Since one of the primary ways that COVID-19 is spread is through
the air by aerosols - airborne liquid particles that may carry
infection - keeping these particles 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
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 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
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
In spaces that are used for extended durations by groups, i.e.
offices and classrooms, CO2 measurements reflect the air quality
with a greater accuracy and give a good indication of ventilation
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
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
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 to
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 CO2 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 COVID-19 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