600-750ppm is fine for a closed space, even Aranet4 (the best home sensor) goes yellow only after 1000ppm. I maintain it under 800ppm in winter because that's r

600-750ppm is fine for a closed space, even Aranet4 (the best home sensor) goes yellow only after 1000ppm. I maintain it under 800ppm in winter because that's roughly the value from where serious cognitive decline starts: https://dash.harvard.edu/server/api/core/bitstreams/7312037d-db2a-6bd4-e053-0100007fdf3b/content https://dash.harvard.edu/server/api/core/bitstreams/7312037d...