I bought a CO2 sensor, and found that the CO2 levels rise to unhealthy levels in like half an hour in my room without opening my window. Since then I only sleep with a window slightly open, and open it periodically during the day.
I also run an air purifier all the time, but I cannot yet measure it's efficacy, because a particle counter is like 2-3× more expensive than a CO2 sensor, so I haven't invested in it yet.
A PM sensor is 2-3 times cheaper than a decent CO₂ sensor if you're willing to get your hands dirty. Look for PMS5003 (or PMS7003 which is pretty much the same beast in a different package), and any ESP32 dev board (they're dirt cheap) + ESPHome as firmware.
Yep, I leave the fan on all the time to circulate air in the house. The levels seem to range from 450-550ppm. I was surprised at how low that is considering this is a newer house.
450-550 is great, especially since ambient levels outside are about 420. I live in an old house and with windows closed and no hvac fan running, rooms only take about 30 minutes to get up to 800ppm with one person in them. After an hour or two they get up to about 1050ppm. With two people, they get > 1000 pretty quickly.
Looking at metrics from the sensor is pretty interesting too. You can see, to the minute, when a room went from unoccupied to occupied.
I also run an air purifier all the time, but I cannot yet measure it's efficacy, because a particle counter is like 2-3× more expensive than a CO2 sensor, so I haven't invested in it yet.