The effect of indoor carbon dioxide on cognition

There is an increasing evidence that the level of carbon dioxide (CO2) inside
buildings can have a negative effect on the cognitive performance and long-term
health of occupants. While in the past, CO2 has been used as a proxy for other
harmful substances in the air, more research indicates that CO2 itself could
directly cause cognition impairments and adverse physiological responses. It is
assumed that a high level of ambient CO2 leads to an increased concentration of
CO2 in the blood, caused by a changing breathing pattern which results in
insufficient lung ventilation. This higher level of blood CO2 leads to a
respiratory acidosis, characterized by a blood ph-level below 7.35 and an
increased concentration of bicarbonate in the blood. Ultimately, this can lead
to certain chemical and biological reactions in the brain, which impair
cognitive performance. Nevertheless, the underlying physiological mechanisms in
the brain are still unknown. Moreover, the cognitive impairment due to CO2
exposure might lead to changes in individual behavior, in particular regarding
individuals* risk taking and time preferences which are key determinants in
economic decision-making. However, current evidence is still inconclusive at
which concentration levels CO2 starts to become harmful. There is also only
limited knowledge about the effect of moderately high CO2-levels over a period
of several hours. Additionally, from the best of our knowledge, there is
currently no study which investigated the effect of moderate indoor CO2
concentration on human energy metabolism. Human energy metabolism is an
important health factor in determining the risk for individuals to develop
metabolic diseases such as cardiovascular disorders, stroke and type 2
diabetes. A lower level of energy metabolism is also associated with increasing
risk for obesity.
This interdisciplinary study aims to contribute new insights to different
streams of literature by examining the isolated effect of medium exposure (over
several hours) to CO2 on both, cognitive and behavioral responses a well as
physiological channels. This contributes to closing several research gaps.
Firstly, the study will enhance our understanding if a typically occurring,
moderate indoor CO2 concentration of 0.3% affects cognition and health.
Secondly, the close monitoring of subjects* physiological conditions will allow
insights into underlying mechanisms causing cognitive impairments. Thirdly, we
will investigate the role of long-term exposure (several hours) to elevated
levels of CO2 that are representative for conditions in offices, which allows
translating the results into direct implications for firms and policy.
Understanding under what conditions health, cognitive capacities and optimal
decision-making of office workers can be optimized is key for future
productivity growth and sustainable employment.

Primary Objective:
The main research question of this study is: What is the impact of an
eight-hour exposure to an elevated concentration of indoor carbon dioxide on
cognitive performance of individuals?
This leads to the following hypothesis: The exposure to an indoor carbon
dioxide concentration of 3000 ppm over a time period of eight hours
significantly reduces the test scores on the CANTAB cognition tests, compared
to an exposure to a concentration of 800 ppm of carbon dioxide.

Secondary Objective(s):
Does an eight-hour exposure to an elevated concentration of indoor carbon
dioxide leads to:
1. A higher breathing rate per minute?
2. An elevation in blood pressure and arterial blood CO2 concentration?
3. A decrease in heart rate variability and arterial blood O2 saturation level?
4. A greater difference in proximal and distal skin temperature?
5. A change in substrate oxidation?
6. More risk-averse behavior of individuals?
7. A higher internal discounting factor individual use to evaluate future
payments?
8. A higher tendence to make decisions in an unconscious, automatic
response.

A randomized cross-over design is used. This is preferred over a
treatment-control group design, because physiological parameters such as heart
rate variability, blood pressure and skin temperature differ between
individuals. A cross-over design allows to determine if the indoor level of CO2
has an influence on these factors within an individual. Furthermore, also the
economic preferences a person has to make when deciding about investment
decisions, determined by his or her risk and time preferences, differ between
individuals. Nevertheless, this study allows to determine if a change in indoor
CO2 concentration has an impact on these economic preferences in terms of
shifting a person*s behavior towards becoming more or less risk averse, as well
as more or less patient to wait for a future payoff. The amplitude of this
shift will differ among individuals. To draw generalized conclusions, it is
important that the shift is going into the same direction for each individual,
e.g. every participant is becoming more risk avers when the level of indoor CO2
is higher. A cross-over design allows to investigate this aspect.
In this study, participants will be exposed to two different conditions on two
separate days in the respiratory chambers of the Metabolic Research Unit of
Maastricht University (MRUM) (see figure 1 below for a timeline). In both
conditions, participants will spend eight hours in the respiratory chamber,
conducting cognition tests multiple times. The light condition chosen in the
climate chamber will be set at 4000 Kelvin and 500 lux which is recommended as
visual comfortable office lighting based on the European Lighting Standard. The
temperature level is set at 21°C, which is according to a recent review about
the impact of temperature on cognitive performance the level for which human
performance is closest to optimal. The only difference between the two
conditions is the level of carbon dioxide (CO2) in the chamber, which is either
800 ppm of CO2 concentration in the air (low-CO2 condition) or 3000 ppm of CO2
concentration in the air (high-CO2 condition). Studies have confirmed that CO2
intoxication and serious harm for humans occur only at levels greater than
100,000 ppm (10% of ambient air) and no immediate damage on health can be
expected from prolonged exposure to CO2 levels below 30,000 ppm (3% of ambient
air).

The indoor CO2 concentration in the respiratory chambers will vary between the
two tests days. It will be either 800 ppm (0.08%) high, or 3000 ppm (0.3%)
high.

Participants have to come once for a screening session (3 hours) and two times
to the MRUM lab and stay there for a period of 11.5 hours. They have to conduct
a cognition test, fill out a questionnaire and play a lottery game two times
per day. Additionally, they have to wear devices to measure heart rate
variability, blood pressure, CO2 blood concentration, O2 saturation level, skin
temperature, and breathing rate. Human energy expenditure will be measured
using indirect calorimetry. The CO2 levels in the room will stay within a range
for which no immediate danger for health is expected. The highest concentration
of CO2 of 3000 ppm is a concentration commonly observed in schools or office
buildings with relatively poor ventilation. Minor symptoms like headache,
dizziness and sleepiness could occur at the end of the 8.5 hours session.