London: Researchers at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have found that even moderately elevated levels of indoor carbon dioxide (CO2) can significantly impair people's decision-making performance.

On nine scales of decision-making performance, test subjects showed significant reductions on six of the scales at CO2 levels of 1,000 parts per million (ppm) and large reductions on seven of the scales at 2,500 ppm.

The most dramatic declines in performance, in which subjects were rated as "dysfunctional," were for taking initiative and thinking strategically.

The results were unexpected and may have particular implications for schools and other spaces with high occupant density.

"In our field we have always had a dogma that CO2 itself, at the levels we find in buildings, is just not important and doesn't have any direct impacts on people. So these results, which were quite unambiguous, were surprising," said Berkeley Lab scientist William Fisk, a co-author of the study.

The study was conducted with researchers from State University of New York (SUNY) Upstate Medical University.
The primary source of indoor CO2 is humans. While typical outdoor concentrations are around 380 ppm, indoor concentrations can go up to several thousand ppm. Higher indoor CO2 concentrations relative to outdoors are due to low rates of ventilation, which are often driven by the need to reduce energy consumption.

In the real world, CO2 concentrations in office buildings normally don't exceed 1,000 ppm, except in meeting rooms, when groups of people gather for extended periods of time.

In classrooms, concentrations frequently exceed 1,000 ppm and occasionally exceed 3,000 ppm. CO2 at these levels has been assumed to indicate poor ventilation, with increased exposure to other indoor pollutants of potential concern, but the CO2 itself at these levels has not been a source of concern. Federal guidelines set a maximum occupational exposure limit at 5,000 ppm as a time-weighted average for an eight-hour workday.

Fisk decided to test the conventional wisdom on indoor CO2 after coming across two small Hungarian studies reporting that exposures between 2,000 and 5,000 ppm may have adverse impacts on some human activities.

Berkeley Lab scientists Mark Mendell and William Fisk, Mendell, and their colleagues, including Usha Satish at SUNY Upstate Medical University, assessed CO2 exposure at three concentrations: 600, 1,000 and 2,500 ppm.

They recruited 24 participants, mostly college students, who were studied in groups of four in a small office-like chamber for 2.5 hours for each of the three conditions.

Ultrapure CO2 was injected into the air supply and mixing was ensured, while all other factors, such as temperature, humidity, and ventilation rate, were kept constant. The sessions for each person took place on a single day, with one-hour breaks between sessions.

Although the sample size was small, the results were unmistakable. Another novel aspect of this study was the test used to assess decision-making performance, the Strategic Management Simulation (SMS) test, developed by SUNY. In most studies of how indoor air quality affects people, test subjects are given simple tasks to perform, such as adding a column of numbers or proofreading text.

"It's hard to know how those indicators translate in the real world. The SMS measures a higher level of cognitive performance, so I wanted to get that into our field of research," said Fisk.

Strategic thinking and taking initiative showed the most dramatic declines in performance at 2,500 ppm carbon dioxide concentrations.

Data from elementary school classrooms has found CO2 concentrations frequently near or above the levels in the Berkeley Lab study. Although their study tested only decision making and not learning, Fisk and Mendell say it is possible that students could be disadvantaged in poorly ventilated classrooms, or in rooms in which a large number of people are gathered to take a test.

"We cannot rule out impacts on learning," their report said. The next step for the Berkeley Lab researchers is to reproduce and expand upon their findings. "Our first goal is to replicate this study because it's so important and would have such large implications," said Fisk.

"We need a larger sample and additional tests of human work performance. We also want to include an expert who can assess what's going on physiologically."

Until then, they say it's too early to make any recommendations for office workers or building managers. The study was published in Environmental Health Perspectives online last month.


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