Feeling a little sluggish? Researchers correlate heat with lowered cognitive function

During a heat wave, thinking skills can be slowed down, even in the young and healthy, according to results from a study published in PLOS Medicine.

Researchers found that daily cognitive tests in students living in dormitories without air conditioning (AC) were consistently lower over a span of 1 week compared with scores from students living in buildings with AC. Their results show that heat waves can have devastating effects on public health globally.

This study differed from previous ones in that it factored in indoor temperatures during a heat wave. Buildings can compound the problem of heat exposure during heat waves because they tend to stay warm overnight, despite lower ambient temperatures. Because adults in the United States have been shown to spend more than 90% of their time indoors, inclusion of this particular variable was important.

“While previous experimental studies have documented the effects of ambient temperatures on cognitive function, few have observed HW (heat wave) effects on indoor temperatures following subjects’ habitual conditions. The objective was to evaluate the differential impact of having AC on cognitive function during a HW among residents of AC and non-AC buildings using a prospective observational cohort study,” wrote these researchers, led by Jose Guillermo Cedeno Laurent, ScD, associate director, Healthy Building Program, Exposure, Epidemiology, and Risk Program, Department of Environmental Health, Harvard T.H. Chan School of of Public Health, Boston, MA.

For this study, Dr. Laurent and colleagues included 44 university students in their late teens and 20s. Twenty-four students lived in buildings equipped with central AC, and 20 in buildings with no AC.

Students were followed for 12 days, including 5 days of seasonable temperatures, followed by a 5-day heat wave, and 2 days of cooler weather. Researchers used the National Oceanic and Atmospheric Administration’s definition of a heat wave: a period comprised of 2 or more days of “abnormally high air temperature and humidity.”

On their smartphones, students completed two tests of cognition each morning. In one, students were asked to correctly identify the color of words, in which their reaction speeds and ability to concentrate were measured. The second test was comprised of basic math problems to measure mental quickness and working memory.

Buildings without AC averaged 79.3° F, but could be as hot as 86.7° F. In the air-conditioned buildings, the average temperature was 70.5° F, with a high of 77° F.

Researchers found 13.4% slower reaction times for the color/word tests in students living without AC, and 13.3% lower math scores compared with those living in AC. They also had a reduction from baseline in two measures of throughput (-9.9% and -6.3%, respectively) compared with ”air conditioned” students.

Researchers found that the optimum reaction and throughput times occurred at 71.6° F. An increase of 1.8° F in overnight indoor temperature exposure caused a 2.74-minute decrease in total sleep time (P < 0.001).

Behavioral factors and personal characteristics also had significant effects on cognitive function. Students with a liquid intake below the mean of less than 6 glasses/day showed significant deficits in the visual addition/subtraction test. Drinking more than one caffeinated beverage per day had an arousal effect on both math and reaction times, which were also affected by the span of time between waking and taking the tests. Poorer performance was associated with longer time lapses.

Previous studies have been focused on the effects of extremely hot weather in vulnerable populations, including only the very old and the very young. In this study, researchers included young, healthy subjects.

“Cognitive function deficits resulting from indoor thermal conditions during heat waves extend beyond vulnerable populations. Our findings highlight the importance of incorporating sustainable adaptation measures in buildings to preserve educational attainment, economic productivity, and safety in light of a changing climate,” concluded Dr. Laurent and colleagues.

This study was funded by the Harvard University Climate Change Solutions Fund.