As climate change pushes global temperatures higher, attention has focused predominantly on the threat that heat poses to older adults, whose physiology makes them more susceptible to health complications.
But a study published on Friday in the journal Science Advances found that certain types of young people — including seemingly hardy working-age adults — may also be particularly vulnerable to rising temperatures.
The researchers analyzed deaths in Mexico from 1998 to 2019 and discovered that people younger than 35 accounted for three-fourths of heat-related fatalities.
“These age groups are also quite vulnerable to heat in ways that we don’t expect even at temperatures that we don’t think of as particularly warm,” said Andrew Wilson, a first author of the paper and an environmental social scientist at Stanford University.
Measuring heat-related deaths is complicated, since death certificates rarely name heat as a cause. A proximate cause of death, like cardiovascular failure, is often listed instead.
To get around this, Dr. Wilson and his colleagues used a common statistical approach to estimate how daily mortality rates across Mexico change in response to fluctuations in the “wet bulb” temperature, a measurement that uses humidity and air temperature to capture how well humans can cool their bodies through sweating.
Keeping with decades of research, the researchers found that adults older than 70 were vulnerable at very hot temperatures.
Unexpectedly, the team also found that a large number of children under 4 and adults aged 18 to 34 were dying from the heat — but at more moderate conditions than those affecting older adults.
Most of the deaths occurred at wet-bulb temperatures around 75 degrees Fahrenheit, roughly equivalent to 88 degrees Fahrenheit with 50 percent humidity.
That’s concerning, Dr. Wilson said, because while climate change is likely to increase the number of extreme heat waves, the moderately hot temperatures that seem to be causing young people to die will be far more common.
Because of this, his team projected that deaths among children and young adults would increase by 32 percent by the end of the century.
“You’re going to increase the number of moderately warm days much more than you’re going to increase the number of extremely hot days,” he said.
People aged 50 to 70 experienced the least amount of heat-related mortality, the research found.
While this study did not give insight into the mechanism by which heat killed people, the paper pointed to several theories that might explain why younger age groups are at risk.
Children under 4 are worse at thermoregulation and don’t have the agency to remove clothing or change environments when they’re under heat stress.
Adults between 18 and 34 are more likely to engage in vigorous activity in the heat — for example, sports or strenuous outdoor jobs. One analysis of Mexican death certificates found that working-age men were more likely to have extreme weather listed as the cause of death.
“It’s not just about your physiological vulnerability,” said R. Daniel Bressler, a graduate student at Columbia University who is also a first author of the paper. “It’s about the economic and the social factors that make it so that you’re more exposed.”
If heat-related deaths among young adults are a result of working conditions, instituting more stringent occupational safety rules could be a fairly straightforward solution, said Drew Shindell, a climate scientist at Duke University who was not involved with this study.
“It’s not like, what do we do about the sea level rising or something where we wouldn’t know what to do,” he said. “We could probably get rid of virtually all these heat-related deaths.”
This year, the Biden administration announced new regulations that, if carried out, would require employers to establish heat safety plans and provide rest areas and water to workers in hot weather.
It is still not clear whether this trend among young people in Mexico, which the researchers chose to study because of the country’s hot climate and detailed mortality data, will be seen in wealthy countries like the United States, where air-conditioning is more common and labor practices are different, said R. Jisung Park, an environmental and labor economist at the University of Pennsylvania.
The researchers are now analyzing similar data in the United States and Brazil.
But Mexico may act as an indicator of how poorer, warmer countries that more often have younger workers performing manual labor will fare as temperatures continue to climb.
“We project, as the climate warms, heat-related deaths are going to go up and the young will suffer the most,” Mr. Bressler said.
