Ventilation and Covid-19

One of the key routes of possible infection with Covid-19 is through the inhalation of the virus through the air.

Government guidance is clear that despite the removal of Covid-19 restrictions, ventilation is still a key mitigation against transmission of Covid-19 and similar respiratory viruses and all schools should continue to maximise ventilation.

Adequate ventilation reduces how much virus is in the air. It helps reduce the risk from aerosol transmission, when someone breathes in small particles (aerosols) in the air after a person with the virus has been in the same enclosed area.

The risk is greater in areas that are poorly ventilated. However, whilst ventilation reduces the aerosol risk, it has minimal impact on droplet transmission, where people are within two metres of each other, and contact transmission, touching surfaces.

Identifying poorly ventilated areas is very important and can be done using carbon dioxide (CO2) monitors, which are recommended by the HSE and have been supplied to publicly funded schools by governments and administrations.

Where CO2 levels rise above 800ppm occasionally, this should be viewed as a sign that ventilation needs to be increased. This is usually achieved by opening doors or windows. Air can be ‘purged’ between lessons/sessions by fully opening doors and windows.

Whilst opening doors can be effective, internal fire doors must not be propped or wedged open unless fitted with automatic closing devices. External fire exit doors can be opened and propped or wedged open, provided they open into a secure location and do not pose a safeguarding risk.

Where a room has levels consistently above 800ppm, this should be reported to school management, as if normal opening of windows etc. is not effective, other measures such as mechanical ventilation, limiting the numbers of people in the room, or repairing windows that no longer open will need to be considered.

Where levels exceed 1500ppm, the air in the room should be purged. School management should be informed if levels cannot be lowered sufficiently, as additional measures such as detailed above may be required. If the levels cannot be quickly lowered to acceptable levels, school management should be informed and an alternative room should be provided. The room should also have an additional risk assessment carried out.

The HSE has confirmed to the NASUWT that the Department for Education expects remedial work to be undertaken where levels exceed 1500ppm. The HSE has also made it clear that it will continue to respond to specific concerns about individual settings.

High CO2 levels are not just hazardous in terms of Covid-19, they also impact on the ability of pupils to learn. Therefore, ensuring adequate ventilation is an educational issue just as much as it is a health and safety issue.

Irrespective of the availability of CO2 monitors, rooms without sufficient natural/mechanical ventilation, where there is a feeling of ‘stuffiness’ or bad odours, or where there are any other concerns about ventilation will require measures to be taken.

Further guidance on ventilation is available on the HSE Ventilation in the workplace overview web page.

Where there are concerns about ventilations, members and NASUWT Representatives should raise these with management in the first instance. If the response in insufficient, you should contact the NASUWT for further advice.

Placement of CO2 monitors

CO2 levels vary within an indoor space. The HSE recommends that CO2 monitors are placed 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 monitors are placed too close they may give a misleadingly high reading.

HSE video on ventilation

Cold weather

The HSE is clear that providing adequate ventilation does not mean people have to work in an uncomfortably chilly or cold workplace. Again the use of CO2 monitors will be useful to measure the levels of ventilation and prevent the need for windows and doors to be open unnecessarily.

In winter conditions, with higher outside wind speeds, windows and doors partially open can still provide acceptable ventilation while keeping the workplace comfortable. Air filtration units could also be utilised.

Heating systems should be turned up and turned on earlier in the morning so that the fabric of the building is warm. Fan convector heaters can also be used if the area is well ventilated or if they are equipped with HEPA filters.

Risk assessments

Health and Safety Regulations already state that all workplaces must be adequately ventilated and the HSE guidance emphasises this in terms of Covid-19 and the potential for airborne transmission in workplaces that are not well ventilated.

If they have not already done so, all employers should urgently undertake a risk assessment of all rooms, which should include the levels of ventilation in order to identify poorly ventilated rooms. This is especially important given the removal of most other mitigations.

Any existing risk assessment must also be reviewed prior to changes in other mitigations.

Members should avoid working in rooms that are poorly ventilated. Poorly ventilated rooms will often have:

  • no mechanical ventilation and no or limited natural ventilation;

  • mechanical systems that do not provide outdoor air, i.e. only recirculate air;

  • a feeling of stuffiness or bad odours.

If members have concerns around the ventilation in their school, they should raise this with their line manager, drawing their attention to the HSE/CIBSE guidance and request that a CO2 detector is provided.

Guidance on the use of air conditioning systems is also available from the Chartered Institute of Building Services Engineers (CIBSE).

Use of air cleaning and filtration units

Air cleaning and filtration units which employ either high efficiency particulate air (HEPA) filters or ultraviolet-based devices can also help remove viruses and other pathogens and allergens from the air.

However, they are not a substitute for ventilation and the HSE has been clear that employers should prioritise any areas identified as poorly ventilated for improvement in other ways before considering using an air cleaning device.

The NASUWT would strongly recommend that they are used in addition to, not instead of, adequate ventilation. The Government has announced it is conducting a trial of filtration units in schools, but this is not expected to report until 2022.

If you are unhappy with the response from their school, you should contact the NASUWT for further advice.


The benefits of ensuring and using adequate ventilation are clear during the Covid-19 pandemic, but the benefits extend beyond Covid-19 and any investment will be effective post Covid. For example, decreasing CO2 levels will have a positive impact on the ability of pupils to learn.

Ventilation does not only remove Covid-19, but also many other viruses and pathogens. Increasing ventilation, combined with the use of air filters and enhanced cleaning routines, could reduce levels of general sickness absence, again improving educational outcomes and reducing costs.


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