In the study, published in Lancet Planetary Health, researchers simulated the extent to which human-caused global warming will expose older people to health risks.
Previous research estimating how much heat a human body can tolerate has primarily been conducted on young, healthy adults.
But older people are generally more vulnerable, because increasing age means that people sweat less, more often have weaker hearts, and more often have other underlying diseases.
Several degrees lower
The researchers, working at Stanford University in California, therefore used data from recent experiments that tested how people over 60 years of age tolerate higher heat and humidity.
The results show that older adults are exposed to various forms of heat stress at temperatures 4.7 to 7.5 degrees lower than young adults.
The conclusion is that if the world becomes 1.5 degrees warmer than in pre-industrial times, 22 percent of all people over 60 years of age in the world will be exposed to dangerous heat for at least one month each year.
If warming reaches 3 degrees, older people in the worst-affected regions - South Asia and the Persian Gulf - could be exposed to dangerous heat around the clock for three months straight every year, according to the study.
Poor countries
In connection with the fact that demographic developments in many countries will lead to a larger proportion of older people, health risks linked to heat are something that needs to be taken seriously, according to the researchers.
The countries facing the greatest threat were stated to be those with high poverty and poor access to cooling such as India, Pakistan, Bangladesh, Myanmar and Niger.
But the population of Europe, which is one of the continents predicted to warm the fastest, will also be affected.
