Elevated carbon dioxide may impair reasoning

Carbon dioxide has been vilified for decades as a driver of global warming. A new study finds signs that CO₂, exhaled in every breath, can exert an equally worrisome threat — impaired cognition — in nearly every energy-efficient classroom, meeting hall or office space.

The work assessed decision-making in 22 healthy young adults. Their performance on six of nine tests dropped notably when researchers raised indoor carbon dioxide levels to 1,000 parts per million from a baseline of 600 ppm. On seven tests, performance fell substantially more when the room’s CO₂ was boosted to 2,500 ppm, scientists report in a paper to be published in Environmental Health Perspectives.

These data are surprising, says Roger Hedrick of Architectural Energy Corp. in Boulder, Colo., because “1,000 ppm of CO₂ used to be considered a benchmark of good ventilation.” Hedrick, an environmental engineer, chairs the committee that drafts commercial ventilation standards through the American Society of Heating, Refrigerating, & Air-Conditioning Engineers.

Carbon dioxide levels are often substantially higher in buildings than the 350 to 400 ppm typically found outdoors. Indoor values of 600 ppm are considered very good. But depending on how many people inhabit a room and how many times per hour its air is exchanged with outdoor air through ventilation, “there are plenty of buildings where you could easily see 2,500 ppm of CO₂ — or close to it — even with ventilation designs that are fully compliant with current standards,” Hedrick says.

“We’ve seen higher CO₂ levels associated with increased student absences and poorer performances on school-type tasks,” says study coauthor William Fisk of Lawrence Berkeley National Laboratory in California. “But we never thought CO₂ was actually responsible. We assumed it was a proxy for other [pollutants].”

His group recruited college students to spend much of a day in a room with computers. Individuals could read or do what they wanted for much of the time. But for part of each 2.5-hour period, participants completed role-playing tests on a computer that required them to manage an organization as it underwent a series of problems or crises. Throughout each of three test segments, conducted in random order, room ventilation was kept very high. Only carbon dioxide levels varied by segment: 600 ppm, 1,000 ppm or 2,500 ppm.

The role-playing tests are more complicated than most used to measure cognitive abilities, notes epidemiologist and coauthor Mark Mendell of Lawrence Berkeley. But they offer a gauge of important real-world skills, he says. “And the magnitude of effects measured at 2,500 ppm was astonishing — so astonishing that it was almost hard to believe,” he says.

If these trends are confirmed in follow-up studies, Hedrick says, “it would be very strong evidence that ventilation rates need to be increased.” Carbon dioxide standards were developed largely with the aim of controlling body odor, he notes. High levels of CO2 were viewed as suggesting occupancy levels were climbing to where “a place may begin smelling bad.”

Classrooms are fairly densely occupied, he notes, so their carbon dioxide frequently exceeds 1,000 ppm. With an increasing push to reduce heating and cooling costs, “there are plenty of school districts using lesser amounts of ventilation,” he says. “So I would not be at all surprised to find 2,500 ppm in a lot of school districts.”

In fact, “CO₂ levels are not that hard to control,” says Jack Driscoll of PID Analyzers in Sandwich, Mass. Building managers just need to measure them on a regular basis, he notes. At the American Chemical Society’s fall national meeting in Philadelphia, he described a new rapid, hand-held CO₂sensor designed for classrooms and office buildings.