In 2023, heat killed around 50,000 people in Europe alone. Year after year, temperatures keep rising and extreme weather events get more and more common. Historically, heat-related mortality was assumed to affect older individuals more severely. But a new study says otherwise. The study, carried out in Mexico, reports that people under 35 bear the brunt of heat-induced deaths, accounting for 75% of these fatalities in Mexico.

“It’s a surprise. These are physiologically the most robust people in the population,” said study coauthor Jeffrey Shrader of the Center for Environmental Economics and Policy, an affiliate of Columbia University’s Climate School. “I would love to know why this is so.” The research appears this week in the journal Science Advances.

High wet-bulb temperature

Mexico provided a unique dataset for this study, offering mortality records across age groups alongside granular temperature and humidity data. The researchers analyzed 13.4 million deaths over 21 years, correlating them with local wet-bulb temperature readings.

Wet-bulb temperature is a measure of heat that combines air temperature and humidity, reflecting the ability of the human body to cool itself through sweating. Unlike dry-bulb temperature, which is essentially just air temperature alone, wet-bulb temperature accounts for the critical role of sweat evaporation in regulating body temperature. It’s the “real feel” of heat.

At a wet-bulb temperature of 35°C (95°F), or even lower under some conditions, the body can no longer dissipate heat effectively. So, prolonged exposure to such conditions can cause heat stress, heat stroke, organ failure, and death, even for healthy individuals.

The team found that on average, Mexico suffered approximately 3,300 heat-related deaths per year. And the findings found stark disparities based on age:

• Children under five: Mortality risk increases by 45% with a single day of extreme humid heat.
• Ages 18 to 34: This group experiences higher mortality rates due to occupational exposure and lack of access to cooling mechanisms like air conditioning.
• Elderly populations: While still at risk, they are more vulnerable to cold-related deaths.

Why this is happening

The study only looked at age-related distribution and didn’t zoom in on the mechanisms behind this. But researchers suspect one of the important factors is occupational hazard. Younger adults are more likely to be engaged in outdoor labor, especially physical labor, such as farming or construction. This leaves them more exposed to dehydration and heatstroke.

“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,” said Shrader. A previous separate analysis by Mexican researchers showed that death certificates of working-age men were more likely to list extreme weather as a cause than those of other groups.

It’s not clear if the findings from Mexico carry over to the rest of the world, but the findings have global implications. Countries in the tropics and subtropics, home to the youngest populations, face similar threats. Many of these nations are also among the least equipped to adapt to rising heat levels. For instance, countries in South Asia, Sub-Saharan Africa, and parts of Latin America are likely to experience disproportionate heat-related mortality due to their climatic conditions and demographic profiles.

As temperatures continue to rise, the burden is also expected to rise. The only good news is that by knowing which groups are most at risk, we can take measures to adapt and reduce the threat for the most at-risk population. Protecting younger populations will require multifaceted approaches, including better urban design, workforce protections, and affordable cooling solutions. Not least, public awareness on the dangers of humid heat are also important, as many young adults may not be aware of the risks.

Journal Reference: Andrew J. Wilson, R. Daniel Bressler, Catherine Ivanovich, Cascade Tuholske, Colin Raymond, Radley M. Horton, Adam Sobel, Patrick Kinney, Tereza Cavazos, Jeffrey G. Shrader. Heat disproportionately kills young people: Evidence from wet-bulb temperature in Mexico. Science Advances, 2024; 10 (49) DOI: 10.1126/sciadv.adq3367