CO₂: when a room needs airing

Carbon dioxide itself is not a poison. It is the most honest sign that a closed room has not had fresh air for a long time.

Which numbers are normal

The zones on the device screen. Colour appears only where attention is needed.

GOST 30494-2011 air quality classes 1–4; EN 16798-1:2019; UBA 2008

Real frames from the device, captured on 4 September 2026.

Outside is about 420 ppm. A closed bedroom gathers 2000–2500 overnight: every breath adds, the window and door let nothing out.
In a controlled trial people who slept at 850 ppm instead of 2400 were more rested in the morning and did better on attention tasks.
In schools every +100 ppm in the classroom goes with slightly more absences; one extra litre of fresh air per second per pupil — with better test results.
The reverse is also true: pure CO₂ up to 3000 ppm barely affects attention on its own. What harms is everything else that builds up with it in an unaired room. That is why CO₂ is a ventilation indicator.

Strøm-Tejsen et al. Indoor Air 2016; Shendell 2004; Haverinen-Shaughnessy 2011; Zhang et al. Indoor Air 2017

Hints in words on the screen. Actions about the room, not about the body.

“A good moment to air the room” — when CO₂ is above 800 and it is no hotter outside than inside, with clean air.
“You can close the window” — when CO₂ has dropped to outdoor level and the room has started to cool.
“Keep it shut” — when there is smog, pollen or frost outside: better a purifier, or wait.

Why is an empty room at 450 by day and 900 in the evening?
Because in the evening there are people in it and a closed window. The device does not show “bad air”; it shows that the room has been closed longer than it should.
Why is an infrared sensor better than a cheap one?
Cheap devices do not measure CO₂; they guess it from volatile compounds, with errors of hundreds of ppm. FAVEO uses a Sensirion SCD40 that counts CO₂ molecules by light absorption.