and the fact that air quality usually gradually worsens over time. i think it's easier to tell bad air quality when you move in between rooms/spaces.

but i agree with the op that 'looking' at a display already subconsciously tells you something about the air quality and your relation to it. my air quality sensor [0] has a green/orange/red light system though, so i can sometimes just tell that the air quality is bad by a red led flaring up in the corner of my eye.

[0] https://www.tfa-dostmann.de/en/product/co2-monitor-airco2ntr...

previous discussion on the above meter: https://news.ycombinator.com/item?id=22764603