Heat-related deaths – on the rise from climate change – are generally thought to threaten older adults the most.

But a study of Mexican data challenges that view, finding that over the past 20 years, people under 35 have accounted for a disproportionately high number of deaths.

The study, published in Science Advances, finds that 75% of heat-related deaths happen in under-35s.

“It’s a surprise. These are physiologically the most robust people in the population,” says co-author Assistant Professor Jeffrey Shrader, a researcher at Columbia University, USA.

“I would love to know why this is so.”

Mexico boasts a “unique combination” of high-quality mortality data and high levels of humid heat, according to the researchers, which is why they decided to use it to examine the relationship between age and heat vulnerability.

The researchers examined mortality data from 1998-2019, comparing it with wet bulb temperatures, which combine measures of humidity and heat.

Humidity is considered a particularly deadly facet of heat stress, because it lowers the body’s ability to cool itself down through sweating.

The team used the wet-bulb temperatures and mortality data to calculate excess deaths, which they could attribute to heat stress.

They found that 3,300 heat-related deaths occurred each year over the time period.

Almost a third of those deaths were in people aged 18-35, which is a much larger proportion than the total number of Mexicans in that age bracket.

The researchers don’t know exactly why this is the case, but they have several theories. A major factor is probably their jobs: young adults are more likely to be labouring outdoors, or indoors with limited air conditioning.

“These are the more junior people, low on the totem pole, who probably do the lion’s share of hard work, with inflexible work arrangements,” says Shrader.

Children under 5, especially infants, were also particularly vulnerable. This finding has been seen in other research: it’s generally considered that their physiology and limited ability to voice their distress are big factors.

People in the 50-70 age bracket were the least vulnerable to heat. But they were vulnerable to cold: the researchers also found that people over 50 accounted for 96% of cold-related deaths, and 80% of years of life lost from cold-related deaths.

This has also been seen in other studies. Research has suggested it’s for physiological, behavioural and social reasons, such as lower shivering temperature thresholds, vulnerability to respiratory infections, and living alone with lower access to healthcare.

“We project, as the climate warms, heat-related deaths are going to go up, and the young will suffer the most,” says co-lead author R. Daniel Bressler, a PhD candidate at Columbia University.

The researchers are now looking to see if the trend is reflected in other countries, including the USA and Brazil.