In the paper I linked, I think they directly add relatively pure carbon dioxide. And the total concentration is 0.1%. So the concentration of oxygen in the air is not really changing.
Can't you apply that argument to oxygen and nitrogen as well? If you are willing to believe that adding a small absolute amount of carbon dioxide can have large effects on the brain, then I don't see why you would not also be willing to believe that decreasing oxygen (a critical fuel for the brain's metabolism) by a small absolute amount might have large effects on the brain. Injecting CO2 as they do does control for air variables like mold and temperature and humidity, but I didn't see anything about also injecting oxygen and nitrogen to independently manipulate the air composition in all 9 possible ways to disentangle which it is. It could be that CO2 is inert, but by pushing out oxygen and reducing oxygen levels has effects; it could be that CO2 is inert but it's both oxygen and nitrogen, or CO2 is poisonous but is combining with lack of oxygen.
The texas natural experiment seems like an especially convincing complement to the more artificial setting, thanks for pointing it out.
I found it interesting that the anti-mold renovations had such large apparent effects compared to the ventilation and other renovations.
The sample size is small, but given the effect size I don't think it even matters that much. The error seems like less than a factor of 2.
Small study effects go beyond just sampling error, so they are untrustworthy.
The CO2 intervention is doubling the density of CO2, and decreasing the density of oxygen by < 1%.
Small proportional changes seem unlikely to drive big effects, unless there is some feedback mechanism that is keeping the level precisely balanced. But 1% changes in oxygen should be happening all over the place. It seems much more plausible for doubling the density of CO2 to have a direct effect.
Similarly, the nitrogen intervention is a significant proportional change.
One or two research groups have published work on carbon dioxide and cognition. The state of the published literature is confusing.
Here is one paper on the topic. The authors investigate a proprietary cognitive benchmark, and experimentally manipulate carbon dioxide levels (without affecting other measures of air quality). They find implausibly large effects from increased carbon dioxide concentrations.
If the reported effects are real and the suggested interpretation is correct, I think it would be a big deal. To put this in perspective, carbon dioxide concentrations in my room vary between 500 and 1500 ppm depending on whether I open the windows. The experiment reports on cognitive effects for moving from 600 and 1000 ppm, and finds significant effects compared to interindividual differences.
I haven't spent much time looking into this (maybe 30 minutes, and another 30 minutes to write this post). I expect that if we spent some time looking into indoor CO2 we could have a much better sense of what was going on, by some combination of better literature review, discussion with experts, looking into the benchmark they used, and just generally thinking about it.
So, here's a proposal:
Some clarifications:
(Thanks to Andrew Critch for mentioning these results to me and Jessica Taylor for lending me a CO2 monitor so that I could see variability in indoor CO2 levels. I apologize for deliberately not doing my homework on this post.)