What a CO2 Reading Says About Ventilation in an Office or Classroom

When a meeting room, office or classroom feels stuffy, a CO2 reading can help show whether the space is getting enough fresh air for the number of people inside. It is not a direct measure of air quality on its own, but it is a practical indicator of how well indoor air is being diluted and replaced.

For facility managers, mechanical contractors and school operators, CO2 is one of the simplest ways to get a snapshot of ventilation performance during real occupancy. Used properly, it can help you decide whether a room needs more outside air, a system check, or a closer look at airflow distribution and filter condition.

Why CO2 is used as a ventilation proxy

People exhale carbon dioxide as they breathe, so occupied rooms naturally see CO2 rise as more people stay in the space and as fresh air delivery struggles to keep up. In that way, CO2 acts as a proxy for how much rebreathed air is building up indoors.

A higher reading does not automatically mean a fault in the plant, and a lower reading does not guarantee perfect performance. It simply gives a useful clue about the balance between occupancy, outside air supply, room volume and how effectively air is moving through the space.

CO2 readings are most useful when you look at them in context:

• •How many people are in the room
• •How long the room has been occupied
• •Whether doors and windows are open or closed
• •Whether the HVAC system is running as intended
• •Whether supply and return paths are clear

In a classroom or office, a reading taken during full occupancy is usually more informative than a reading taken after the room has been empty for a while.

What elevated CO2 readings usually indicate

If CO2 rises and stays elevated while the room is occupied, the most common explanation is that the fresh air rate is too low for the occupancy or the air is not mixing and exhausting effectively. That can happen for several reasons.

Common causes to check

• •Outdoor air intake is restricted, closed or set incorrectly
• •Dampers are out of position or controls are not responding as expected
• •Supply fans are running but airflow is low because of dirty or loaded filters
• •Return air paths are blocked, reducing circulation through the room
• •The space is over-occupied for its intended ventilation arrangement
• •The room layout or partitions are disrupting airflow patterns

Elevated readings can also happen when the CO2 sensor itself is poorly located, poorly calibrated or affected by local airflow. A sensor right next to a person’s breathing zone, a supply diffuser or an open doorway may not reflect the average condition of the whole room.

A good reading is one that represents the occupied zone, not a spot with unusual air movement.

How to interpret readings in occupied rooms

CO2 interpretation is less about chasing a single number and more about watching how the reading behaves over time. In an occupied office or classroom, look for the pattern rather than the snapshot.

What the trend can tell you

• •Steady rise after occupancy begins may suggest the room is not getting enough outside air for the load
• •Readings that level off at a moderate point may indicate the ventilation system is keeping pace with occupancy
• •Readings that keep climbing through the day can point to insufficient outside air, poor distribution, or a system issue
• •Sudden drops may mean windows have been opened, occupancy has changed, or the sensor is near a supply air stream

For practical use, compare the occupied room with nearby spaces that use the same system. If one room consistently reads much higher than others under similar conditions, that is often worth investigating.

It also helps to compare occupied and unoccupied periods. If the room does not come back down when people leave and the system is running, the issue may be less about occupancy and more about ventilation supply, return air balance or sensor placement.

What to check when CO2 stays high

A high reading is a prompt to inspect the ventilation system systematically. Start with the simple items before moving to controls and ductwork.

A practical fault-finding approach

• •Confirm the room occupancy is within the intended use of the space
• •Check that supply air is actually reaching the occupied zone
• •Inspect return air grilles, transfer paths and any obvious obstructions
• •Review the outside air setting and demand control sequence if fitted
• •Check fan operation, belt condition and control response where relevant
• •Inspect filters for loading and confirm they are seated properly
• •Look for dirty coils, blocked intakes or other airflow restrictions

Filters are only one part of the picture, but they matter because a loaded filter can increase resistance to airflow and reduce system delivery if the plant is already close to its limit. If the system has pressure gauges, use them to monitor pressure drop and compare against the equipment documentation. Inspection frequency depends on the system, site conditions and dust load, so follow the filter and equipment manufacturer instructions rather than relying on a fixed calendar.

If the room uses specialised filtration, or if you are selecting replacement media for a specific application, you can review our products or discuss custom air filters for the system in use.

CO2, ventilation and standards in Australia

In Australia, ventilation design and operation are commonly considered against AS 1668.2:2024, but site requirements still need to be confirmed with the mechanical services engineer or building owner. A CO2 reading can support that discussion, yet it does not replace design review, commissioning or a proper assessment of the HVAC system.

That matters because one room may be performing exactly as intended while another, with a different occupancy pattern or layout, needs a different approach. Classrooms, open-plan offices, enclosed meeting rooms and shared breakout spaces can all behave differently even when they are connected to the same central plant.

If you are using CO2 data to inform maintenance or upgrades, it is best treated as one input alongside airflow checks, occupancy patterns, filter condition and control settings. For sites with ongoing service needs, filter servicing and commercial filter servicing can help keep airflow issues visible before they turn into a comfort complaint.

Using CO2 readings well in day-to-day operation

The most useful CO2 program is usually simple and consistent. Measure in the same part of the room, at similar times, under similar occupancy conditions, and record what else was happening at the time.

Good practice for regular checks

• •Use the same sensor location where possible
• •Avoid placing sensors directly in front of supply diffusers or open windows
• •Note the number of occupants during each reading
• •Compare readings before, during and after occupancy
• •Keep an eye on whether the system is running as designed

For schools, this can be especially helpful during timetable changes, exam periods and event days when room occupancy varies. For offices, it can highlight rooms that are repeatedly overused for meetings or used in ways that do not match the ventilation setup.

If you are unsure whether a reading reflects the room or a sensor issue, it may be worth checking the instrument location and the air distribution around it. A reading is only as useful as the setup behind it.

Talk to Total Air Filtration about the right filter setup

If your CO2 readings suggest a ventilation problem, the next step may include checking filters, airflow and system components rather than changing one item in isolation. Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009, and we can help supply or make the right filters for your system.

Call 07 3800 3448 or request a quote. You can also contact us to discuss replacement filters, custom air filters or the right product fit for your site.