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Scientists report a surprising link between indoor carbon dioxide levels and cognitive function


This story has been updated.

In a development likely to sharpen debate about indoor environment conditions in office buildings, a study published Monday reports that workers showed significantly lowered cognitive functioning after spending a day in a simulated office environment featuring high concentrations of carbon dioxide and volatile organic compounds. By contrast, in conditions set to simulate a green building — or better yet, a green building with enhanced ventilation — cognitive performance was higher.

The study was led by Harvard’s environmental health researcher Joseph Allen in collaboration with colleagues from Harvard, SUNY Upstate Medical School, and Syracuse University, and was published in Environmental Health Perspectives. And it notes that the research was designed to mimic “conditions that are commonly encountered every day in many indoor environments.” Carbon dioxide is exhaled by humans, and volatile organic compounds are emitted by numerous indoor materials and products.

“What we found were quite shocking results,” Allen said. “We saw a doubling of cognitive performance scores in the environments that started with a green building, and we enhanced the outdoor ventilation rate.”

In the study, which was conducted in Syracuse at the university’s Center of Excellence in Environmental and Energy Systems, 24 professionals spent 6 full work days in a controlled office experimental setting where they performed their regular jobs. The space was equipped with Internet, printing facilities, phones — all the usual office stuff.

The big difference, though, was that at 3 pm each day, the subjects were given a 1.5 hour test called the Strategic Management Simulation, which tests “higher-order decision making.” Thus, for instance, respondents might have to figure out how to handle a tough problem — “handling a township in the role of a mayor or emergency coordinator” is one example provided in the study.

Unbeknownst to the participants, however, researchers manipulated the indoor environmental conditions in which they were working. Sometimes, levels of volatile organic compounds — including formaldehyde, benzaldehyde, and acetaldehyde — were higher or lower (green building designs aim to lessen the presence of these compounds). And sometimes, there was greater or lesser ventilation of the workspace with outdoor air. Indoor carbon dioxide levels were also varied, from 550 parts per million, to 945 parts per million, to as high as 1,400 parts per million.

The goal was to simulate conditions of a “conventional” building, versus a green building and, with the 550 parts per million carbon dioxide case, a “Green Building +” condition. And the study found a large effect of these indoor conditions on test performance. “Cognitive function scores were higher in Green building conditions compared to the Conventional building condition for all nine functional domains,” the researchers wrote, including crisis response, information usage, and strategy.

Carbon dioxide levels, in particular, seemed to have a major effect. “Cognitive function scores were 15% lower for the moderate CO2 day (~945 ppm) and 50% lower on the day with CO2 concentrations around 1400 ppm than on the two Green+ days,” the researchers wrote.

In a statistical sense, the study reported a quite strong link between the two variables under examination — office air quality and cognitive performance. The researchers claimed that they could explain 81 percent of the variability in the study subjects’ cognitive test scores based on the variations in the office’s indoor conditions, leaving only a quite small 19 percent attributable to other factors not controlled in the study, such as “diet, previous night sleep quality, and mood.”

The carbon dioxide part of the story is particularly striking, as there has in the past been little worry about this molecule in the air we breathe. The researchers note, however, that their work is consistent with a prior 2012 study, also published in Environmental Health Perspectives, which found that “statistically significant decrements” occurred in performance on decision-making tasks as carbon dioxide concentrations indoors increased from 600 to 1000 parts per million.

“Carbon dioxide at the concentrations we typically find indoors was long thought to be benign,” says Allen. “But our understanding of this is changing. We’re starting to see that carbon dioxide has direct effects.”

The new study says that CO2 levels in offices and building spaces can be higher than these levels where detrimental effects are now being detected, including greater than 1000 parts per million in some previously measured classrooms and apartment buildings. It also observes more generally that there has often been a trade-off in the past between ventilation and energy efficiency in building designs — tighter building envelopes can contain heat or cooling, lessening electricity use, but can also lessen air circulation.

In reaction to the work, two experts praised the study, although they also said there’s more work to be done.

Margo Oge, the former head of the Environmental Protection Agency’s Office of Transportation and Air Quality, called the work a “a small but a very well designed study” in comments by email, and added that it “should be repeated in other indoor environments.” Oge, who also previously headed indoor air program at the EPA in the early 1990s, was not involved in the research.

William Fisk, who leads the indoor environment group at Lawrence Berkeley National Laboratory and was a coauthor of the 2012 study mentioned above, also commended the work, calling it “an excellent study with a design that eliminates common sources or error and that employs a sophisticated test of cognitive performance.”

“These findings are consistent with the results of our two prior studies,” Fisk continues. “The underlying mechanisms remain uncertain.”

The authors agree that there remains a great deal more research to be done — Allen concurs with Fisk that the scientists do not know yet how volatile organic compounds or carbon dioxide could impair cognitive performance. Nor do they know whether the change that they detected is a temporary effect that we can bounce back from easily, or a lasting one — another major uncertainty.

Joseph Romm, a physicist who heads the widely read site Climate Progress and is a former Clinton administration energy official, argued in a blog post Monday (and in a forthcoming book, Climate Change: What Everyone Needs to Know), that these results in effect mean that global warming could threaten human cognitive function, by increasing concentrations of carbon dioxide in the atmosphere (they are currently around 400 parts per million).

“We now have evidence that CO2 levels that people are routinely exposed to in buildings – people and their children – impair higher cognitive function,” says Romm. “Outdoor CO2 is the baseline for indoor CO2, so the higher CO2 levels are outdoors, the harder and harder it will become to keep CO2 levels indoors below whatever level the public health research shows is safe.”

Allen agrees that carbon dioxide levels in buildings are controlled both by indoor factors like the number of humans respiring, but also the outdoor baseline concentrations. Romm cites research suggesting these can be significantly above average in cities where there is a lot of traffic.

“This study was designed to reflect indoor office environments in which large numbers of the population work every day,” the researchers conclude. “These exposures should be investigated in other indoor environments, such as homes, schools, and airplanes, where decrements in cognitive function and decision-making could have significant impacts on productivity, learning, and safety.”

“We spend 90 percent of our time indoors, yet we often ignore indoor environmental quality as an important public health issue,” says Allen.