Extreme heat puts stress on everyone's bodies. In recent years, scientists and policymakers have homed in on the risks heat poses to older people, whose bodies become more sensitive to heat with increasing age.

But a new study in the journal Science Advances suggests that there is another group at risk, and one that gets less attention.

"Young people are disproportionately vulnerable to heat," says Andrew Wilson, an environmental scientist at Stanford University and an author of the new analysis.

The study, which focused on Mexico, found that people under 35 made up three-quarters of the country's deaths related to heat in recent decades, with risks concentrated amongst children under 4 years old and young adults from 18 to 35. That percentage is likely to increase in the future as human-caused climate change intensifies the number of sticky, humid heat days in the country.

The warming climate will also drive a drop in deaths related to cold weather, the study finds, at the same time as heat-related deaths rise. The study projects that overall, deaths influenced by temperatures will drop within Mexico. But who is dying will likely change. The study finds that in the past, temperature-related deaths have been concentrated among seniors and driven mostly by cold weather. In a hotter world, the burden of temperature-related deaths is likely to shift toward younger people.

The result is "a really surprising inequality across age groups," says Wilson.

The results underscore the complexity of temperature-related deaths in a changing climate, says Tamma Carleton, an environmental economist at the University of California, Berkeley, who was not involved with the study. In many mid-latitude and northern regions, the number of overall deaths from extreme temperatures is projected to drop. But in places that are already hot, deaths from heat are projected to soar.

In aggregate, "in most of the world we're going to see net increases under warming because those heat increases are going to overpower declines on the cold side," she says. "But it is a dance that can look very different in different regions of the world." And the impacts, Carleton says, are generally much greater in countries that have historically contributed the least to human-driven climate change.

What heat does to people's bodies

The study used detailed death records from across Mexico, which keeps more complete records than most other countries. The researchers compared the number of deaths with those that might be expected under normal conditions. Sometimes, there were more than expected. By linking that "excess mortality" with weather data like temperatures and humidity conditions, they could see how hot and cold weather influenced deaths. Looking at the relationship between deaths and temperature for different age groups, they could see who was most sensitive—and then see, using forecasts from climate models, what the impact on those different groups would be in the future.

The human body reacts differently to heat at different ages. Babies and very young children generate more heat at rest, and their shape—smaller and rounder than adults, with less surface area to shed heat—makes them more prone to overheating. "You're basically a bigger ball that can absorb more heat easier," says Dan Vecellio, a climate and health expert at the University of Nebraska, Omaha, who was not involved in the study, compared to "the candle sticks like older adults are."

In contrast, young people and those in the middle of their life are often better at managing heat. They sweat efficiently and their blood vessels, hearts, and lungs adjust relatively effectively under heat stress. In older bodies, those systems often become less effective: Older people sweat less, and their hearts can't pump as hard, restricting their ability to move blood to their skin and cool down.

Humidity adds another curveball. People cool themselves by sweating; the evaporation of that sweat sucks heat out of their bodies. But if that sweat doesn't evaporate, the body can't remove the heat, which slowly builds up, pushing people away from the safe core temperature and into potential heatstroke. Evaporation slows, or even stops, when the air becomes saturated with water—high humidity conditions.

The risks of dying creep up well before extreme heat and humidity levels. Death rates, the study finds, increase even at temperatures in the 80s when humidity levels are also high.

Physiologically, "young people normally aren't as vulnerable if they are just doing things normally," says Vecellio. But that doesn't hold if they're exercising, playing, or working hard days outside in the heat. "Those are things that are going to increase the vulnerability and increase the risk for even younger adults," he says.

Solutions for every age

Many programs and policies designed to protect people from heat focus on older people. In the U.S., for instance, many outreach campaigns during heat waves target people over 65.

But "sometimes we forget to talk about this population in the middle age range," says Jenni Vanos, a heat and climate expert at Arizona State University who was not involved in the study.

In Vanos's home county of Maricopa, Ariz., public health officials have drilled deep into their own data to pinpoint who is dying in the county's routinely record-breaking and pervasive heat. She sees parallels with the groups identified in the new Mexico-focused study: Many of Maricopa's heat deaths also occur in younger people, primarily men, who work outdoors, unhoused people, or those without access to air conditioning.

Mexico is hotter than the U.S., and its demographic makeup and economic situation are different from the U.S.. But many of the key outcomes from the study are likely relevant in the U.S. as well, Wilson says. For example, it is true in both places that young people are often working in hot conditions, indoors or outside. That means they are, as a group, at higher risk in both countries.

The Occupational Health and Safety Administration in the U.S. is working on a workplace heat rule that would apply to millions across the country "Things like that would address the risks that these kinds of young adults face at work in a way that would improve overall health," Wilson says.

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