'Normal' CO2 Levels Slow Brain Function
Magnitude of CO2-associated deterioration in decision-making performance ranged from 11 percent to 94 percent.
Medically Reviewed by Robert Jasmer, MD and Dorothy Caputo, MA
WEDNESDAY, Sept. 26, 2012 (MedPage Today)
— Indoor concentrations of carbon dioxide
(CO2) within the normal range correlated with
significant declines in human decision-making performance, results of a small
clinical study showed.
As the median CO2 concentration increased from
600 to 2,500 ppm, performance on seven of nine scales of decision-making
decreased significantly, Mark Mendell, PhD, of the Lawrence Berkeley National
Laboratory in Berkeley, Calif., and colleagues reported. A post-hoc pairwise
comparison showed that performance deteriorated significantly between
CO2 concentrations for all seven scales.
The magnitude of CO2-associated deterioration
in decision-making performance ranged from 11 percent to 94 percent, the
researchers wrote online in Environmental Health
"While the modest reductions in multiple aspects of decision
making seen at 1,000 ppm may not be critical to individuals, at a societal
level or for employers, an exposure that reduces performance even slightly
could be economically significant," the authors wrote. "The
substantial reductions in decision-making performance with 2.5-hour exposures
to 2,500 ppm indicate ... impairment that is of importance even for
individuals," they added. "These findings provide initial evidence
for considering CO2 as an indoor pollutant, not just a
proxy for other pollutants that directly affect people."
Typical indoor concentrations of CO2 exceed
those in the outdoor environment. Indoor CO2
concentrations increase as ventilation decreases. As a result, peak indoor
CO2 concentrations have been used to estimate
outdoor-air ventilation rate per indoor occupant, the authors noted.
Efforts to reduce energy consumption have the potential to reduce
ventilation rates, increasing indoor CO2 concentration
in the process.
Previous studies of the health effects of CO2
have focused on concentrations that far exceed those found in
normal indoor settings. Concentrations greater 20,000 ppm can lead to deeper
breathing. Visual disturbances and tremors have been reported in association
with indoor CO2 levels greater than 100,000 ppm, and
death can occur at CO2 concentrations greater than
The Occupational Safety and Health Administration recommends a maximum
indoor CO2 concentration of 5,000 ppm for an 8-hour
workday, the authors continued.
Several studies have shown that indoor CO2
concentrations at the upper limits of normal are associated with perceived poor
air quality, increased acute health symptoms, decreased work performance, and
increased absenteeism. However, the associations usually have been attributed
to higher levels of indoor pollutants resulting from decreased outdoor
"Thus, CO2 in the range of concentrations
found in buildings (up to 5,000 ppm) has been assumed to have no direct impacts
on occupants' perceptions, health, or work performance," the authors
Mendell and colleagues conducted a controlled clinical study involving
22 adults, primarily college students.
To assess the possible effects of indoor CO2
concentration on decision-making, investigators used the Strategic Management
Simulation (SMS) test, a computer-based instrument that provides data on
individuals' decision-making performance under different conditions.
According to the authors, the SMS measures complex behaviors required
for effectiveness in workplace settings. The test system assesses basic
cognitive and behavioral responses to tasks, as well as cognitive and
behavioral components of executive functioning. The study assessed
decision-making performance on nine SMS scales.
The study was conducted in a chamber facility that allowed
manipulation of the indoor environment, including ventilation. Each participant
completed three study sessions on separate days. During each session,
participants the SMS and a questionnaire about health symptoms and perceived
Participants completed one session during which indoor CO2
was maintained at 600 ppm, one at a concentration of 1,000 ppm, and
one at a concentration of 2,500 ppm.
Compared with the 600 ppm condition, performance on seven of nine SMS
scales declined as CO2 concentration increased (basic
activity, applied activity, task orientation, initiative, information usage,
breadth of approach, and strategy).
As compared with the 600 ppm condition, performance during the 1,000
ppm condition deteriorated by 11 percent to 23 percent across the seven scales
and by 44 percent to 94 percent during the 2,500 condition.
Additionally, the authors found a performance decrement of 35 percent
to 93 percent between the 1,000 ppm and 2,500 ppm conditions.
In contrast to the other scales, scores for the "focused
activity" scale increased with the CO2
concentration in the test chamber. "Focused activity is
important for overall productivity, but high levels of focus under
non-emergency conditions may indicate 'over-concentration,'" the authors
"The direct impacts of CO2 on performance
indicated by our findings may be economically important, may disadvantage some
individuals, and may limit the extent to which outdoor air supply per person
can be reduced in buildings to save energy," the authors wrote.
They concluded that the causality of the observed effects was clear,
but that this study in college students many not apply to the general
"Replication of these study findings, including use of other
measures of complex cognitive functioning and measures of physiologic response
such as respiration and heart rate, is needed before definitive conclusions are
drawn," they said.
'Normal' CO2 Levels Slow Brain Function