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

There are some simple ways to identify poorly ventilated areas:

• Look for areas where people work and where there is no mechanical ventilation or natural ventilation such as open windows, doors, or vents.
• Check that mechanical systems provide outdoor air, temperature control, or both. If a system only recirculates air and has no outdoor air supply, the area is likely to be poorly ventilated
• Identify areas that feel stuffy or smell bad

Using carbon dioxide (CO2) monitors

People exhale carbon dioxide (CO2) when they breathe out. If there is a build-up of CO2 in an area it can indicate that ventilation needs improving.

Checking levels of carbon dioxide (CO2) using a monitor can help you identify poorly ventilated areas.

Types of CO2 monitor to use

There are many different types of CO2 monitors available. The most appropriate portable devices to use in the workplace are non-dispersive infrared (NDIR) CO2 monitors.

How to use a CO2 monitor

CO2 levels vary within an indoor space. It’s best to place CO2 monitors at head height and away from windows, doors, or air supply openings.

Monitors should also be positioned at least 50cm away from people as their exhaled breath contains CO2. If your monitors are too close they may give a misleadingly high reading.

Measurements within a space can vary during the day due to changes in the numbers of occupants, activities, or ventilation rates. Doors and windows being open or closed can also have an effect.

The amount of CO2 in the air is measured in parts per million (ppm). If your measurements in an occupied space seem very low (around 400ppm) or very high (over 1500ppm), it’s possible your monitor is in the wrong location and you should move it to another location in the space to get a more accurate reading.

Instantaneous or ‘snapshot’ CO2 readings can be misleading, so you should take several measurements throughout the day frequently enough to represent changes in use of the room or space. Then calculate an average value for the occupied period.

You may need to repeat monitoring at different times of the year as outdoor temperatures change and this will affect worker behaviour relating to opening windows and doors when your space relies on natural ventilation.

Your readings will help you decide if a space is adequately ventilated.

How to get the most accurate readings

• Check your monitor is calibrated before making CO2 measurements. Follow the manufacturer’s instructions, including the appropriate warm-up time for the device to stabilise
• Know how to use your portable monitor correctly, including the time needed to provide a reading
• Take multiple measurements in occupied areas to identify a suitable sampling location to give a representative measurement for the space. In larger spaces it is likely that more than one sampling location will be required
• Take measurements at key times throughout the working day and for a minimum of one full working day to ensure your readings represent normal use and occupancy
• Record CO2 readings, number of occupants, the type of ventilation you’re using at the time and the date. These numbers will help you use the CO2 records to decide if an area is poorly ventilated

How the measurements can help you take action

CO2 measurements should be used as a broad guide to ventilation within a space rather than treating them as ’safe thresholds’.

Outdoor CO2 levels are around 400ppm.

Indoors, a consistent low CO2 reading is likely to indicate a better ventilated space with a higher air quality:

Air quality level	Measurement
High indoor air quality	700-750ppm
Medium indoor air quality	850-900ppm
Moderate indoor air quality	1150-1200ppm
Low indoor air quality	1550-1600ppm

Source: BS EN 13779

An average of 1500ppm CO2 concentration over the occupied period in a space is an indicator of poor ventilation. You should take action to improve ventilation where CO2 readings are consistently higher than 1500ppm.

However, where there is continuous talking or singing, or high levels of physical activity (such as dancing, playing sport or exercising), providing ventilation sufficient to keep CO2 levels below 800ppm is recommended.

Where CO2 monitors will be less effective

CO2 monitors are not suitable for use in areas that rely on air cleaning units because these remove contaminants (such as coronavirus) from the air but do not remove CO2.

In large, open spaces and spaces with higher ceilings, such as food production halls or warehouses, you can’t be sure the air is fully mixed and CO2 monitors may be less representative.

Monitors are of limited use in less populated areas. These include fitting rooms or large offices with one or two occupants.

The Scientific Advisory Group for Emergencies (SAGE) has published a paper on the use of CO2 monitoring. The table below gives examples of spaces where monitors may be useful.

Although this table gives some examples, every space is different, and you need to consider whether a CO2 monitor will be appropriate for you.

Suitability of CO2 monitoring in different types of space

Characteristics of space	Examples	Suitability of CO2 monitor
Small spaces up to 50 square metres floor area. Occupied by a consistent number of people for more than an hour.	Small offices and meeting rooms.	Can be used, but results should be treated carefully as concentrations can be affected by the differences between individual breathing rates.
Small spaces up to 50 square metres. Occupancy varies over short periods	Changing rooms and small retail premises.	Unlikely to give reliable measurements.
Mid-sized spaces of 50-320 square metres. Occupied by a number of people for more than an hour.	Larger office and meeting rooms, classrooms, restaurants/bars, and some indoor sports (low aerobic activity).	Often well suited to monitoring as the higher number of occupants provides more reliable values.
Mid-sized spaces of 50-320 square metres.  Occupancy varies over short periods.	Larger office and meeting rooms, classrooms, restaurants/bars, and some indoor sports (low aerobic activity).	Often well suited to monitoring as the higher numbers of occupants provides more reliable values.
Mid-sized spaces of 50-320 square metres. Occupancy varies over short periods.	Some retail spaces.	Can be used, but results should be treated carefully as concentrations may be affected by variations in occupancy levels.
Large spaces over 320 square metres.  Occupied by a consistent number of people for a longer period of time.	Indoor concert venues, large places of worship and airport concourses.	Can be appropriate for monitoring in occupied areas, but might require multiple sensors to provide meaningful measurements.
Can be appropriate for monitoring in occupied areas, but might require multiple sensors to provide meaningful measurements.	Rail concourses and shopping malls.	Unlikely to give reliable measurements.