Ventilation and air conditioning during the coronavirus (COVID-19) pandemic

Removal of most coronavirus (COVID-19) restrictions

This page is up to date following the removal of most restrictions in England. There are different regulations and guidance to follow for Scotland, Wales and Northern Ireland.

The latest advice on keeping workplaces safe will help you assess the risks and continue to reduce COVID-19 transmission. This includes links to information on vaccination, as well as on testing, tracking and tracing (including where variants are spreading).

2. Identifying poorly ventilated areas and using CO2 monitors

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

You should prioritise these areas for improvement to reduce the risk of aerosol transmission.

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.

Although CO2 levels are not a direct measure of possible exposure to COVID-19, checking levels 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 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 (far below 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 levels are around 400ppm and indoors a consistent CO2 value less than 800ppm is likely to indicate that a space is well ventilated.

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 consistent 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
Large spaces over 320 square metres.
Occupancy varies over short periods
Rail concourses and shopping malls Unlikely to give reliable measurements

We review and update this page regularly to reflect any changes in guidance.

Page last reviewed: 30 September 2021

Next review due: 31 October 2021

 
Updated 2021-10-18