After it was recommended here on HN, I bought this CO2 meter [0] and I've been very satisfied with my purchase. It reminds me to open the window and that has had a tremendous impact on my health. I've gotten to the point where I now keep the window slightly open at all times.
I'll share an anecdote which would appear to support the hypothesis that higher CO2 levels affect cognition. I play Heroes of the Storm and my performance in the game becomes notably worse as CO2 levels rise. It's not a huge thing, but I start to make more mistakes and my decision-making gets worse. This is important because at my skill-level (top 100 to top 250 globally, depending on my activity levels) it only takes one mistake in the late-game to throw the match. I've had the CO2 meter start beeping in the middle of a match, and after opening the window I've managed to turn things around because I can think with greater clarity.