Identifying poorly ventilated areas by using CO2 monitors

HSE outlines how to go about identifying poorly ventilated areas by using CO2 monitors .

Carbon dioxide (CO2) monitors help employers identify poor ventilation so they can improve it and reduce the risk of spreading COVID-19.

This page covers:

• how CO2 monitors help identify poor ventilation
• types of monitor to use
• how to use them to measure CO2
• deciding if a space is suitable for monitors

How CO2 monitors help identify poor ventilation

The priority for your risk assessment is to identify areas of your workplace that are usually occupied and poorly ventilated. CO2 monitors can help you do this.

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

CO2 monitors don’t measure levels of coronavirus but using them can tell you if an area needs improved ventilation.

Types of monitor to use

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

How to use a monitor to measure CO2

Follow the manufacturer’s instructions to understand how to use your monitor 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 ventilation, for example from opening windows and doors
• 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 amount of CO2 in the air is measured in parts per million (ppm). A consistent CO2 value below 800ppm is likely to indicate that an indoor space is well ventilated.

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

CO2 levels below 800ppm are recommended for areas with continuous:

• talking
• singing
• high levels of physical activity such as sport or dancing

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 work spaces.

They are not suitable in areas that rely on air cleaning units because these remove contaminants (such as coronavirus) from the air but do not remove 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
• 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 work spaces 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 changes to the amount of people in a space

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 (PDF) .

This article forms part of HSEs ventilation during the coronavirus (COVID-19) pandemic guidance and you can view it here.