ASHRAE Journal - March 2013 - 84

IAQ APPLICATIONS

Is CO2 Indoor Pollutant?
By William J. Fisk, Fellow ASHRAE; Usha Satish; Mark J. Mendell; Toshifumi Hotchi; Douglas Sullivan

ecause humans produce and exhale carbon dioxide (CO2), concentrations of CO2 in occupied indoor spaces are higher than

concentrations outdoors. As the outdoor air ventilation rate per person
decreases, the magnitude of the indoor-outdoor difference in CO2

concentration increases.
Prior research has found that with
higher indoor levels of CO2, indicating
less outdoor air ventilation per person,
people tend to be less satisfied with
indoor air quality, report more acute
health symptoms (e.g., headache, mucosal irritation), work slightly slower
and are more often absent from work or
school. It has been widely believed that
these associations exist only because the
higher indoor CO2 concentrations are a
proxy for higher levels of other indoorgenerated pollutants that directly cause
the adverse effects. Therefore, CO2 in
the range of concentrations found in
buildings (i.e., up to 5,000 ppm, but
more typically in the range of 1,000
ppm) has been assumed to have no direct
effect on occupants’ perceptions, health,
or work performance.
A small study from Hungary1 casts
doubt about this assumption. The authors
reported that controlled human exposures
to CO2 between 2,000 ppm and 5,000
ppm, with outdoor air ventilation rates
unchanged, had subtle adverse impacts
on proofreading of text in some trials, but
the brief report in conference proceedings
provided limited details.
These results from Hungary stimulated
our effort to evaluate effects of variation in CO2 alone on potentially more
sensitive high-level cognitive functioning. We investigated a hypothesis that
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ASHRAE Journal

higher concentrations of CO2, within the
range found in buildings, and without
changes in outdoor air ventilation rate,
have detrimental effects on occupants’
decision-making.
To test this hypothesis, 22 subjects
completed tests of decision-making performance when exposed to low, medium,
and high CO2 concentrations for 2.5
hour periods in an exposure chamber.
During sessions with low CO2, subjects
and outdoor air were the only sources of
CO2, and measured CO2 concentrations
were approximately 600 ppm. In sessions
with CO2 at the medium and high levels,
99.9999% pure CO2 was added to the
chamber supply air at the rate needed to
increase the CO2 concentration to either
1,000 or 2,500 ppm. All other conditions (e.g., ventilation rate, temperature)
remained unchanged.
Each subject experienced all three
CO2 conditions on the same day, and the
order of sessions (low, medium, high;
medium, low, high, etc.) was varied, as
needed, among the subjects to cancel out
effects of order of exposure. Three different versions of the decision-making test
were used in a manner such that ﬁndings
were not affected by differences in test
difﬁculty. Subjects were not informed of
the CO2 conditions.
Performance in decision making was
evaluated using the Strategic Manageashrae.org

ment Simulation (SMS) designed to assess complex cognitive functioning. The
parameters of decision making determine
day-to-day productivity and are important
for overall functioning.
SMS users are exposed to diverse
computer-generated real-world-equivalent scenarios that match real-world
day-to-day challenges and convey their
decisions through a computer interface.
The validated measures of task performance vary from relatively simple
competencies such as activity and task
orientation, through intermediate level
capabilities such as initiative and use of
information, to highly complex thought
and action processes such as breadth of
approach to problems and strategy.
SMS scores predict real-world success
as judged by peers and as demonstrated
by income, job level, promotions, level in
organizations. In other words, the SMS
assesses capacities required for day-today routine tasks as well as higher level
decisions people make at home and work.
The SMS, described in greater detail in
the documents cited in Reference 2, is
available for research through the State
University of New York, Upstate Medical
University.
The main results are depicted in Figure 1. The data indicate that at 1,000
ppm CO2, relative to 600 ppm, there
were moderate and statistically signiﬁcant decrements in six of nine scales of
decision-making performance. At 2,500
ppm, large and statistically signiﬁcant
reductions occurred in seven scales of
decision-making performance, but a
small increase in performance was seen
in the focused activity scale. For some
scales of performance, the reductions
were dramatic. More details are provided
in Reference 2.
March 2013

ASHRAE Journal - March 2013

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