High heat preferentially kills young people, not old people, study finds

Many recent studies assume that older people are at particular risk of dying from extreme heat as the planet warms. A new study on mortality in Mexico overturns this hypothesis: it shows that 75% of heat-related deaths occur in people under the age of 35 – a significant percentage of them being aged 18 to 35, the same group one might expect. be the most resistant to heat.

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

The research appears in the journal Scientific advances.

The researchers chose Mexico for the study because it collects very granular geographic data on mortality and daily temperatures. The researchers reached their conclusions by correlating excess mortality, that is, the number of deaths above or below average, with temperatures on the so-called wet bulb scale, which measures the amplified effects of heat when combined with humidity.

The analysis found that between 1998 and 2019, the country recorded around 3,300 heat-related deaths per year. Of these, almost a third concerned people aged 18 to 35, a figure far disproportionate to the figures for this age group. Also very vulnerable: children under 5 years old, especially infants. Surprisingly, people aged 50 to 70 suffered the least heat-related mortality.

Based on this, “we predict that as the climate warms, heat-related deaths will increase and young people will suffer the most,” said study co-lead author R. Daniel Bressler, who holds a Ph.D. candidate in Columbia’s sustainability program.

Researchers say several factors may be at work. Young adults are more likely to work outdoors, particularly in agriculture and construction, and are therefore at greater risk of dehydration and heat stroke. The same goes for indoor manufacturing in spaces without air conditioning.

“It’s the younger guys, at the bottom of the totem pole, who are probably doing the lion’s share of the hard work, with inflexible work arrangements,” Shrader said. Young adults are also more likely to participate in strenuous outdoor sports, the researchers point out.

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

The vulnerability of infants and young children was somewhat less surprising. We already know that their bodies absorb heat quickly and that their ability to sweat, and therefore cool down, is not fully developed. Their immune systems are also still developing, which can make them prey to illnesses that become more common with humid heat, including vector-borne and diarrheal diseases.

Humidity temperatures are often converted by popular media to “real” heat ratings on the Fahrenheit scale, where the numbers can vary depending on the exact combination of heat and humidity. According to the study, humid temperatures of around 13°C (equivalent to 71°F with 40% humidity) are ideal for young; within this range, they experience minimal mortality.

Previous research has suggested that workers begin to have difficulty when humid temperatures reach around 27 degrees Celsius, which would be equivalent to 86 to 105 degrees Fahrenheit, depending on humidity. However, the new study found that the greatest number of deaths occurred at humid temperatures of only 23 or 24°C, in part because these temperatures were much more common than higher temperatures, and thus cumulatively exposed more people. people in dangerous conditions.

Using the same daily temperature and mortality data, the researchers found that older people primarily died not from heat, but rather from moderate cold. (Mexico is primarily tropical and subtropical, but has many climate zones, including high altitude areas that can get relatively cold.)

Among other things, older people tend to have a lower core temperature, which makes them more sensitive to cold. As a result, they might be inclined to stay indoors, where infectious diseases spread more easily.

Despite all the attention paid to the dangers of global warming, extensive research has found that cold, not heat, is currently the leading cause of temperature-related mortality worldwide, including in Mexico. However, the proportion of heat-related deaths has been increasing since at least 2000, and this trend is expected to continue.

The new study has global implications, the researchers say. Mexico is a middle-income country; Compared to the population under 35, it is average and around 15% of workers are employed in agriculture.

In contrast, many poorer, hotter countries, mainly in Africa and Asia, have much younger populations working in much higher proportions. So, if Mexico is to be believed, heat-related mortality in these countries could be massive.

A study published in 2023 showed that farmworkers in many poor countries are already planting and harvesting amid increasingly oppressive heat and humidity.

Bressler said the team was now looking to confirm their findings by expanding their research to other countries, including the United States and Brazil.