Ever feel during one of these recent sweltering days that it’s just so hot you can’t think straight?
Well, maybe you can’t.
Harvard researchers say that they studied students in dorms with and without air conditioning and during a heat wave. They found that the students suffering through the heat performed worse on a series of cognitive tests.
The researchers from the Harvard T.H. Chan School of Public Health published their results Tuesday in the journal PLOS Medicine.
Most research on the health effects of heat has focused on vulnerable groups such as the elderly. That may have created the perception that most people aren’t affected by heat waves, Jose Guillermo Cedeño-Laurent, research fellow at Harvard Chan School and lead author of the study, said in a statement from the university.
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Knowing how the heat affects other groups is critical, he said, “considering that in many cities, such as Boston, the number of heat waves is projected to increase due to climate change.”
Extreme heat is the leading cause of death of all meteorological phenomena in the United States, the researchers said. Global temperatures are on the rise. The National Oceanic and Atmospheric Administration says 2017 was the third-warmest year ever recorded globally, while 2016 was the warmest, and 2015 was the second-warmest.
Researchers studied 44 students in Boston in their late teens and early 20s. Twenty-four lived in air-conditioned buildings. The other 20 lived in buildings that did not have air conditioning.
The study was conducted over a 12-day period in the summer of 2016. The first five days temperatures were seasonable, then came a five-day heat wave, then a two-day cooldown, the researchers said. The researchers did not release the names of the buildings or the institutions the students attended.
The students were asked to take two tests on their smartphones right after waking up each day. The test results showed that during the heat wave students without air conditioning experienced decreases across five measures of cognitive function. The students, for example, experienced 13.4 percent longer reaction times on a test where they were asked to correctly identify the color of displayed words. They also had a 13.3 percent lower scores on basic arithmetic questions.
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The study has “implications for basically millions of people that could be suffering this detriment to cognitive function,” Cedeño-Laurent said in a telephone interview.
He said he hoped the study results could “drive a change in the way we approach climate change by making it personal.”
During the cooldown period after the heat wave, the differences continued, the researchers said, warning that the effects of a heat wave linger in buildings — and American adults spend 90 percent of their time indoors.
“Indoor temperatures often continue to rise even after outdoor temperatures subside, giving the false impression that the hazard has passed, when in fact the ‘indoor heat wave’ continues,” Joseph Allen, co-director of the Center for Climate, Health, and the Global Environment at the Chan School.
“In regions of the world with predominantly cold climates, buildings were designed to retain heat. These buildings have a hard time shedding heat during hotter summer days created by the changing climate, giving rise to indoor heat waves,” Allen, one of the study’s senior authors, said in the statement.
Massachusetts has been baking in the heat since late June. Seven of the past 12 days in Boston have seen temperatures rise to 90 or above as the mercury hit the mid-90s on Tuesday. Areas in the interior haven’t benefited from the sea breeze that sometimes helps Boston. A break in the heat is expected in the days ahead.
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Martin Finucane can be reached at Martin.Finucane@globe.com