I asked Erik @bernhardsson why high CO2 levels in your office are such a big deal: "I'm not a health nut. But one of the things I've been radicalized on is CO2 levels. There's a real relationship between CO2 levels, productivity, and cognitive performance. And CO2 levels are usually way too high in offices and schools. Normal air CO2 levels are 300-500 PPM. In offices it often hits 1,000 or 2,000. Airplanes can get up to 2,500. You start getting brain damage at 5,000. So I went and bought a bunch of CO2 monitors for the office. And I look at them every day. And I open windows anytime we get too high."