[Content note: reading this post might cause feelings of suffocation or provoke panic attacks in susceptible individuals. Epistemic status is very speculative.]

Last month I moved into a small cottage behind a big group house. The cottage is lovely. The big group house is also lovely, but the people in it started suffering mysterious minor ailments. Headaches, fatigue, poor sleep – all the things that will make your local family doctor say "Take two placebo and call me in the morning". Using my years of medical training and expertise, I was able to…remain completely unaware of the problem while my housemates solved it themselves.

There's been a flare-up of research interest in indoor carbon dioxide levels, precipitated by a Berkeley study (paper, popular article) finding that increasing CO2 concentration from the level of a well-ventilated building to the level of a poorly-ventilated building had profound effects on cognitive ability, cutting various test scores by as much as 50%. This was so dramatic as to be implausible, but seems to match the result of previous Hungarian studies and a later Harvard study on the same subject. The Harvard team later replicated their result with real workers in real offices and found that, controlling for other factors, workers in the best-ventilated offices scoredabout 25% better on cognitive tests than in the worst-ventilated ones. NASA got really interested in this research because spaceships require a lot of intellectual work and don't have a lot of open windows. They're still running tests but they say that "preliminary results suggest differences" between better- and worse- ventilated environments.

On the other hand, a 2017 study failed to find the effect, possibly because their cognitive tests were easier. And bloggers have pointed out that submarines have more CO2 than the worst terrestrial buildings, but don't have any problems overt enough for the Navy to notice or worry. So it's a crapshoot of contradictory results and considerations, just like everything else.

Aware of this research, my housemates tested their air quality and got levels between 1000 and 3000 ppm, around the level of the worst high-CO2 conditions in the studies. They started leaving their windows open and buying industrial quantities of succulent plants, and the problems mostly disappeared. Since then they've spread the word to other people we know afflicted with mysterious fatigue, some of whom have also noticed positive results.

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