Reopening safely: How CO₂ monitoring can improve ventilation and lower infection risk

Assessing the efficacy of ventilation with CO₂ monitors has been identified as a key strategy for enabling a safe return to work, school and leisure.

Keeping airborne liquid particles that may carry infection out of the nearby air by regularly introducing clean and fresh air into communal spaces (i.e. ventilating) is vital for preventing the spread of disease. ²

So how do you ventilate spaces effectively? And how do you know if ventilation practices are even working?

The UK Government's Scientific Advisory Group for Emergencies (SAGE), along with The Chartered Institute of Building Service Engineers (CIBSE) (UK) and the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) (Europe), have issued guidelines on how to improve ventilation in rooms. Their advice includes opening windows, ensuring regular breaks for occupants and increasing mechanical ventilation.

All three reports also highlight CO2 monitoring as a key tool for any effective ventilation strategy, both to help improve ventilation and to help assess its efficacy.

Why is CO2 monitoring important?

Feeling that a room has become 'stuffy' or feeling drowsy after hours sat in a meeting room are sensations that are familiar to many-and they're caused by a build-up of CO2.

Just like CO2, aerosols carrying infectious diseases that are exhaled by occupants will linger in the air without sufficient ventilation. The longer that a room is occupied, the greater the volume of infectious aerosols that will build up, and the greater the risk of infection.

It is possible to evaluate the level of ventilation available in a room just by assessing how occupants are feeling. For example, if a person perceives that a room feels 'stuffy', they may proceed to open a window to increase air flow rates. However, this method is ultimately reliant on guesswork, and occupants may not be able to detect simply by feeling alone when extra ventilation is needed until it is too late.

Monitoring CO2 levels eliminates guesswork from ventilation assessment and enables a systematic approach to increasing ventilation. By accurately measuring the volume of CO2 in the air, CO2 monitors provide hard evidence of when extra ventilation is needed - which might be a long time before occupants feel that the room could do with some fresh air.

The report from SAGE identifies that CO2 monitoring is particularly important in workplaces and schools, where it is most likely to be an effective indicator of ventilation rates.

In spaces that are used for extended durations by groups, e.g. offices, classrooms and public transport, CO2 measurements reflect the air quality with a greater accuracy and give a good indication of ventilation conditions.

By contrast, CO2 monitoring is not as effective at indicating ventilation rates in large volume or low occupancy spaces, where airflow and dispersion of occupants may vary. In these areas, however, SAGE still recommends aiming to keep CO2 levels low (below 800ppm) as a good rule of thumb.

How to improve ventilation with CO2 monitoring

After establishing what ventilation is available in a room, CO2 monitors should be used for two purposes:

  • to alert when extra ventilation is needed
  • to evaluate the efficacy of ventilation

CO2 monitors should be placed on an inside wall, with sensors facing away from windows or grilles so that the readings are as accurate to the inside conditions as possible. It is also recommended that they are placed in visible positions c. 1.5m off of the ground so that responsible persons can easily act upon their alerts.

Alarms show when extra ventilation is needed

Alarms on CO2 monitors can alert occupants of the immediate need to increase ventilation in a room by signalling that the CO2 concentration has exceeded acceptable levels.

As a visual indicator, CO2 alarms also help to boost awareness of the importance of ventilation. The CIBSE report suggests that involving school pupils in responding to CO2 alerts can help to increase ventilation efficacy by improving awareness.

The reports suggest that alarms should be set to alert when the CO2 concentration measures at or above 1000ppm. If or when an alarm is triggered, action should be taken to immediately increase air flow into the occupied space. Opening high windows is an effective strategy, especially in colder months, as it will sufficiently increase ventilation rates while not significantly lowering room temperature or causing a draught.

For schools with mechanical ventilation systems, the reports recommended maximising fresh air flow at all times.

Take informed action with long-term recorded data

With data loggers, CO2 levels can be recorded over time so that data can be viewed and analysed, and further action taken based on this evidence.

Data from CO2 monitoring should be used to assess whether ventilation measures are taking effect.

Time-stamped data from one room might show that instances of increasing natural ventilation-for example, by opening windows or doors-is effective at lowering CO2 levels. For a different room, with different dimensions and different available ventilation routes, analysis of data may reveal that there is a need to implement more significant measures to fully protect the health of occupants. Rooms which are shown to regularly record above 1500ppm should be identified and prioritised for improvements to ventilation.

A further advantage of monitoring with a CO2 data logger is that long-term recorded data can provide evidence of the need for significant or structural changes to be made, helping to support funding applications.

The Tinytag CO2 data logger

Robust, easy-to-use and quick to deploy, the Tinytag CO2 data logger is a simple and effective solution for indoor air quality monitoring.

Using a self-calibrating non-dispersive infrared sensor (NDIR), the data logger accurately measures and records carbon dioxide concentrations with outstanding long-term stability. The TGE-0010 model measures from 0 to 2000ppm and is ideal for measuring indoor air quality in busy classrooms or offices. A model measuring up to 5000ppm is also available for more specialised applications.

The data logger has an LED on its front which will flash red when an alarm has been triggered, enabling responsive action to high CO2 levels. The data logger can be wall-mounted or placed on surfaces to continuously monitor CO2 concentrations wherever is required. Compact and light-weight, the logger can be situated anywhere where there is a mains supply, and can easily be transferred between different locations for versatile monitoring throughout a building.

Data is offloaded and viewed in user-friendly Tinytag Explorer software, where it can then be exported to third-party programmes in a variety of popular file formats (.xls, .xml, .csv, .txt) for further analysis and presentation.

Gemini Data Loggers also offers a certificated calibration service that is traceable to National Standards. CO2 data loggers can be calibrated prior to shipment, or returned to Gemini at any time for calibration.

While low levels of CO22 are a good indication of effective ventilation, they are not confirmation of low aerosol transmission by themselves. Other transmission factors, including occupant density, occupant duration and the type of activity being performed in a room should also be taken into account when assessing transmission rates. A good risk management strategy should also comprise mitigation efforts for other transmission routes, for example, mask-wearing, social distancing and reducing occupancy time in rooms.


'Role of Ventilation in Controlling SARS-CoV-2 Transmission', SAGE EMG, 2020

'REHVA Covid-19 Guidance', REHVA, 2020

'CIBSE Covid-19 Ventilation Guidance', CIBSE, Oct 2020

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