As the 21st century progresses, rising atmospheric carbon dioxide (CO2) concentrations will cause urban and indoor levels of the gas to increase, and that may significantly reduce basic decision-making abilities and complex strategic thinking, according to a study from the University of Pennsylvania and the University of Colorado Boulder published in the journal GeoHealth.
By the end of the century, people could be exposed to indoor CO2 levels as high as 1,400 parts per million (ppm)—more than three times today’s outdoor levels, and well beyond what humans have ever experienced.
“It’s amazing how high CO2 levels get in enclosed spaces,” says Kris Karnauskas, a fellow of the Cooperative Institute for Research in Environmental Sciences, associate professor at CU Boulder, and lead author of the study. “It affects everybody from little kids packed into classrooms to scientists, businesspeople, and decision makers to regular folks in their houses and apartments.”
Shelly Miller, a professor in CU Boulder’s school of engineering adds that “building ventilation typically modulates CO2 levels in buildings, but there are situations when there are too many people and not enough fresh air to dilute the CO2.” It can also build up in poorly ventilated spaces over longer periods of time, such as overnight while sleeping in bedrooms, she says.
Put simply, when we breathe air with high CO2 levels, the CO2 levels in the blood rise, reducing the amount of oxygen that reaches the brain. Studies show that this can increase sleepiness and anxiety and impair cognitive function.
It’s a common feeling: Sit too long in a stuffy, crowded lecture hall or conference room and drowsiness or dullness begins to set in. In general, CO2 concentrations are higher indoors than outdoors, the authors write. And outdoor CO2 in urban areas is higher than in pristine locations. The CO2 concentrations in buildings result from both the gas that is otherwise in equilibrium with the outdoors and the CO2 generated by building occupants as they exhale.
Atmospheric CO2 levels have been rising since the Industrial Revolution, reaching a peak of 414 ppm at NOAA’s Mauna Loa Observatory in Hawaii in 2019. In the ongoing scenario, one in which people do not reduce greenhouse gas emissions, the Intergovernmental Panel on Climate Change predicts that outdoor CO2 levels could climb to 930 ppm by 2100. And urban areas typically have around 100 ppm CO2 higher than this background.
Karnauskas and colleagues, which included Anna Schapiro, an assistant professor of psychology at Penn, developed a comprehensive approach that considers predicted future outdoor CO2 concentrations and the impact of localized urban emissions, a model of the relationship between indoor and outdoor CO2 levels, and the impact on human cognition. They found that if the outdoor CO2 concentrations do rise to 930 ppm, that would nudge the indoor concentrations to a harmful 1,400 ppm.
“At this level, some studies have demonstrated compelling evidence for significant cognitive impairment,” says Schapiro, a coauthor on the study. “Though the literature contains some conflicting findings and much more research is needed, it appears that high-level cognitive domains like decision-making and planning are especially susceptible to increasing CO2 concentrations.”
In fact, at 1,400 ppm, CO2 concentrations may decrease basic decision-making ability by 25%, and complex strategic thinking by around 50%, the authors found.
The cognitive impact of rising CO2 levels represents what scientists call a “direct” effect of the gas’s concentration, much like ocean acidification. In both cases, elevated CO2 itself—not the subsequent warming it also causes—is what triggers harm.
According to the researchers, there may be techniques to adapt to higher indoor CO2 levels, but the best way to prevent levels from reaching those that are harmful is to reduce fossil fuel emissions. This would require globally adopted mitigation strategies such as those set forth by the Paris Agreement of the United Nations Framework Convention on Climate Change.
Schapiro and her coauthors say they hope these findings will spark further research on ‘hidden’ impacts of climate change such as that on cognition.
“This is a complex problem, and our study is at the beginning. It’s not just a matter of predicting global [outdoor] CO2 levels,” Karnauskas says. “It’s going from the global background emissions to concentrations in the urban environment to the indoor concentrations and finally the resulting human impact. We need even broader, interdisciplinary teams of researchers to explore this. Investigating each step in our own silos will not be enough.”
Coauthors of the study include Kristopher B. Karnauskas, a fellow of the Cooperative Institute for Research in Environmental Sciences and an associate professor at the University of Colorado Boulder Atmospheric and Oceanic Sciences and Colorado School of Public Health, and Shelly L. Miller, a professor of mechanical engineering at the University of Colorado Boulder.