Why is CO2 monitoring useful?
Good ventilation is an important mitigation strategy for COVID-19 transmission as it reduces exposure to small aerosols that can remain airborne for long periods in shared indoor spaces. However it can be challenging to determine how effective ventilation is in many buildings.
CO2 monitoring is an approach that can be used to both assess and manage ventilation. CO2 is in our exhaled breath and emitted through our normal activities. In an indoor environment, providing there are no other significant sources (e.g. combustion devices) it represents the fraction of air that has been exhaled by people in that space. The higher the CO2 value, the more of the air has previously been breathed by people in the room.
The amount of CO2 in the air increases with the number of people in the space and the time that they spend there, and is reduced by ventilation with outdoor air. A CO2 measurement in an indoor space can therefore be used as a proxy for both occupancy and ventilation. It is important to remember that, although improving ventilation can reduce transmission risk, the level of CO2 in the air is not a direct measure of infection risk.
CO2 measurement is already widely used in mechanically ventilated buildings and some naturally ventilated buildings with automated controls – the CO2 value is used as a measure to control the ventilation system. This section focuses on stand alone use of CO2 monitors rather than those installed as part of building management systems.
What are appropriate CO2 levels?
SAGE-EMG have looked at the relationships between ventilation and infection risk and considered guidance being offered worldwide. Their current consensus is:
- A CO2 value that is normally < 800ppm (absolute value) is likely to indicate that a space is well ventilated. This corresponds to a ventilation rate of around 10 l/s/person in most spaces, which is in line with the current building regulations recommendations
- CO2 values that are regularly >1500ppm are likely to indicate that a space has too high and occupancy or has poor ventilation. Mitigating actions are likely to be required in these spaces.
It is also recommended that spaces where there are enhanced risks, such as those where there is regular sustained singing/speaking or spaces where aerobic activity takes place, that higher ventilation rates may be needed, and it is more important to maintain CO2 < 800ppm.
Application of CO2 monitors
CO2 monitors can be used in two different ways in buildings
- 1. To assess ventilation. This would normally be used by those responsible for managing the building to identify parts of a building that have adequate ventilation, and areas where action may need to be taken to improve ventilation or change occupancy. An organisation may have a small number of monitors that can be used on an occasional basis to check ventilation. Where CO2 monitors are used to assess ventilation, they would normally be positioned in a space for a period from a few hours to a few days to record data. By assessing this data alongside occupancy information it is possible to provide an evaluation of the ventilation in the space. Typically this would be done by considering mean values over periods of time. Other parameters including temperature and humidity can be evaluated at the same time, with the assessment providing a judgement on ventilation efficacy compared to thermal comfort. This type of use can be used to give guidance to occupants to improve ventilation, or if this is not possible to identify spaces which will need more significant action to improve ventilation.
- 2.To actively manage ventilation. By having a CO2 monitor in a space with a visible display, occupants of a room can use it to manage the ventilation by adjusting window openings or ventilation settings to keep CO2 levels within an appropriate range. This can also be used to monitor temperature and to manage the balance between effective ventilation and this is usually best suited to naturally ventilated spaces where occupants have control.
Applying CO2 monitors in this way requires them to be positioned where the occupants of the room can easily see them, but where they are not intrusive. It will also be critical to give occupants guidance on what the CO2 monitor is telling them and what action they should take in response to the level in the room. Some monitors provide a display which just states the CO2 concentration, while others include “traffic light” warnings and even alarms.
However the sensors are used, it is important to have a strategy for what to do if readings suggest poor ventilation – it is no use measuring if action is not taken. This may be a staged plan where initial actions focus on guidance for managing ventilation or changing occupancy (lower numbers, more frequent breaks) with a second stage to take more significant action if the environment can’t be improved.
Factors that affect CO2 monitoring
CO2 measurements are a useful indicator, but should not be treated as exact readings or a direct measurement of ventilation rate unless used by a professional as part of a well-controlled ventilation measurement assessment.
Multiple factors can influence the CO2 reading within a space including:
- The age, gender and health of occupants and their activity. CO2 emissions depend on the metabolic rate of people. Children tend to have lower CO2 emission rates than adults, women may have slightly lower emissions than men, and activities such as sustained singing or aerobic exercise can significantly increase CO2 emission rates.
- Background concentrations. Outdoor baseline concentrations are normally 400-450ppm, but can be higher in some urban settings especially if there are combustion sources nearby.
- Airflow pattern within a space. Even when a room has a ventilation system that provides good mixing there will be variations in CO2 concentrations in a space. Values close to ventilation inlets or windows tend to be lower, while those in close proximity to people will be higher. Different readings can therefore be obtained at different locations in a room.
- The accuracy of the sensor. Sensors that are based on a Non-Diffusive Infra-Red (NDIR) sensor are the most accurate – some other sensors do not measure CO2 directly, but calculate it from other measurements and are therefore not accurate. Even a good sensor will have a range of typically +/- 50 ppm.
- Duration of occupancy. When a room is occupied CO2 concentrations will build up over time to a steady state concentration that depends on the size of the room, the rate of CO2 emission and number of people and the room ventilation. In a room with good ventilation the CO2 will build up quickly to the steady state, which would normally be around 800ppm or lower. An example of this is shown in Figure 1 below. In a poorly ventilated room, CO2 builds up more slowly taking longer to reach a steady state. The steady state concentration would be a higher value than the equivalent room with good ventilation. A CO2 concentration that may appear low, but continues to rise consistently over a long period of time is a sign of a poorly ventilated room.
Figure 1: Measured CO2 concentration in a well mixed room (black line) with occupancy (red bars).
Practical considerations to using CO2 monitors
In most spaces the CO2 level will fluctuate with the occupancy, activity and variation in ventilation. Occasionally sensors will record short spikes which are much higher than the average reading – this may be a short increase in occupancy, or someone located too close to a sensor for a short period. In using CO2 data to assess ventilation, mean values typically over periods of 15-30 minutes are generally more representative than short duration fluctuations.
Sensors should be positioned in the occupied region of the room as far as possible, and ideally at breathing height. It is a good idea to position them away from windows, doors and ventilation supply points. They should also be positioned at least 50 cm away from people.
If sensors are used to actively manage ventilation then greater consideration needs to be given to the display, location and robustness of the sensor. Those that use mains power are likely to be more reliable as they will not require maintaining as frequently. There may also be benefits in fixing sensors to a wall or other surface so they are not removed as easily.
A maintenance plan should be in place to regularly check sensors are working and to recalibrate if necessary. Spurious readings may indicate a faulty sensor. It is important that there is clear responsibility for the sensors.
Not all spaces in buildings are suitable for CO2 monitoring. They are likely to be most effective in medium sized spaces with regular occupancy by a reasonably consistent number of people. These may include offices, meeting rooms, classrooms, restaurants/bars, some retail, indoor sports in smaller spaces and many workspaces. Spaces with very low numbers of occupants or those that are occupied very transiently are less likely to give reliable readings. Large spaces are unlikely to be fully mixed and may require several sensors to get an appropriate measurement. This is described in more detail by SAGE-EMG1.
In spaces which have high levels of filtration/air cleaning, such as through application of a stand alone air cleaner or HEPA/UVC in ducting, a CO2 monitor will not be representative of the air from a viral risk perspective. A HEPA filter or UVC device will remove virus and other contaminants from the air, but will not remove gases such as CO2.