I got a DIY basic kit, and was pretty surprised when I saw that after 8 hours of sleep the CO2 level was 3600 ppm in the bedroom. My wife still jokes about the way I woke her up. I immediately said "We're basically suffocating". Maybe it was a little bit too dramatic sure, but kind of true.

The levels go above 1000 ppm in less than an hour with both of us in there.

Opening the windows to ventillate doesn't really solve this, as you'd need to keep opening and closing them every hour. I'm thinking about installing a HRV system, but struggling to find a model that could be integrated into Home Assistant. (ERV not necessary as I live in a european country with temperate climate, the humidity is mostly OK)

In the meantime I have two windows tilted open permanently in another room with the doors open. This keeps CO2 under 1000 ppm, but pretty bad for energy efficiency.

Overall pretty happy with the product, definitely made me much more aware of the air quality. I want to build a portable one with a display and logging to check out the car, the office, maybe the hotel rooms on a trip, the plane, etc.