CO2 can rise quickly in a meeting room when occupants generate carbon dioxide faster than outside air replaces or dilutes it. Small room volume, high occupancy, doors/windows closed and low outdoor-air delivery can contribute. A fast increase is a reason to examine the room’s ventilation conditions, not a standalone verdict about health risk.
Occupancy and outdoor air should be logged together
The same room can show a steep CO₂ rise in a meeting and a shallow rise during empty hours without the ventilation equipment changing at all. Record people, room use, supply/return operation and sensor position. An improving trend after occupancy ends helps characterize dilution but does not by itself establish outdoor-air delivery rate.
The balance of people, air and time
People exhale CO2 continuously. During a meeting, occupant-generated CO2 mixes with indoor air while outdoor ventilation and infiltration remove or dilute it. A crowded room can show an upward trend even when the initial concentration is low. Changes in outdoor air, occupancy and HVAC operation can cause the trend to level off or fall. For related guidance, see CO2 Monitoring for Indoor Air Quality: Sensor Placement, Trends.
Where to measure and what to log
- Choose a representative location away from breath plumes, supply diffusers and open windows.
- Verify the CO2 sensor’s start-up condition, calibration information and recording interval.
- Log baseline CO2 before the room fills, with occupancy and HVAC operating status.
- Continue through the meeting, noting entry/exit, door changes and ventilation mode.
- Record conditions after people leave or the ventilation operating mode changes.
- Review the trend with the building’s ventilation professional; do not treat one ppm reading as a universal pass/fail result.
How to interpret common patterns
| Trend | What to investigate |
|---|---|
| Rapid rise immediately after occupancy begins | Occupant count, room volume, outdoor-air supply and sensor placement |
| Rise slows toward a plateau | Balance of ongoing generation and removal at that condition |
| Concentration drops after ventilation increases | Changed outdoor-air delivery and possible changes in occupancy |
| One large short spike | Breathing near sensor, movement or other local disturbance |
Hypothetical trend example
A conference room begins at 490 ppm before occupancy and reaches 1,030 ppm after a busy meeting starts. The net increase is 540 ppm. That is a meaningful trend to investigate, but without the occupancy, outdoor baseline, room mixing and ventilation rate, it is not enough to calculate airflow per person or declare a specific hazard. For a more reliable airflow estimate, see our guide to air-velocity readings and duct traverses.
Suitable Testo equipment
The testo 440 measuring instrument accepts a compatible CO2 probe that can also measure temperature and humidity. Record the exact probe configuration and location rather than implying the base meter alone detects CO2. NIOSH guidance emphasizes that CO2 monitoring can support ventilation assessment but cannot predict transmission risk on its own.
Corrective-action limits
Operational responses should follow building design and responsible engineering review. Do not disable ventilation safety systems or infer that opening a door necessarily achieves the required outdoor-air rate. CO2 is one diagnostic indicator among many.