Why does high CO2 lead to declining performance? Two of our best neurological guesses in anticipation!

Unraveling the biological and psychological effect of indoor air quality on human performance has been on the radar of the most innovative research groups, including MCRE. What we know, and are confirming repeatedly, is that high indoor CO2 results in poor performance (Durán et al., 2021; Allen et al., 2019; Kuhn et al. 2019). We are also getting better at identifying and distinguishing which factors are causal and which are correlational (Stroom et al., 2021; Allen et al., 2016; Du et al., 2020). One of the most important factors is the concentration of carbon dioxide (CO2) in buildings, a gas whose primary indoor source is human exhalation. What remains uncharted territory, however, is the biological mechanism: how do these factors influence the way our brain works. We already know it’s not a lack of oxygen (Shriram et. al. 2019, Zhang et. al. 2017). In anticipation of our discovery journey, we discuss two of our most current working hypotheses.*

A brief crash course in neuropsychology is warranted. When thinking of a human brain, you will most likely think of a wrinkly gray mass. This part is called the cortex and is responsible for most of our complex abilities and higher cognitive performance (visual-audio, memory, mathematical skills, planning). Right below the surface, which can only be seen by cutting through, is our ‘white mass’. The white mass consists of pathways, connecting the cortex areas with each other and the brainstem. If the gray mass are train stations, the white mass are railroad tracks. At the heart of our brain, all the way via our brainstem to the spin, are our subcortical (‘below-cortical’) areas. These areas direct the cortex, and the most primal functions of our body (breathing, heartbeat, reflexes, emotions). Communication by these very distinct subcortical areas mostly runs through neurotransmitters, which also affects the communication in the white mass pathways.

Evidently, it is of the utmost importance that the subcortical areas remain functioning. One might lose (working) memory after drinking too much, but losing your heartbeat will be less of a funny story a day later. And our body makes sure this will not happen. Our brain prioritizes resources (oxygen in the blood) in line of importance, like layers of an onion. When under stress, our complex thinking suffers first, but our primal reactions remain largely unscathed (Arnsten, 2009). This might sound familiar, as our body does the same in reaction to reaction to hypothermia: in extreme cold, our body automatically prioritizes heat to the vital organs. The fight or flight response is another extreme example: under high levels of stress and danger, our subcortical brain decides what to do instantly (Jansen et al., 1995). The first hypothesis of CO2 on performance follows this line of thought. Performance, located in the most complex cortical areas, suffers from the body’s stress reaction to the high CO2. Both scenarios that the body cannot produce enough resources due to lesser quality air (resource deficiency) or needs to compensate for the worsened general attention (fatigue) will lead to the same outcome: overall stress lowering the higher cognitive performance – the outer layer of the onion suffers first. If the normally performing cortex is a well-tuned orchestra, consider high CO2 as lowering the budget: it puts your crazy uncle on stage with the trombone to play at your wedding.

The second hypothesis has the exact opposite chain reaction reasoning. As previously mentioned, the subcortical structures control the most primal reactions, but also influence the communication of the higher cortical areas. One of its key communication jobs includes maintaining a healthy go (dopamine) or rest (serotonin) balance. CO2 is likely to cause shallow breathing, leading to decreased pH-level of the blood (Shriram et al., 2019). This acid blood, in turn, triggers more serotonin release (Wemmie, 2022) – now the inner layer of the onion suffers first. The orchestra is all present and ready to play, but the conductor tells them to take a break. But then why does high CO2 lower performance, and not our reflexes or breathing? First, it probably does: we are as we speak testing how the rest of our body (such as breathing) is reacting to high levels of CO2. Second, our subcortical brain is very skilled in heartbeats. It could literally do this with our eyes closed. Being the conductor of the cortical areas, however, is a more refined job.

Both hypotheses have their merit. Brain imaging (fMRI) experiments show that high-level cortex activation lowers first under many forms of endured stress (even depression; Drevets, 2000). On the other hand, the first results in CO2 research show a good indication that high CO2 might not only lead to stress to cope with it but also lowers the pH in the blood. Thus, high CO2 levels either lead to either a stress reaction or fatigue (Zhang et. Al. 2017). We perform badly either by being too aroused/annoyed or too sleepy. Unfortunately, the brain is not an easy research subject. Most measurements measure activation indirectly, are delayed, and with an inaccurate location. However, all great research starts with great hypotheses. And fortunately, our efforts are close to spotting insights on the horizon.

* Note that these explanations are presented in a simplified manner. Let us know if you are interested to get to know both processes at a deeper technical level!

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