The cognitive ability of humans is varies with the concentration of CO₂ in breathing air. But how does it vary?

A very high concentration causes sleepiness, and sleepiness is a state of reduced cognitive ability.

That means it does vary, but I think there is no sudden onset of the reduction.

I would be interested in, ideally, a function of some experimental correlate of cognitive ability by level of CO₂, but any theoretical or experimental information about the shape of the function is interesting.

I'm aware that the signal to noise ratio in an experimental measurement is high, but qualitative results are the most important.

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    $\begingroup$ This review might help formulate an answer. $\endgroup$
    – Fizz
    Nov 6 '19 at 11:41
  • $\begingroup$ Are you sure the CO2 is relevant? I read your related question as well, and it seems far more likely that the effects you are measuring are not caused by the increase of CO2 concentration but instead are caused by the decrease of relative oxygen concentration that will be caused by the CO2. In other words, increase of any non-oxygen atmospheric gas concentration would have the same effect. The drowsiness comes from a lack of oxygen AFAIK, not from an increase of CO2. $\endgroup$
    – terdon
    Nov 7 '19 at 12:38
  • $\begingroup$ @terdon oxygen concentration is 209500ppm. Just adding 1000ppm of another gas would only reduce oxygen to ~209300ppm. $\endgroup$
    – DavePhD
    Nov 8 '19 at 14:11
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    $\begingroup$ @terdon I found it counter intuitive myself, and had expected it should all be about oxygen. I do not yet understand the exact interaction, but there is one very strong hint: Oxygen is transported by hemoglobin, but carbon dioxide is also transported by hemoglobin, it connects in a different way thought. It could be that the CO2 somehow competes with O2 on hemoglobin, with CO2 having priority, which should be the case in the bloodstream. Too much CO2 is possibly more problematic than not enough oxygen, because CO2 makes the blood acidic. $\endgroup$ Nov 8 '19 at 14:28
  • $\begingroup$ @VolkerSiegel what you describe is about carbon monoxide (CO) not dioxide (CO2). That does indeed compete with oxygen for the 4 hemoglobin binding sites. But apparently, CO2 concentration is the main sensor for respiration, so there is indeed something that can detect it. $\endgroup$
    – terdon
    Nov 8 '19 at 14:45

According to Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance Environ Health Perspect. 2012 Dec; 120(12): 1671–1677:

of 9 different cognitive tests performed at 600ppm (green), 1000ppm (blue) and 2500ppm (red) several show very marked decrease in scores at higher C02 concentration.

enter image description here

Authors summarized findings as:

Relative to 600 ppm, at 1,000 ppm CO2, moderate and statistically significant decrements occurred 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

Four years after the above was published, the negative effects of carbon dioxide were confirmed in Associations of cognitive function scores with carbon dioxide, ventilation, and volatile organic compound exposures in office workers: a controlled exposure study of green and conventional office environments. Environ Health Perspect (June 2016) 124:805–812, which was mentioned on Earth SE by Ken Fabian, although there is one common author Usha Satish.

On the other hand, a very recent article Effects of acute exposures to carbon dioxide on decision making and cognition in astronaut-like subjects NPJ Microgravity. (June 2019) 5: 17, which also has Usha Satish as an author, tends to contradict the above:

enter image description here

  • $\begingroup$ Oh... That makes what I was thinking about significantly more scary... Proportional to the significance reflected in the diagrams... Secular [bleep*]! (*American English) $\endgroup$ Nov 8 '19 at 14:36
  • $\begingroup$ @VolkerSiegel I added data from a very recent study might lessen your fear. $\endgroup$
    – DavePhD
    Nov 8 '19 at 15:01
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    $\begingroup$ @VolkerSiegel This is another very recent article that found no negative effect ncbi.nlm.nih.gov/pubmed/29789085 They tested all the way up to 15,000ppm $\endgroup$
    – DavePhD
    Nov 8 '19 at 15:10
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    $\begingroup$ @VolkerSiegel Make sure you aren't getting your decimal places in the wrong place. 2000 ppm is 0.2%, not 2%. 2000 ppm CO2 is not the onset of sleepiness. $\endgroup$ Nov 8 '19 at 15:18
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    $\begingroup$ Zhang, X., Wargocki, P., & Lian, Z. (2016). Human responses to carbon dioxide, a follow-up study at recommended exposure limits in non-industrial environments. Building and Environment, 100, 162-171. is another negative result, showing even 5000 ppm CO2 doesn't cause sleepiness. $\endgroup$ Nov 8 '19 at 15:39

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