Young adults in Mexico are among those most at risk from the heatwave. According to a new study, this cohort are especially hard hit by its intensity.

Recent studies suggest that elderly individuals are at particular risk from extreme heat as temperatures heat up around the planet, yet a new Mexico mortality study contradicts this notion: 75% of heat-related deaths among individuals under 35 are occurring among 18 to 35-year olds – whom would normally be expected to be most resistant.

“This finding comes as quite a shock, since these individuals appear to be among the healthiest members of society,” noted study coauthor Jeffrey Shrader of Columbia University’s Climate School’s Center for Environmental Economics and Policy affiliate. He would love to understand why it occurs this way; their research appeared this week in Science Advances journal.

Researchers selected Mexico for this research because it gathers highly detailed geographical information about mortality and daily temperatures. Their conclusions were reached through correlating excess mortality-that is, deaths above or below average-with temperatures measured using the so-called wet-bulb scale which measures heat’s magnifying effects when coupled with humidity.

From 1998 to 2019, Australia experienced approximately 3,300 heat-related deaths annually from 1998 through 2019. Of this figure, almost one-third occurred among people between 18 to 35 – an outsized amount considering their numbers – while children under 5 especially infants were highly at-risk of heatstroke deaths. Surprisingly though, people 50 to 70 experienced less heat related mortality.

“Based on this analysis, we project that as global temperatures warm, heat-related deaths will increase with age – particularly among younger populations,” stated R. Daniel Bressler (co-lead author and PhD. candidate at Columbia’s Sustainable Development program).

Researchers suspect a variety of contributing factors. Young adults are more likely to engage in outdoor labor like farming and construction that exposes them to dehydration and heat stroke; similarly indoor manufacturing spaces without air conditioning pose more of an exposure risk; “these more junior people, lower on the totem pole, probably do most of the hard work” said Shrader. Additionally, young adults tend to participate more in strenuous outdoor sports and extreme weather is more often listed on death certificates of working-age men than it is other groups.

Infants and small children were less of a surprise; we already knew their bodies absorbed heat quickly while their sweat glands hadn’t fully matured to provide relief from heat stress. Furthermore, their immune systems weren’t fully developed at that point either, leaving them susceptible to vector-borne and diarrheal illnesses more easily spreading during humid heatwaves.

Popular media often convert wet bulb temperatures to “real-feel” heat indexes on the Fahrenheit scale, where numbers may differ based on heat and humidity conditions in each location. According to this research study, wet-bulb temperatures of around 13 C (equivalent to 71 F with 40% humidity) are optimal for young people’s survival; at these levels they experience minimal mortality risk. Recent research indicates that workers begin to feel stressed when wet-bulb temperatures reach 27 degrees C – equivalent to between 86 to 105 F depending on humidity levels. However, according to this new research study, deaths due to wet-bulb temperatures as low as 23-24 C were the leading cause. These temperatures occurred more often than higher ones and thus exposed more people than before to potentially hazardous conditions.

By using daily temperature and mortality data from Mexico City, researchers discovered that elderly deaths occurred predominantly not from extreme heat but rather mild cold. (Mexico City is generally tropical or subtropical with multiple climate zones including high-elevation areas that can get quite cool.) Older adults tend to have lower core temperatures making them more sensitive to chillier environments – often choosing to remain indoors where infectious disease spread more readily.)

Although much attention has been directed at global warming’s possible dangers, extensive research demonstrates that cold is currently leading to temperature-related mortality worldwide and in Mexico; although heat deaths have steadily been increasing since at least 2000. It remains uncertain how this trend may play out over time.

According to its researchers, this new study could have global ramifications. Mexico is an example of an upper middle income country; by share of population under 35 it falls somewhere within average and 15% of workers employed in agriculture; many poorer countries in Africa and Asia with much younger populations working manual labor at much higher percentages may see vastly greater heat-related mortality as evidenced in Mexico’s case study; one published last year showed farmworkers in such nations already planting and harvesting under increasingly oppressive temperatures and humidity conditions.

Bressler stated that her team intends to bolster its conclusions by expanding research efforts across other nations – specifically the US and Brazil.

Andrew Wilson of Stanford University co-led this study. Cascade Tuholske of Montana State University; Colin Raymond from UCLA; Patrick Kinney from Boston University and Teresa Cavazos at Centro de Investigacion Cientifica y Educacion Superior de Ensenada in Baja California were co-authors, alongside Catherine Ivanovich Radley Horton and Adam Sobel from Columbia Climate School as coauthors.