https://astralcodexten.substack.com/p/eight-hundred-slightly-poisoned-word

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In 2012, a Berkeley team found that indoor carbon dioxide had dramatic negative effects on cognition (paper, popular article). Subjects in poorly ventilated environments did up to 50% worse on a test of reasoning and decision-making. This is potentially pretty important, because lots of office buildings (and private houses) count as poorly-ventilated environments, so a lot of decision-making might be happening while severely impaired.

Since then people have debated this on and off, with some studies confirming the effect and others failing to find it. I personally am skeptical, partly because the effect is so big I would expect someone to have noticed, but also because submarines, spaceships, etc have orders of magnitude more carbon dioxide than any civilian environment, but people still seem to do pretty hard work in them pretty effectively.

As part of my continuing effort to test this theory in my own life, I played a word game eight hundred times under varying ventilation conditions.

…okay, fine, no, I admit it, I played a word game eight hundred times because I'm addicted to it. But since I was playing the word game eight hundred times anyway, I varied the ventilation conditions to see what would happen.

The game was WordTwist, which you can find here (warning: potentially addictive). You get a 5x5 square of letters and you have to find as many words as possible (of four letters or more) within three minutes. You can move up, down, right, left, or diagonal, and get more points for harder words. A typical board looks like this: