The practical answer
AS 1668.2 uses a mechanical ventilation design framework based on the enclosure use and required ventilation pathway. It is not simply a CO₂ pass/fail Standard.
CO₂ is most useful when trended over time with occupancy and system operation. A rising pattern during a known high-occupancy session can be strong evidence that the current ventilation path deserves investigation.
The engineering conclusion should connect the measurement to what the system was designed to do and what it actually delivers. A calibrated sensor, representative location and reliable occupancy context matter.
When this question usually matters
Office or classroom occupants complain of stuffiness or headaches during busy periods.
A room use or occupant density has changed since the original HVAC design.
Portable sensors show recurring peaks but no formal ventilation assessment has been completed.
A VAV or demand-control system may be reducing airflow during occupied periods.
The owner wants to know whether a measured CO₂ problem means immediate HVAC replacement is necessary.
Red flags that make the question more significant
The sensor is uncalibrated or mounted beside a person, door or supply diffuser.
The room occupancy during the reading is unknown.
Only one short reading is available.
The outdoor CO₂ concentration was not considered.
The system operating state and airflow at the time of the reading are unknown.
How to use CO₂ properly in a ventilation assessment
Treat CO₂ as one line of evidence in a system investigation, not as the entire engineering method.
Confirm the room and occupancy
Record how many people are present, how long they stay, what the room is used for and whether the use differs from the original design.
Check the sensor and location
Use suitable equipment and place it where it represents the occupied zone rather than one person’s breathing plume or a supply diffuser.
Trend the pattern
Compare CO₂ against occupancy, door position, HVAC operation and time of day instead of relying on one spot value.
Review the mechanical ventilation basis
Check the AS 1668.2 pathway, outdoor-air source, terminal airflow, VAV settings and control sequence.
Measure the air system
Where needed, verify airflow and pressure so the measurement evidence can be linked to actual system performance.
Decide the remedy
The solution may be balancing, control changes, operating schedule, increased capacity or a change to room use — not automatically a new AHU.
Detailed practical examples
These examples show how the question changes a real engineering decision. They are not universal answers; they show what needs to be clarified before the right answer becomes obvious.
CO₂ rises every afternoon during four-hour training sessions
A converted meeting room now holds twenty-five trainees. CO₂ rises steadily during afternoon sessions, but the room is comfortable in temperature and the central AHU appears to be running.
The trend is useful evidence that the ventilation path may not match the new occupancy, but it does not identify whether the issue is outdoor-air quantity, VAV turndown, control sequence or central capacity.
What made the job difficult
- The room was designed for lower occupancy.
- The VAV box minimum setting is unknown.
- The central outdoor-air damper serves many rooms.
How the review should proceed
- Confirm the current room use and occupancy profile.
- Trend CO₂ together with occupancy and terminal airflow.
- Check VAV minimum settings and central outdoor-air operation.
- Determine whether local rebalancing, controls or central changes are required.
The CO₂ data becomes the trigger and supporting evidence for an AS 1668.2 ventilation assessment, not the compliance verdict by itself.
A desk sensor reads very high beside one employee
A single desktop CO₂ sensor near an employee repeatedly shows high values. The facility manager concludes the floor HVAC is non-compliant.
Local placement can bias the reading. Before escalating to a whole-building conclusion, the measurement method and representativeness should be checked.
What made the job difficult
- The sensor is within the breathing zone of one person.
- No calibration record is available.
- No comparison readings are taken elsewhere.
How the review should proceed
- Relocate or supplement the sensor to obtain representative occupied-zone data.
- Compare readings across time and locations.
- Record occupancy and HVAC state.
- Only then decide whether detailed airflow investigation is justified.
The investigation may still find a ventilation problem, but the conclusion is based on representative evidence rather than one poorly located device.
Information to gather before deciding
Room use and current occupancy
CO₂ sensor type, calibration and location
Time-series CO₂ data
Outdoor CO₂ reference where relevant
HVAC operating schedule
Terminal and outdoor-air airflow information
VAV or demand-control settings
BMS trend data
Recent room-use or fit-out changes