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
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
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
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
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
Alarms show when extra ventilation is needed
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
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.
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
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
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.
TGE-0010 model measures from 0 to 2000ppm and is ideal for
measuring indoor air quality in busy classrooms or offices. A model
up to 5000ppm is also available for more specialised
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.
of Ventilation in Controlling SARS-CoV-2 Transmission', SAGE
Covid-19 Guidance', REHVA, 2020
Covid-19 Ventilation Guidance', CIBSE, Oct 2020