A new study shows strong effects of what should be a negligible CO2 concentration.

The paper "Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance" exposed people to three levels of CO2: 600, 1000, and 2500 ppm while keeping constant other factors such as ventilation, humidity, temperature, and indoor pollutant concentration. Of course, the participants didn't know how much CO2 was in the air.

At the 2500 ppm level, most cognitive performance measures were temporally reduced (they used the SMS management test). Some changes were drastic.

Since normal air is 400 ppm, 2500 ppm sounds like a lot. However, it is dwarfed by internal production. Humans exhale 50,000 ppm CO2 in the lungs, meaning that the lung air averages 25,000 ppm (as a rough approximation). The extra 2500 ppm is only a 10% increase in the average "atmosphere" that the blood is exposed to. Furthermore, people partially compensate increased P(CO2) with deeper ventilation (the effect takes much larger concentrations to be noticeable). Why would this extra 10% have such a strong effect?

Personally I am skeptical. This study has such high impact (millions of people's performance) that it needs replication. Maybe the hissing sound of the injector got people worried (more hissing at a higher concentration!). Maybe an experimenter (who knew the concentration) was watching the people and that subconsciously affected them.

  • $\begingroup$ Interesting question. I will take a look at the original paper, but this can take a day or two. $\endgroup$
    – Chris
    Jun 20, 2014 at 15:15
  • $\begingroup$ @Chris: Thanks. I am not the best at including links and other formatting. Can editors be rewarded 5 rep when the asker likes it? $\endgroup$ Jun 20, 2014 at 15:22
  • $\begingroup$ You only get rewards for accepted) edits when you are below 1000 reputation points (and only 2). Good that you like it. :-) $\endgroup$
    – Chris
    Jun 20, 2014 at 15:26
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    $\begingroup$ I have read it now but I am not decided yet. The first and biggest problem I see is the relatively small number of participants (only 24). This is pretty small to do real statistics (it can be done, but needs extra care and is not as conclusive as larger groups). And I am not sure, if the differences are really so big. If you look on the single results in figure 1, than these results vary quite a lot. So personal factors seem to play an important role here, too. I am not too convinced by this study. $\endgroup$
    – Chris
    Jul 1, 2014 at 10:12
  • $\begingroup$ I think that the diffusion of carbon-di-oxide in the alveolar surface depends on the residual P(CO2) in the inhaled air. If the partial pressure of CO2 is high in the inhaled air then the amount of CO2 that diffuses out of the blood decreases $\endgroup$
    – One Face
    Jan 7, 2015 at 12:34


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