Assessing ventilation

Assessing ventilation

Assessing ventilation

There are several ways to quickly assess whether a space is well ventilated. Your senses can give a good immediate indication – 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 your CO2 monitor.

If you can’t resolve ventilation issues using our guidance, or you are concerned about ventilation in your service, you can contact the Ministry’s ventilation team on

Carbon dioxide (CO2) monitoring

Measuring CO2 levels indicates how well-ventilated a space is when it is occupied. Elevated CO2 levels means fresh air isn’t flowing into a space quickly enough to meet the needs of the space’s occupants. If sustained at elevated levels this can cause drowsiness and concentration issues for the people in the space.

Also, if the air in a space is not replaced quickly enough, pollutants and airborne particles including viruses stay in the air for longer. That means if anyone in the room is infectious with COVID-19, the risk of airborne transmission of the virus is increased. 

To help services assess CO2 levels, we have detailed how you can purchase portable CO2 monitors at a discounted price. Some spaces may already have fitted CO2 monitoring devices (via Internal Environment Monitors).

With winter approaching it is important to have a regular routine in place of checking CO2 levels to gauge how well the supply of fresh air is being balanced with maintaining comfortable indoor temperatures.  

How to set up an Aranet4 Home CO2 monitor

  1. Follow the manufacturer’s instructions in the box.  To activate the device, fit the two supplied AA batteries. There is no on/off switch or other controls on the device that need to be set.
  2. Prior to first use, sit the device outside for 15-30 minutes to let it calibrate to the outdoor fresh air CO2 levels (approx. 420ppm).
  3. Download the Aranet smartphone app from Google Play or the Apple App Store.  The app allows you to connect to the device via Bluetooth to track readings over time and download the readings for up to seven days, in CSV format.  Doing this will avoid you having to manually record the readings.
  4. With the app there is the option to change the measurement interval to our recommended setting of every 2 minutes, and to adjust the warning levels to match our advice presented below.

Using your 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 also consider if you should monitor the space’s CO2 levels over a longer duration.

  1. Take the device to each space and place it somewhere about 500mm high, 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 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 device in the room for a longer period for it to automatically gather its readings.

  1. Take the device into the space and place it somewhere about 500mm high, 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, use the app to view and download the CO2 readings.  If downloading 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. If you need to, discuss your concerns with the Ministry’s ventilation team on, provide a copy of the downloaded data.
Sustained CO2 Levels What to do
Less than 800 ppm

Your space is very well ventilated – continue with your current approach.

800-1250 ppm

Open all windows and doors as much as possible, and whenever it is practical to do so each day while maintaining comfortable indoor temperatures.

Ensure all exterior windows are functional and can be opened as originally intended.   This may require some maintenance or minor property improvements.

Consider briefly vacating the room, changing activity, or lowering the occupancy at times through the day to purge and refresh the air in the space.

1251-2000 ppm

Short peaks above 1250ppm throughout the day are common.  If there are consistent and sustained elevations in CO2 levels over 1250ppm over the day, consider:

  • briefly vacating the room at regular intervals (e.g. 5 minutes each hour) with all windows and doors fully open, to purge and refresh the air in the space
  • lower the occupancy or the level of vigorous activity performed in the room

If the elevated CO2 levels continue, you can contact

Over 2000 ppm

Peaks of high CO2 levels can also occur. If you have followed the above advice and continue to have sustained CO2 levels over 2000ppm, you can contact

Purchasing CO2 monitors

The portable CO2 monitor the Ministry has selected is the Aranet4 Home device.

If you would like to purchase the Aranet4 device, they can be sourced direct from our supplier Butler TechSense Ltd at a discounted price via the web site at using the voucher code '3DUCATION'.

If purchasing another CO2 monitor brand, we recommend ensuring the device has a nondispersive infrared (NDIR) CO2 sensor.