A surprising study of temperature-related deaths in Mexico upends conventional thinking about what age group is hit hardest by heat. Researchers found at higher temperatures and humidity, the heat kills far more young people under 35 than those older than 50.

For decades, health and weather experts have warned that the elderly and the youngest children were most vulnerable in heat waves. But this study looking at all deaths in Mexico from 1998 to 2019 shows that when the combination of humidity and temperature reach uncomfortable levels, around 30 degrees Celsius and 50% relative humidity, there were nearly 32 temperature-related deaths of people 35 years old for every temperature-related death of someone 50 and older.

The study in Friday’s journal Science Advances shows an especially surprising spike of heat-related deaths in an age group thought to be young and robust: people between 18 and 35. That age group alone had nine times as many temperature-related deaths as those older than 50.

Study authors and outside experts are scrambling to figure out why. Demographics alone don’t explain why more young adult Mexicans are dying in high heat than their elders. Two theories: Outdoor workers who can’t escape the heat, and young people who don’t know their limits.

The trend is likely to widen as the world warms from human-caused climate change, according to computer simulations run by the study team.

“We found that younger people are especially vulnerable to humid heat,” study co-author Jeffrey Shrader, a climate economist at Columbia University, said. “As the climate warms, we’re really going to be shifting the burden of temperature-related mortality towards younger individuals and away from older individuals who tend to be more vulnerable to cold temperatures.”

Data from cold weather shows more than 300 deaths of Mexican residents 50 and older for every young person dying from cold temperatures, according to the study.

“People of all ages are increasingly at risk from the rising temperatures, and this study shows that those that we might have considered relatively safe from heat-related adverse health outcomes might not be so much so,” said Marina Romanello, executive director of the Lancet Countdown that monitors health effects of climate change. She was not part of the study team.

“Heat is a much more dangerous silent killer than most people acknowledge it to be, and that heat is increasingly putting our health and survival at risk,” Romanello said in an email.

Study authors decided to examine weather-related deaths in Mexico because that country not only has detailed mortality data, but it has a variety of different climates making it an ideal place to study in depth, Shrader said.

Researchers also want to figure out whether this is just a situation in Mexico or other warmer sections of the globe have similar spikes in young adult deaths in high heat and humidity.

Initially the team just wanted to look at deaths and what scientists call wet-bulb globe temperatures, but when they looked at age differences, they were surprised and looked in more detail, Shrader said. Wet-bulb temperature, which is intended to mirror how the body cools itself, is derived using a complicated measurement system that factors in humidity and solar radiation. A wet-bulb globe temperature of 35 degrees Celsius is thought to be the limit for human survivability. Most places don’t reach that level.

Researchers determined temperature-related mortality by complex statistical analysis that compares numerous factors in the number of deaths and removes everything they can except temperature fluctuations, said study co-author Andrew Wilson, a Columbia climate economics researcher.

Researchers also calculated the ideal temperature for when there are the fewest excess deaths at each age group. Younger adults’ sweet temperature spot is about 5 degrees Celsius cooler than it is for older people, Shrader and Wilson said.

Some outside health and climate experts were initially puzzled at the higher youth mortality seen in the study. Co-author Patrick Kinney, a professor of urban health and sustainability at Boston University, said it was likely the study included a higher proportion of outdoor workers exposed to heat than prior studies did.

Study co-author Tereza Cavazos, a climate scientist at the Ensenada Center for Scientific Research and Higher Education in Mexico, said she remembers her father’s generations taking siestas in the high heat of the day and that was healthy. That doesn’t happen so much now, she said.

“There is a lot of population that is vulnerable in the future. Not even in the future, right now,” Cavazos said. She mentioned three Mexican heat waves this year that hit in the middle of the country and kept the deadly heat going overnight so people had little relief. Usually cool nights allow a body to recover.

Younger people often have a sense of invulnerability to weather extremes and do things that increase their risk, such as play sports in high heat, Cavazos said.

“High humidity makes it a lot harder for the body to cool itself through sweating – which is how our body primarily stays cool,” said Dr. Renee Salas, an emergency medicine physician and climate change expert at Massachusetts General Hospital and Harvard Medical School. She was not part of the study team. “So someone young and healthy working outside in heat and high humidity can reach a point where the body can no longer cool itself safely – causing a deadly form of heat injury called heat stroke.” 
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