If you have ever been in extreme heat and felt sluggish, slow, and dimwitted – almost as though your brain has melted – you are not alone.
Past research in the lab has proven that the human brain ceases to function normally when exposed to high temperatures, and now, an investigation by the Harvard T.H. Chan School of Public Health shows that this phenomenon holds true for real-world situations.
Their study, published in PLOS Medicine, tested changes in cognitive performance of 44 college students living in air-conditioned and non-air-conditioned dorms in Boston during July 2016 – the summer of the hottest year on record. Using cellphone-based tests of selective processing, working memory, and inhibitory control – the ability to focus on relevant stimuli in the presence of irrelevant stimuli – the authors revealed that subjects without air conditioning (AC) had significantly reduced mental capacity on the extra hot days compared with those blessed with AC units.
Though this work examined just a small group of people in one area of the world, lead author Jose Guillermo Cedeño-Laurent and his colleagues write that the findings bring attention to an often-overlooked side effect of climate change.
"Most of the research on the health effects of heat has been done in vulnerable populations, such as the elderly, creating the perception that the general population is not at risk from heat waves," Cedeño-Laurent said in a statement. “Knowing what the risks are across different populations is critical considering that in many cities, such as Boston, the number of heat waves is projected to increase due to climate change.”
The study data was collected on 12 consecutive days: five of normal summer temperatures (15-31°C/59-88°F range) followed by five abnormally hot (28-36°C/ 82-97°F) days, capped off by two cool down days (28°C/82°F). Daily cognitive performance was assessed via a Stroop color-word test and a visual addition/subtraction challenge that subjects completed on their phones after waking up and before they left their bedrooms. To rule out other factors that could impact brain power, the students were asked to wear biosensors to track overall activity and sleep, and monitors in each dorm recorded indoor temperature, humidity, noise levels, and carbon dioxide levels.
Unsurprisingly, students in the AC group experienced significantly lower indoor temperatures, humidity, and noise (because they didn’t need to run fans) throughout the 12 days. After adjusting for the learning effect, wherein students got better at the tests over time, the researchers discovered that cognition between groups was matched during the five normal days, but those without AC experienced 13.4 percent slower reaction times and received 13.3 percent worse arithmetic scores on the five heat wave days.
And unexpectedly, the difference in cognition between groups continued during the cool-down period.
"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," co-author Joseph Allen explained. "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 and Cedeño-Laurent conclude that in the face of increasing global temperatures, temperature-smart buildings and homes will be a necessity for public health, especially considering that Americans spend about 90 percent of their time inside.
“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.”