How heat affects the brain

In July 2016, a heat wave hit Boston, with daytime temperatures averaging 92 degrees for five consecutive days. Some local college students who stayed in town for the summer were lucky and lived in dorms with central air conditioning. The other students, not so much – they were stuck in older dorms with no air conditioning

José Guillermo Cedeño Laurent, then a researcher at Harvard, decided to take advantage of this natural experiment to see how heat, and especially night heat, affected the cognitive performance of young adults. He asked 44 students to take math and self-control tests five days before temperatures rose, every day during the heatwave and two days afterward.

“Many of us think we are immune to heat,” said Dr. Cedeño, now an assistant professor of environmental and occupational health and justice at Rutgers University. “So something I wanted to test was whether it was really true.”

It turns out that even young and healthy students are affected by high temperatures. On the hottest days, students in non-air-conditioned dorms, where the nighttime temperature averaged 79 degrees, performed significantly worse on tests they took each morning than did air-conditioned students, whose rooms remained at a pleasant temperature of 71 degrees.

A heat wave is once again affecting the Northeast, South and Midwest. High temperatures can have an alarming effect on our bodies, increasing the risk of heart attacks, heat stroke and death, especially in older adults and people with chronic illnesses. But heat also has detrimental effects on our brains, impairing cognition and making us irritable, impulsive and aggressive.

Many laboratory studies produced results similar to those of Dr. Cedeño’s research, with scores on cognitive tests decreasing as the scientists increased the temperature in the room. Investigation found that a simple four-degree increase—which participants described as still being comfortable—resulted in an average 10 percent decline in performance on tests of memory, reaction time, and executive functioning.

This can have real consequences. R. Jisung Park, an environmental and labor economist at the University of Pennsylvania, examined high school standardized test scores and found that they decreased by 0.2 percent for every degree above 72 degrees Fahrenheit. That may not sound like much, but it can add up to a lot for students taking an exam in a non-air-conditioned room during a 90-degree heat wave.

In another study, Dr. Park found that the more hot-than-average days there were during the school year, the worse students performed on a standardized test, especially when the thermometer exceeded 80 degrees. He thinks this could be because greater exposure to heat was affecting students’ learning throughout the year.

The effect was “more pronounced among low-income and racial minority students,” Dr. Park said, perhaps because they were less likely to have air conditioning, both at school and at home.

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