On a sweltering summer afternoon almost a decade ago, Meenu Tewari was visiting a weaving company in Surat in western India. Tewari, an urban planner, frequently makes such visits to understand how manufacturing companies operate. On that day, though, her tour of the factory floor left her puzzled.

“There were no workers there … only machines,” says Tewari of the University of North Carolina at Chapel Hill.

The missing employees weren’t far away; they were resting in the shade under a nearby awning. Scorching temperatures had been causing workers to make mistakes or even faint near the dangerous machinery, Tewari’s guide told her. So the company had mandated that workers come in earlier and leave later so that they could rest during the midday heat.

Physiologically, people’s bodies aren’t built to handle heat beyond wet bulb temperatures — a combined measure of heat and humidity — of around 35° Celsius, or about 95° Fahrenheit (SN: 5/8/20). Mounting evidence shows that when heat taxes people’s bodies, their performance on various tasks, as well as overall coping mechanisms, also suffer. Researchers have linked extreme heat to increased aggression, lower cognitive ability and, as Tewari and colleagues showed, lost productivity.

With rising global temperatures, and record-breaking heat waves baking parts of the world, the effects of extreme heat on human behavior could pose a growing problem (SN: 6/29/21).

And lower-income people and countries, with limited resources to keep cool as climate change warms the world, are likely to suffer the most, researchers say.  “The physiological effects of heat may be universal, but the way it manifests … is highly unequal,” says economist R. Jisung Park of UCLA.

Heat and aggression

Scientists have been documenting humans’ difficulties coping with extreme heat for over a century. Much of that work, however, has taken place in laboratory settings to allow for a high degree of control.

For instance, a few decades ago, social psychologist Craig Anderson and colleagues showed undergraduates four video clips of couples engaged in dialog. One clip was neutral in tone, while the remaining three showed escalating tension between the duo. The undergraduate students watching the clips were each sitting in a room with the thermostat set to one of five different temperatures, ranging from a cool 14° C to a hot 36° C. The researchers then asked the students to score the couples’ hostility level. Anderson, now of Iowa State University in Ames, found that students in uncomfortably warm rooms scored all the couples, even the neutral one, as more hostile than students in rooms with comfortable temperatures did. (Interestingly, students in uncomfortably cold rooms also scored the couples as more hostile.)

Heat tends to make people more irritable, says Anderson, whose findings appeared in the 2000 Advances in Experimental Social Psychology. And as a result, “they tend to just perceive things as being more nasty when they’re hot than when they’re comfortable.”

Research suggests that such perceptions can give way to actual violence when people lack an escape hatch. But this “heat-aggression hypothesis” has been hard to demonstrate outside the lab because teasing out the effect of heat from other environmental or biological variables linked to aggression is tricky in the messy real world. Studies in the last few years, however, have started confirming the idea.