Slightly off-topic but a few years ago I bought a
Netatmo weather station for my home. Among other things it measures and graphs the CO2 levels in my bedroom and home office. When no one is in the bedroom the CO2 reading is about 300-400. When my wife and I are sleeping in there with all the windows and door shut it climbs to 1000-1200. My home office is in a small wing added to the house and is very tightly sealed. When I'm working in there with the windows closed the CO2 soars to 2000 or more. If I open the window the level starts dropping.
It was quite an eye-opener that CO2 levels climbed so high in sealed rooms without ventilation. In years past I've slept in my office and wonder how I didn't pass out after breathing such high CO2 levels all night.
The Netatmo demonstrated how levels of harmful gases can easily soar when sealed in.