Ventilation and COVID-19

Ventilation and COVID-19

The law says employers must make sure there's an adequate supply of fresh air (ventilation) in enclosed areas of the workplace. This has not changed during the pandemic. You can do this by using:

  • natural ventilation - fresh air comes in through open windows, doors or air vents. This is also known as 'passive airflow', or
  • mechanical ventilation - fans and ducts bring in fresh air from outside

Ventilation isn't the only way of making sure you're working safely. You should also make sure workers are keeping the workplace clean and washing their hands frequently. You can also identify other control measures by your risk assessment.

This bulletin will help you and your workers:

  • identify poorly ventilated areas
  • assess the risk from breathing in small particles of the virus (aerosol transmission) in enclosed areas
  • decide on the steps you can take to improve ventilation

Why ventilation is important

Adequate ventilation reduces how much virus is in the air. It helps reduce the risk from aerosol transmission. Aerosol transmission can happen when someone breathes in small particles in the air (aerosols) after a person with the virus has been in the same enclosed area. The risk from aerosols is greater in areas that are poorly ventilated.

Although ventilation reduces the risk from aerosols, it has minimal impact on:

  • droplet transmission (from people being in close contact)
  • contact transmission (touching surfaces)

Your ventilation is likely to be adequate to minimise the risk of COVID-19 aerosol transmission if the rooms or spaces in your building(s) are:

  • used within the occupancy limits specified in the building design, and
  • have a sufficient fresh air supply to meet the current minimum building standard.

You can get advice from a competent ventilation engineer or, as a precautionary approach, operate your system on the maximum air flow rate


Assessing the risk of aerosol transmission

Adequate ventilation can look different depending on the workplace or setting.

You can reduce the risk of aerosol transmission by:

  • making sure infected workers (or anyone with COVID-19 symptoms) do not come into the workplace
  • providing adequate ventilation with fresh air
  • Deciding what adequate ventilation looks like in your workplace should be part of a risk assessment.

When you have completed an assessment, there are examples of improving ventilation to reduce transmission available on the HSE website

You should also make sure any control measures you identify by your risk assessment take account of the public health regulations and guidance for the nation you are working in.

Video on using ventilation to tackle COVID-19

This following video gives basic advice on how you can use ventilation to help reduce the risk of COVID-19 transmission in your workplace.

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.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

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.


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.

Balancing ventilation with keeping people warm at work

Providing adequate ventilation does not mean people have to work in an uncomfortably chilly or cold workplace. There are simple steps you can take to make sure your workplace is adequately ventilated without being too cold:

  • Partially opening windows and doors can still provide acceptable ventilation while keeping workplace temperatures comfortable
  • Opening higher-level windows will probably create fewer draughts
  • In occupied rooms relying on natural ventilation, air the space by opening windows and doors as fully as possible to regularly provide additional fresh air
  • This can be done while people leave the room for a break. For example, 10 minutes an hour can help reduce the risk from virus in the air, depending on the size of the room
  • If the area is cold, relax dress codes so people can wear extra layers and warmer clothing
  • You could set the heating to maintain a comfortable temperature even when windows and doors are open

Consider providing additional sources of heating if required. Only use fan convector heaters if the area is well ventilated

Ventilation in vehicles

Make sure workers switch on ventilation systems while they're using work vehicles. They should be set to draw in fresh air and not to recirculate it. Encourage your employees to keep vehicle windows open. If it's cold they can leave the heating on to keep the vehicle comfortable.

If it's safe to do so, opening doors of vehicles between different passengers will help to change the air quickly. Opening vehicle windows fully for a few minutes can also help clear the air before anyone else gets in.

You should also make sure any control measures you identify by your risk assessment take account of the public health regulations and guidance for the nation you are working in.

Source: Health and Safety Executive

Should you require any further information, clarification or assistance please contact us on info@wpsafety.co.uk or 01268 649006.