"It's a surprise. These are physiologically the most robust people in the population," said Jeffrey Shrader of the Center for Environmental Economics and Policy, affiliated with Columbia University's Climate School. The study, published in Science Advances, highlights a significant shift in understanding the demographics affected by extreme heat.
Using Mexico's detailed geographical data on mortality and daily temperatures, researchers analyzed excess mortality-deaths exceeding average expectations-correlated with wet-bulb temperature. This metric reflects the combined effects of heat and humidity. Between 1998 and 2019, heat-related deaths in Mexico averaged 3,300 annually, with one-third involving individuals aged 18 to 35. This is an outsized proportion compared to their share of the population. Infants and young children under five were also found to be particularly vulnerable. Meanwhile, individuals aged 50 to 70 showed the lowest levels of heat-related mortality.
"As the climate warms, heat-related deaths are going to increase, with the young bearing the brunt of the impact," said R. Daniel Bressler, co-lead author and a PhD candidate in Columbia's Sustainable Development program.
The study attributes young adults' susceptibility to their involvement in physically demanding activities. These include outdoor labor in sectors like farming and construction, as well as indoor work in poorly ventilated, uncooled environments. Strenuous outdoor sports and limited flexibility in work arrangements also heighten their exposure. Supporting this, separate research from Mexican scientists found that death certificates for working-age men often listed extreme weather as a cause.
For infants and young children, the findings align with existing knowledge about their physiological vulnerabilities. Their bodies absorb heat more rapidly, and their cooling mechanisms, such as sweating, are underdeveloped. Immature immune systems also increase susceptibility to heat-aggravated illnesses like vector-borne diseases and diarrhea.
The analysis revealed that young people experience the lowest mortality at wet-bulb temperatures of about 13 C (71 F with 40% humidity). However, the highest number of deaths occurred at wet-bulb temperatures of 23 - 24 C, primarily because these conditions were more frequent, leading to prolonged exposure.
Interestingly, the study found that elderly individuals in Mexico predominantly succumb to moderate cold rather than extreme heat. Factors like lower core body temperatures and an increased tendency to stay indoors, where infections spread more easily, make this group particularly vulnerable to colder conditions.
Globally, cold-related deaths remain the leading cause of temperature-related mortality, but the proportion of heat-related deaths has been steadily climbing since 2000. The implications of this study are vast, particularly for lower-income countries with younger populations and higher rates of manual labor. Similar heat-related mortality trends could be even more pronounced in nations across Africa and Asia, where agricultural work is already being performed in intensifying heat.
The research team plans to expand their studies to other countries, including the United States and Brazil, to validate their findings and understand broader patterns.
Research Report:Heat disproportionately kills young people: Evidence from wet-bulb temperature in Mexico
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