Thread by @CathNoakes

Now more business are going to have more people in them ventilation becomes an even more important mitigation strategy
You can use CO2 monitoring to help.
Here's another thread that may be useful if you are looking to use CO2 1/

You use a CO2 monitor for a short period to check the ventilation in an occupied space. This would often be done by facilities management and is a good check in regularly occupied spaces with constant mechanical ventilation 2/

You can also have a monitor displaying all the time and use it to actively manage the ventilation – this is good where you have opening windows or other controls that people in the room can use 3/

Its important to remember CO2 is only a guide. The numbers are not exact and they don’t tell you actual risk from virus.
CO2 increases when there are more people in a space and decreases when ventilation is increased. So the value tells you about ventilation AND occupancy 4/

If CO2 is normally 800ppm or less then ventilation is probably OK
If the CO2 is regularly over 1500ppm then you really should do something about ventilation
Between the two, try to get the value lower , but don’t be a slave to the numbers 5/

There are lots of things that affect CO2:
People– some breath out more, some less. Kids are generally lower and if you are doing intensive exercise you can exhale much more
Sensor position – there are always higher and lower regions in a room
Quality & calibration of sensor 6/

Also remember CO2 can be a false indicator. If you have other sources, such as a gas fire or cooker then it will be higher. Air filters can remove virus particles but will not lower the CO2. The background CO2 can also be higher in some places, particularly urban areas 7/

So if you are going to go ahead, first chose your monitor. You should always look for an NDIR sensor as they are more reliable. A monitor that also measures temperature and humidity is useful. 8/

CO2 monitoring isn’t good in every space. It is best in small-mid sized spaces regularly occupied by several people. Spaces with infrequent occupancy (e.g. corridors) and very large spaces are less reliable to measure 9/

If you are using a monitor to check spaces, it can be a good idea to get one that logs data, then you can download and graph it/calculate averages in order to have a record of your measurements 10/

If you are actively managing ventilation, look for one with an easy to read display for the people who are using it. Having colours/lights can help so that people don’t have to understand the CO2 values. Some have alarms to give a warning, but some people find these annoying 11/

Locate your monitor as best you can to measure the part of a room that people occupy. Avoid placing by windows/doors or within 50cm of people as you may get false low/high readings. You may have to be pragmatic and place it by a plug socket 12/

To manage ventilation it is a good idea to semi-permanently mount the meter on a wall or a shelf so that people don’t remove it 13/

Check your monitor is reading a sensible value by locating it in an empty room overnight – you should get values below 500ppm when the room has been empty for a long period. You can also check outdoor concentrations too 14/

Watch the data when the room is occupied. It should rise when people come into a room and settle to something fairly steady within around 20-30 min. If the occupancy changes a lot or the ventilation is very variable then it can bounce around quite a lot 15/

If the value rises and rises and rises slowly over a long period of time (more than about 40 min) it is telling you that the room is poorly ventilated, even if the CO2 remains low. If people are there for a long period then this could be a risky space 16/

Don’t worry if you get a few very short peaks at higher values – this can happen for many reasons including people going too close to a sensor. It is sustained high values that are a concern 17/

You can also watch how the CO2 value drops when a room is empty – if it takes several hours to return to background levels then the ventilation rate is low and you may need to improve 18/

One advantage of actively managing a room with a CO2 meter is that you can balance ventilation with temperature and energy. You may find that you only need windows open a small amount to be effective, especially in colder weather 19/

If you find that CO2 is persistently high in a space don’t panic. The infection risks may be higher, but the space usually isn’t instantly dangerous (unless you have crazy high values where the CO2 itself is hazardous) 20/

Before making major changes check a few things. Check the sensor calibration. Move the sensor to another location and see if you still see the same. Check the ventilation is working/vents are open 21/

If you can’t resolve it then time for a solution. Short term you can reduce number and duration of occupancy, take longer breaks etc. Look at whether there are simple modifications to ventilation like increasing flow in a mechanical system or opening stuck windows 22/

You could also install a HEPA filter or UVC air cleaner, but remember that these won't change the CO2 reading so you may need some expert help to calculate the effect 23/

CO2 meters are not infallible. They can break, they can give odd readings sometimes. Look after your sensors – have someone in charge. Check them and calibrate them as needed. And don’t spray clean them – we did that - bad move! 24/

If you are using CO2 meters to check ventilation then write a protocol for doing it. This way everyone who does checking in the organisation will do the same thing and you can compare readings 25/

If you are managing ventilation, you will need to give some information to employees/customers. Perhaps have a simple sign with a QR code link to more details. You could show recorded data too over time. 26/

You may also want to run training or inform people in other ways about what the monitors are for, why they are useful and what people should do in response to them 27/

Remember too that CO2 monitors are not a mitigation themselves – they enable you to ventilate and manage occupancy. There is some more information here gov.uk/government/pub… 28/