Why CO₂ Is the Key Metric
Carbon dioxide concentration is the single most practical indicator of ventilation performance in occupied spaces. Unlike VOCs or particulates, CO₂ is directly linked to occupancy and ventilation rate — making it an objective, measurable proxy for fresh air provision.
What Do the Numbers Mean?
CIBSE Guide A and Building Regulations Part F set clear benchmarks:
- Below 800 ppm — good ventilation, adequate fresh air supply
- 800–1,000 ppm — acceptable but approaching the threshold; ventilation should be reviewed
- 1,000–1,500 ppm — indicates inadequate ventilation; occupants may experience drowsiness and reduced concentration
- Above 1,500 ppm — poor ventilation requiring immediate corrective action
Common Causes of Elevated CO₂
In our 27 years of assessing commercial buildings, the most frequent causes we encounter include:
- AHU fresh air dampers set incorrectly or stuck closed
- Demand-controlled ventilation systems miscalibrated
- Rooms repurposed for higher occupancy without ventilation adjustment
- Building management system (BMS) faults or overrides
What to Do About It
Regular CO₂ monitoring — either spot-check or continuous — provides the evidence needed to identify underperforming ventilation zones and take corrective action before complaints arise. Green Air Monitoring provides both one-off assessments and ongoing monitoring programmes as part of our IAQ assurance contracts.