Some 75% of heat-related deaths occurred amongst people under the age of 35 , and one-third of such deaths were young adults between the ages of 18 to 35, according to a new study on heat mortality in Mexico.

This contradicts previous assumptions that the elderly are the most vulnerable to heat.

“We project, as the climate warms, heat-related deaths are going to go up, and the young will suffer the most,” said R. Daniel Bressler, a PhD candidate in Columbia’s Sustainable Development program and co-lead author of the study, published in Science Advances.

“It’s a surprise. These are physiologically the most robust people in the population,” said the study’s co-author Jeffrey Shrader  from the Center for Environmental Economics and Policy, an affiliate of Columbia University’s Climate School in New York City. “I would love to know why this is so.”

The researchers speculate that several factors may be at work. Young adults are more likely to be engaged in outdoor labour including farming and construction, and thus more exposed to dehydration and heat stroke. Young adults are also more likely to participate in strenuous outdoor sports, the researchers pointed out.

Infants and children under the age of five are also particularly vulnerable to heat as their bodies are not yet efficient in regulating temperature. Deaths of infants and young children, together with those aged 18-35, made for 75% of all heat-related deaths.

Meanwhile, people in the 50 to 70 age bracket suffered the least amount of heat-related mortality to the surprise of researchers.

Using the same daily temperature and mortality data, the researchers found that elderly people were at a higher risk of dying from modest cold, as compared to heat, at least in the Mexican context. Mexico is mainly tropical and subtropical country, but it has many climate zones including high-elevation areas that can get relatively chilly.

Younger people do most manual, outdoor labour

The new study has global implications, said the team of researchers, who along with Shrader, are mostly affiliated with Columbia, as well as Boston University, Stanford and the University of California. Many poor, hot countries, mainly in Africa and Asia, have young populations, with a large proportion working in manual and outdoor labour.

The retrospective analysis assessed heat-related deaths over a twenty-year period, from 1998 to 2019. For individuals under 35, heat causes 2.6 times more deaths than cold whereas for individuals 35 and older, cold causes 56 times more deaths than heat.

Children under five, especially infants, also had a disproportionate number of heat-related deaths, although not as large. Overall, people under 35 years accounted for 75% of historical, heat-related deaths, the researchers found.

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

“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.

The vulnerability of infants and small children came as somewhat less of a surprise. It is already known that their bodies absorb heat quickly, and their ability to sweat, and therefore cool off, is not fully developed. So exposure to temperatures that exceed their body temperature can be rapidly fatal.

Their immune systems are also still developing, which put them at higher risk of ailments that become more common with humid heat, including vector-borne and diarrhoeal diseases.

The researchers reached their conclusions by correlating excess mortality or the number of deaths above or below the average, with average temperatures in the same period, on the “wet-bulb scale” that reflects he magnified effects of heat when combined with high levels of humidity.

Despite being based mostly in the USA, the researchers y chose Mexico for the study because it collects highly granular geographical data on both mortality and daily temperatures.

Older people more likely to die of cold

Older people tend to have lower core temperatures, making them more sensitive to cold. In response, they may be prone to staying indoors, where infectious diseases spread more easily.

Despite all the attention given to the dangers of global warming, other extensive research has generally suggested that to date, excessive cold exposures, not heat, are currently the world’s number one cause of temperature-related mortality, including in Mexico. However, the proportion of heat-related deaths has been climbing since at least 2000, and this trend is expected to continue.

Around 4.1 billion people or roughly half the world’s population has experienced unusually hot temperatures between June and August this year, according to a report from US-based non-profit Climate Central.

Deaths occurred at lower-than-expected heat levels

There is a widespread recognition that temperatures alone are not a good measure of the impact of heat but “wet bulb temperatures” that also factors in humidity, are. Essentially, the same temperature level in conditions of high humidity is more dangerous than at low-humidity because humid conditions reduce the absorption of sweat, which cools off the body.

Wet bulb temperatures are often referred to as the “real-feel” heat indexes where numbers can vary depending on the exact combination of heat and humidity.

Previous research has suggested that workers begin to struggle when wet-bulb temperatures reach about 27°C, which would equate to 86 to 105°F, depending on humidity. However, the new study found that the largest number of deaths occurred at wet-bulb temperatures of just 23 or 24°C, in part because those temperatures occurred far more frequently than higher ones, and thus cumulatively exposed more people to dangerous conditions.

A study published last year showed that farm workers in many poor countries are already planting and harvesting amid increasingly oppressive heat and humidity.

Bressler, the report’s lead author, said the team is now looking to firm up its conclusions by expanding its research into other countries, including the United States and Brazil.

Image Credits: Unsplash, Study: Heat disproportionately kills young people: Evidence from wet-bulb temperature in Mexico.