Assessing ventilation

Assessing ventilation

There are several ways to quickly assess whether a space is well ventilated. 

Assessing ventilation

Your senses can give a good immediate indication of whether a space has good airflow – for example if a room feels stuffy or has lingering smells, it may not be well ventilated. You’ll be able to verify this using a CO2 monitor.

If you can’t resolve ventilation issues using our guidance, or you are concerned about ventilation in your school you can contact your Ministry Property Advisor or

Carbon Dioxide (CO2) monitoring

Monitoring CO2 levels is one way to quickly assess whether a space is well ventilated when it is occupied. Elevated CO2 levels for a sustained period of time (e.g. over an hour or more) indicate that fresh air isn’t flowing into a space quickly enough to meet the needs of the space’s occupants. 

A space under 800ppm is considered well-ventilated and when levels start to climb above this level, associated risk of airborne transmission of illnesses such as COVID-19 also begins to rise. We advise schools to take action to improve ventilation if CO2 levels stay above 800ppm for an hour or more. More detail about these actions is provided in the table below.  

In 2022 we have distributed more than 12,000 portable CO2 monitors to all state and state-integrated schools to help them assess CO2 levels. A number of spaces also have internal environment monitoring devices fitted that include CO2 monitoring. 

To learn more about setting up your portable CO2 monitor, please refer to the Ministry’s Aranet4 Setup and User guide.

Using your portable CO2 monitor to perform spot checks

Spot checks provide an immediate indication of current CO2 levels. If the levels are high, follow our guidance to try to lower them and consider if you should also monitor the space’s CO2 levels over a longer duration as explained below. 

Spot checks only provide a snapshot of the current CO2 level and may not represent peak or sustained levels. Short peaks of elevated CO2 levels are common in all indoor spaces and are not a cause for concern. Spot checks are best carried out toward the end of a teaching period while the space is fully occupied and in use, to provide the most representative spot-check readings. For a more accurate understanding of ventilation, carry out a full day reading as per below. 

  1. Take the CO2 monitor to each space and place it somewhere around student head height, away from doors and windows, out of direct sunlight, and at least 1m away from the closest people. Note breathing directly into or over the device will cause it to report high CO2 levels.
  2. Leave the device in the room for at least 5 minutes before checking the CO2 levels reported on its screen. If temperature readings are also required, extend this to 30 minutes to allow the device to report this accurately.
  3. Repeat this process in a selection of spaces, or all spaces on a regular basis (e.g. fortnightly). Look for patterns and relationships between CO2 levels, who is in the room, doing what, and with windows and doors open or closed.

Using your portable CO2 monitor to gather a full day’s readings

If you have a concern with how the space’s ventilation is changing through the day, you can leave the CO2 monitor in the room for a longer period for it to automatically collect its readings. This will provide a more representative picture of ventilation for that space compared to performing spot checks. The devices will typically store 3-7 days data that can then be downloaded. 

  1. Take the CO2 monitor into the space and place it somewhere around student head height, away from doors and windows, out of direct sunlight, at least 1m away from the closest people and in a place where it will not be disturbed or moved.
  2. At the end of the day or days, use the smartphone app to view and download the CO2 readings. When downloading and assessing the data, ensure you only review the data linked to that space on that day. Take note of how CO2 levels change based on who is in the room, doing what, with windows and doors open or closed at different times through the day.
  3. When discussing any concerns with your Ministry property advisor, provide a copy of the downloaded data.
  Sustained CO2 levels Actions to consider

Under 800ppm

Your space is well ventilated, continue with current approach. 



Open windows more if this can be done while maintaining comfortable indoor temperatures. Consider lowering the level of activity, briefly vacating the room and/or purging and refreshing the air in the space (a “reboot”). 



Take further action to introduce more fresh air, for example: 

  • Open all windows and doors as much as possible, and whenever it is practical to do so while maintaining comfortable indoor temperatures. 
  • Reboot the room by fully opening all windows and doors for a few minutes, preferably while vacating the room, to purge and refresh the air in the space. 
  • Reduce the level of vigorous activity performed in the room or lower the total occupancy. 

To reduce the risk of transmission of airborne illnesses such as COVID-19, also consider increasing other health measures such as face coverings and physical distancing. 


Over 2000ppm 

If you have followed the above advice and still have sustained CO2 levels over 2000ppm, contact your Property Advisor or

In addition, instigate at least four refresh breaks through the school day where you reboot the room with all windows and doors open for a few minutes, and preferably with the room unoccupied.