, compared to those reported in previous studies.
"Older adults face far more frequent and widespread heat exposure beyond the body's ability to compensate, having greater heat risk under 1.5 degrees Celsius of global warming than young adults do under 4 degrees Celsius," the authors wrote.
Dangerous heat is likely to affect more people, across larger regions, and for longer durations than previously recognised, they said.