How CO2 Builds Up and How to Clear It
People exhale CO2 continuously — about 20 liters per hour per person. In a poorly ventilated room, CO2 accumulates until it reaches a steady-state level set by the balance between generation and removal. This calculator uses the well-mixed room model to predict how long ventilation takes to reach your target level.
The CO2 Clearance Formula
Equilibrium CO2 (ppm) = 420 + (occupants × 0.02 m³/h ÷ airflow m³/h) × 10⁶. If your target is below the equilibrium, the concentration will never reach it — more airflow is needed. Otherwise: time (h) = −(room volume / airflow) × ln((target − equilibrium) / (current − equilibrium)).
CO2 Health Thresholds
420 ppm: Outdoor air. 800 ppm: Good — well ventilated. 1,000 ppm: ASHRAE acceptable limit (most building codes). 1,500 ppm: Cognitive performance starts declining. 2,000 ppm: Headaches, fatigue. 5,000 ppm: OSHA 8-hour work limit. Meeting rooms and classrooms regularly exceed 2,000 ppm without proper ventilation.
Practical Ventilation Tips
Open windows 10 minutes before occupants arrive to pre-flush the room. During meetings, crack a window 5–10 cm — even a small opening dramatically increases ACH. A portable CO2 sensor (<$100) takes the guesswork out of knowing when to ventilate.
Frequently Asked Questions
No. A mask slightly delays the release of CO2 but does not reduce the total amount exhaled. The CO2 still enters the room — masks help reduce airborne particles and droplets but have no effect on CO2 accumulation. Ventilation is the only effective CO2 control.
Standard AC systems recirculate indoor air without introducing outdoor air. They have no effect on CO2 levels. To reduce CO2 you need either a fresh-air mechanical ventilation system (ERV/HRV) or open windows/doors. Running AC with a slightly open window is a common compromise.
With no occupants, the generation rate G = 0 and the equilibrium drops to 420 ppm (outdoor air). You can recalculate with 0 people (set people = 0.001 or use the result manually) — the math shows the room flush time is determined solely by the room volume and ventilation rate.