5. Using CO2 monitors

Carbon dioxide (CO2) monitors can help you identify poor ventilation so you can improve it.

How monitors can help identify poor ventilation

The priority for your risk assessment is to identify areas of your workplace that are usually occupied and poorly ventilated.

People breathe out CO2. If there is a build-up of CO2 in an area it can indicate that ventilation needs improving.

There is separate guidance for businesses to help them assess the risk from CO2 as a hazardous substance.

Types of monitor

The most appropriate portable devices to use in the workplace are non-dispersive infrared (NDIR) CO2 monitors.

How to use a monitor

Before using the monitor, read the manufacturer's instructions so you understand how to use it correctly.

Where to place them

CO2 levels vary within an indoor space.

Place them at head height and keep them away from:

  • windows
  • doors
  • air supply openings

Position the monitors over 50cm away from people as their exhaled breath contains CO2. If your monitor is too close it may give a misleadingly high reading.

Try out several locations to find the most representative position for the monitor in the space. In larger spaces more than one sampling location will usually be required.

How to get accurate measurements

  • Follow the manufacturer's instructions, including those on calibrating your monitor
  • Single or 'snapshot' readings can be misleading. Take several measurements throughout the day, when the room is occupied, to represent changes in activities, the number of people using it and ventilation rates
  • As weather changes you may need to repeat monitoring due to differences in natural ventilation, for example from different windspeeds or people opening and closing windows or doors to prevent draughts
  • Record CO2 readings, number of occupants and the type of ventilation you're using at the time. This information will help you decide if an area is poorly ventilated

Understanding the numbers and when to take action

The ventilation rate

The ventilation rate refers to the volume of air that is provided to a room over a period of time and is often stated in building guidance as ' recommended values'.

HSE's Approved Code of Practice states that 'The fresh-air supply rate should not normally fall below 5 to 8 litres per second, per person (l/s/p). A value of 10 litres per second per person is recommended in many guides as a suitable value for most commercial buildings.'

CO2 monitors are a useful way to estimate airflow rates. The amount of CO2 in the air is measured in parts per million (ppm). 1000ppm is equivalent to about 10 litres per second, per person.

CO2 levels consistently higher than 1500ppm in an occupied room indicate poor ventilation and you should take action to improve it.

Remember that CO2 measurements are only a broad guide to ventilation rather than demonstrating 'safe levels'

Deciding if a space is suitable for CO2 monitors

CO2 monitors will only be effective in certain workspaces. They are not suitable in areas with air cleaning units because these remove contaminants from the air but do not remove CO2 or improve ventilation.

Monitors are also not suitable in areas that contain processes that produce CO2. They are of limited use in:

  • areas with not many people in them, including fitting rooms or large offices with one or two occupants or if the numbers of people are inconsistent
  • large, open spaces with higher ceilings such as production halls or warehouses, where you can't be sure the air is fully mixed and CO2 monitors may be less representative

The following examples of different workspaces will help you consider whether a CO2 monitor is appropriate for you.

Small spaces (up to 50 square metres)

  • Monitors can be used for spaces when the same number of people use the space for over an hour, such as small offices and meeting rooms
  • They are unlikely to give reliable measurements where the overall number of people changes over short amounts of time, for example in changing rooms and small retail premises

Mid-sized work spaces (50-320 square metres)

  • Monitors can be used for spaces when a consistent number of people use them for over an hour, such as larger office and meeting rooms, classrooms, restaurants/bars and some retail spaces
  • They can be used for spaces where the overall number of people changes over short amounts of time, for example some retail settings. Treat results carefully as CO2 levels may be affected by these changes

Large spaces (over 320 square metres)

  • Monitors can be used when a consistent number of people use the space for a longer period of time, for example indoor concert venues, large places of worship and airport concourses
  • You may require multiple monitors to get meaningful measurements
  • Where the overall number of people changes over short amounts of time, for example in rail concourses and shopping centres, they are unlikely to give reliable measurements

These examples are based on recommendations in a paper by the Scientific Advisory Group for Emergencies (SAGE) on using CO2 monitors in managing ventilation and reducing COVID transmission but remain valid for general ventilation management.

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