Indoor CO2 measurements help describe occupancy-related ventilation trends, but a CO2 value alone is not an air-quality or infection-risk verdict. Interpretation depends on occupancy, room volume, outdoor CO2, ventilation operation, sensor position and time since people entered the room. Monitor trends under defined conditions instead of treating one number as a universal pass/fail threshold.
A CO₂ reading does not identify the ventilation fault
CO₂ concentration depends on occupancy, background concentration, air exchange and mixing. A high value might reflect an overcrowded room rather than a broken fan. Record sensor location and occupancy changes over time; trend data and outdoor baseline usually explain more than a single threshold comparison. For a focused explanation of rapid meeting-room increases, read why CO₂ rises in a meeting room.
What a CO2 meter actually measures
A suitable sensor reports carbon dioxide concentration, commonly in parts per million (ppm). CO2 tends to accumulate when occupants generate it faster than ventilation removes or dilutes it. Outdoor air, room mixing and occupant activity influence the result. Changes in CO2 do not directly measure all pollutants, filtration effectiveness or the distribution of infectious aerosols.
Where to place the sensor
Place the meter in a representative breathing-zone location rather than immediately beside an occupant’s face, an open window, a supply diffuser or an exhaust grille. Direct breathing plumes and fresh-air jets can create unusually high or low local readings. Follow the probe manufacturer’s operating range and stabilization instructions, and record placement height and room layout.
Practical monitoring sequence
- Record the room size, occupancy and HVAC schedule.
- Check the instrument’s date, calibration status and sensor response after startup.
- Establish a baseline with the room vacant or at a known low occupancy if possible.
- Log CO2, temperature and relative humidity at consistent intervals while occupancy changes.
- Note door/window opening, air-system mode and unusual events.
- Review trends with building ventilation professionals and relevant guidance instead of applying a generic CO2 cutoff.
Worked example: trend, not a safety verdict
Hypothetical: a meeting room begins at 520 ppm, rises to 1,060 ppm over a busy session and later falls to 610 ppm when occupants leave and ventilation continues. The increase during occupancy is 540 ppm. It is evidence of changing CO2 conditions; it does not by itself quantify outdoor-air delivery in CFM per person or show that a specific health threshold was crossed.
Testo measurement configuration
The testo 440 supports indoor-air-quality measurements when paired with an appropriate CO2 probe; its product page lists CO2 probes that also measure temperature and humidity. The base handheld meter should not be described as having an internal CO2 sensor without that probe. Confirm exactly which wired or Bluetooth probe is in the selected kit.
Why comparison with another CO2 meter can be difficult
Two sensors may differ because of warm-up time, calibration, airflow around the inlet or sampling at different locations. Place them side by side under reasonably stable conditions before assuming either has drifted. NIOSH notes that CO2 can inform ventilation assessment but cannot predict transmission risks or replace wider ventilation evaluation. For a practical side-by-side comparison method, see why two CO₂ meters give different readings.