I do feel impaired around 1000ppm. I get headaches. At home I got a few Air Gradient Ones (no affiliation, but they are great) and connected those to my Home Assistant to turn up the ventilation in stages (above 650ppm go up, above 800ppm again, at 1000ppm max). I also do this for the bedrooms, cause in the night it goes up too.

The article talks about "within the hour". With four people in my living room doing normal things it jumps within 20min to around 1000ppm. If I am wrestling with my kids much sooner.

In offices companies often neglect it.

edit: if you are cooking on gas it also has an immediate effect on co2 of course apart from other small particles