Indoor CO₂ is the cleanest ventilation gauge available — it rises predictably with the people in a room and falls predictably when outdoor air gets in. Here is what the numbers mean.
Outdoor baseline. Atmospheric CO₂ in 2026 sits around 420 ppm globally, slightly higher in dense urban areas. Every reading above that comes from local sources — almost always human respiration, sometimes combustion.
The bands that matter.
- Below 800 ppm. Healthy. Outdoor air is reaching the room. Good for sleep, concentration and most cognitive work.
- 800–1,000 ppm. Acceptable. Most occupied rooms sit here. Comfortable for routine work, may notice mild stuffiness at the upper end.
- 1,000–1,500 ppm. Borderline. Cognitive performance starts to decline measurably above 1,000 ppm in controlled studies. Open a window, run mechanical ventilation, or reduce occupancy.
- 1,500–2,000 ppm. Poor. UK Department for Education classifies this as the upper bound for classrooms. Drowsiness, reduced focus, headaches all documented in this range.
- Above 2,000 ppm. Action range. Common in closed bedrooms by morning, packed meeting rooms, small podcast booths.
How quickly CO₂ rises.
A single adult adds ~250–400 ppm of CO₂ per hour to a small room (12 m²) with no ventilation. A 30-student classroom can reach 1,500 ppm within 20 minutes of a lesson starting. A closed bedroom shared by two adults routinely passes 2,000 ppm by 6 AM.
Why CO₂ is the right ventilation metric.
PM2.5 tells you about cooking and outdoor air. VOCs tell you about cleaning and renovation. CO₂ tells you about people and ventilation — and that is what most occupied-space problems come down to.