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In April, during an intense heat wave in the Mexican city of Monterrey, María de Jesús Ávila was leaving work when she spotted a man unconscious on the sidewalk outside a construction site. She rushed over to check if he was still breathing -- but it was too late. Moments later, an ambulance arrived to carry his body away.
Between April and June 2024, Mexico endured three record-breaking heat waves. Temperatures in the hottest areas, like Monterrey, soared to 113°F (45°C). By the end of the third wave, Mexico's Health Ministry reported 1,937 heat-related medical cases and 90 fatalities from heat strokes and dehydration.
"I'll never forget it," Ávila said, reflecting on the April incident. The man looked no older than his early 30s, said Ávila, a sociologist at the Autonomous University of Nuevo León who for years has studied the growing impact of extreme heat on northern Mexico's workers.
While scientists have long monitored vulnerable groups like the elderly during extreme weather events, new evidence reveals a startling trend: Younger workers have become one of the most at-risk groups for heat-related deaths over the past two decades.
A recent study published in the journal Science analyzed heat-related death records in Mexico from 1998 to 2019. Of the 73,000 heat-related deaths during that period, 75 percent occurred among those under 35, with children under 5 and adults aged 18-34 being the most vulnerable.
"The people who are suffering are the ones who are young adults working outside or young children who can't make decisions about how much water they drink or what clothes they wear," said study co-author Andrew Wilson, a researcher at Stanford University's Center on Food Security and the Environment.
Mexico was a key focus for the research not only because it has one of the best public databases on heat-related deaths, but also because it's one of the few countries where wet-bulb temperatures have reached nearly 95°F (35°C) -- the maximum the human body can survive. This measure -- critical for understanding heat's real-world impact -- caught the attention of Wilson and his team.
Wet-bulb temperature takes its name from the way it's measured: A thermometer is wrapped in a wet cloth, simulating how the human body cools itself through sweat. Unlike standard dry-bulb measurements, which only gauge heat, wet-bulb temperature accounts for both heat and humidity. This distinction is vital because humidity dramatically affects how people experience and cope with heat.
"When the air is humid, sweat doesn't evaporate as effectively," Wilson explained. "Evaporation is key to regulating body temperature. Without it, high humidity and heat can overwhelm the body's cooling mechanisms, leading to health issues -- including death."
On a typical day with an average temperature of 90°F and 70 percent humidity, the wet-bulb temperature would be approximately 81°F (27°C). "A 30°C wet-bulb temperature would feel like the most miserable day of your life by far," Wilson said.
The accuracy of Mexico's heat-related mortality records allowed researchers to analyze deaths across six age groups, quickly identifying the most affected populations. "Having the ability to analyze data with this level of precision is unprecedented," said Bernardo Bastién-Olvera, a geographer at the National Autonomous University of Mexico. He was not involved in the new research.
Vulnerability for children under 5 can largely be explained by their heightened susceptibility to overheating and their underdeveloped immune systems, which leave them more prone to climate-related infectious diseases like insect-borne and diarrheal illnesses.
"Younger workers are less aware that heat can knock you down."
Young adults, on the other hand, are more likely than older individuals to work in outdoor occupations with minimal flexibility for adopting precautionary measures. This is the harsh reality for millions of people in low- and middle-income countries worldwide.
In Mexico, roughly 8.8 million people work in construction, and another 5.3 million are employed in agriculture. While outdoor labor contributes significantly to Mexico's economy, experts agree that protections for these workers have been neglected as temperatures rise -- a problem worldwide.
Tereza Cavazos, a climatologist at the Centro de Investigación Científica y de Educación Superior de Ensenada in Baja California, and co-author of the Science study, recalls how citrus farmers like her father would take midday naps to avoid the heat during her childhood in Nuevo León.
"It was the only way they could escape the sun," Cavazos said. But the problem today, she noted, is that "younger workers are less aware that heat can knock you down."
Ávila was not involved in the new study. In a separate 2020 analysis, she and colleagues from the Autonomous University of Nuevo León analyzed deaths from extreme climate events in Mexico between 2000 and 2015. They found that nearly 30 percent of fatalities were caused by extreme heat and cold, but their results didn't reveal such a pronounced disparity in deaths among younger versus older age groups.
Ávila believes that heat-related deaths among the elderly are underreported. Older individuals often have other medical conditions, and when their cause of death is recorded, those conditions are listed -- even when heat was the ultimate trigger, Ávila explained.
The Science study authors acknowledge that Mexico's relatively young population may partially explain the high mortality rates among younger individuals. However, they also point out that many of the world's regions experiencing the most severe temperature increases -- such as Africa, Central America, the Middle East and parts of South and Southeast Asia -- also have some of the youngest populations. These regions could be facing similar trends, but insufficient data makes it difficult to conduct similarly precise analyses.
Patrick Behrer, an economist in the Sustainability and Infrastructure team of the World Bank, has spent recent years studying how governments can implement climate adaptation strategies to protect workers exposed to extreme heat for long hours.
In 2021, Behrer published a study analyzing the conditions of agricultural workers in California. His findings aligned with the Science analysis: Younger workers are the most affected by rising temperatures. Despite this growing trend, public policies aimed at adaptation remain scarce.
One such example is California's 2005 law requiring employers to protect workers when temperatures exceed 80°F. When the daily maximum temperature hits that threshold, employers must provide their employees with shade, water and rest breaks that increase in frequency as temperature goes up.
Yet, Behrer notes, boosting economic opportunities might be the most effective way to help workers cope with the adverse consequences of heat.
"The majority of the people in the world work in environments where they don't have a lot of control over their environmental conditions or their exposure to heat. Thinking about what climate change will mean for those people is a very important question because it will affect millions of people," Behrer said. "We want to provide people with jobs, but a part of good jobs is also being safe at work."
In 2024, Nuevo León's government established hydration stations in crowded areas, offering water and electrolytes to help people endure heat waves. Temperatures were slightly lower than in 2023, but heatstroke deaths fell more dramatically: from 112 that year to 14, Ávila said. While no studies have yet assessed the effectiveness of the hydration strategy, she thinks it's clear that it played a role.
However, targeted actions focused on protecting workers are still urgently needed, she said.
According to the Science study, younger individuals account for 87 percent of heat-related lost life years, a measure that highlights the difference between the years lost when a young person dies compared to someone older. "No life is worth more than another," said Wilson. However, this metric can be used to plan health strategies, set prevention priorities and compare premature mortality across populations.
The study estimates that if current trends continue, heat-related deaths among people under 35 could increase by up to 32 percent by 2100. "The disproportionate burden of this impact on the young is likely an important source of future climate-driven inequality," the authors write.