Last reviewed 6 September 2021

The start of a new term and school year after many months of disruption will probably involve a mix of heady excitement, deep concern and a challenging new safety relationship with the pandemic. Jon Herbert looks at the urgent need to improve natural and mechanical ventilation.

Term-time again! Whether the return to class is hesitant like Shakespeare’s reluctant pupil “creeping like snail unwillingly to school”, or an open welcome to fresh ways of working, there will be a long list of priorities.

One of the most crucial is good natural and mechanical ventilation of school work spaces — or a combination of both — to reduce the airborne transmission of the Covid-19 virus and its Delta variant in particular. An up-to-date working template prepared by the HSE is suggested below.

Cardiff commitment

The Welsh Government has already allocated £6 million to fund air technology. Its aim is to install 30,000 CO2 sensors and 1800 ozone disinfection machines developed by Swansea University in schools, colleges and universities (for more information, see the Welsh Government website).

These are expected to warn of conditions conducive to the airborne spread of virus like “canaries in a coalmine”. A key goal is to reassure staff and pupils about the effectiveness of air flows in buildings. However, this must also be linked to mitigation steps that actually improve air quality.

English equivalent

Classrooms in England need similar investments, seven trade unions have stressed in an August letter to Education Secretary Gavin Williamson. They want air filters and monitoring devices fitted urgently to protect against the virus and further dislocation of the education sector.

The letter, signed by Unite, the National Association of Head Teachers, the Association of School and College Leaders, the NASUWT, the National Education Union, GMB and Unison, is also backed by the Liberal Democrats.

Representing teachers, school leaders, administrators and support staff, it underlines the need for “urgent action” to improve ventilation without the further wearing of masks or pupil “bubbles”.

Specifically, the letter referred to the risks of steeply rising Covid-19 cases, particularly if this coincides with respiratory diseases like flu and RSV that can be worse for children than coronavirus. The signatories add that long-Covid is already affecting “tens of thousands of school staff”.

Government response

The response of the Department for Education (DfE) is that good ventilation has consistently been part of the Government’s guidelines, and that “areas where ventilation is poor should be proactively identified so that steps can be taken to improve fresh air flow if needed.”

The Department of Health and DfE also point out that they are jointly running a £1.75 million pilot programme in Bradford to identify the most effective use of air purifying technology in schools.

HSE ventilation and air conditioning advice

With airborne virus transmission in mind, the Health and Safety Executive (HSE) issued in March 2021 detailed guidance notes — “Ventilation and air conditioning during the coronavirus (Covid-19) pandemic”; this was reviewed on 31 August and will be updated again on 30 September 2021. For the latest version see the HSE website.

Ventilation in most schools currently relies on windows and doors being kept open for long periods. But maintaining an acceptable and workable balance between room temperature and adequate air flows can be difficult.

This is fine in warmer weather when problems are less likely. However, with virus precautions now expected to persist through into the colder winter months, the issue is ongoing.

Legal responsibilities

The law stipulates that employers must ensure adequate fresh air (ventilation) in enclosed workplace areas — both generally and in schools — and this has not changed during the pandemic.

Providing fresh air can be by natural ventilation with passive air flows through windows, doors and vents that are totally or partially open at all times.

Alternatively, good air flows can be as a result of mechanical ventilation with fans and ducts that bring outside air indoors, or through a combination of natural and mechanical systems.

Ventilation is so important that it should rank alongside keeping workplaces clean and frequent hand-washing. The aim of the guidelines is to help in:

  • assessing the risks of aerosol virus transmission in enclosed spaces

  • identifying poorly ventilated areas

  • identifying steps to improve ventilation.

Why ventilation is important

Ventilation reduces the amount of airborne virus and helps to decrease aerosol transmissions — small particles in the air emitted by an infected person/persons.

It is also important to note that good ventilation cuts aerosol risks but not direct droplets or contact transmission between close people.

Assessing the risk of aerosol transmission

This can be reduced by:

  • ensuring infected workers, or anyone with symptoms, do not enter the workplace

  • providing adequate ventilation with fresh air.

But deciding what adequate ventilation is suitable for individual workplaces should be part of a detailed risk assessment.

Identifying poorly ventilated areas using CO2 monitors

Detailed advice is given on:

  • the types and use of monitors

  • how accurate readings can help to define the best course of action

  • looking for workplaces with no natural or mechanical ventilation

  • checking the operation of mechanical systems

  • providing outdoor air, temperature controls, or both (recirculated air is poor ventilation!)

  • remembering that stuffy or smelly air is bad.

Assessment of fresh air (ventilation)

Several factors are involved here. They can include a review of ventilation processes — natural or mechanical. This is in the context of different areas often relying on different ventilation set-ups.

Floor or design plans can be helpful here; walking the floor and taking notes can be an alternative. Changing rooms, break-out areas and canteen facilities are examples. Not being able to tell if ventilation is adequate is often a sign of poor ventilation.

The number of people using a particular area

The greater the number of people, the higher the risk. Reducing numbers reduces risk.

Other factors include length of occupancy. The longer the time people spend in an area, and the higher the turnover of people using it, the greater the risk.

Area size

In general, the larger the area the lower the risk. The reason for this is that large buildings and rooms are usually designed with higher ventilation rates, which dilute the virus.


As already mentioned, deep breathing increases aerosol generation and transmission risks, even with good ventilation. Therefore, moving strenuous activities outside can be a wiser option.

Desk and ceiling fans

These are not a good idea. Air may feel fresher but risks are simply being transferred and spread. However, buildings with local extraction ventilation (LEV) systems are useful, providing they move air within a building to the outside.

Complex ventilation systems

Old buildings, and buildings with multiple floors and rooms, may rely on different systems in different places. Guidance for complex ventilation systems is available here from the Chartered Institution of Building Service Engineers (CIBSE). The expert advice of a ventilation engineer may be needed.

Talking to staff members

Staff should be aware of the findings of any risk assessments so that they know how they can subsequently help to reduce infection risks and report failures, non-compliances and infringements.

Explaining the reasons for enhanced ventilation with staff and pupils is strongly recommended as it usually increases what they find acceptable from a personal comfort point of view.

Other steps to improve a temperature/ventilation balance may include opening high-level windows and relaxing dress codes so that extra layers of warm clothing can be worn.

Improving natural ventilation

This can be a tricky one to monitor on cold, blustery days and will probably depend on how far windows, doors and vents are left open, excluding fire doors of course!

Building are designed to provide adequate ventilation; if this is not possible for any reason then occupation can be suspended until improvements are made. Windows and doors should not be closed completely.

Winter low temperatures and winds will increase natural ventilation through gaps and openings that do not have to be opened as wide as in warmer calmer weather. More advice is available from the HSE here.

Purging or airing rooms

Airing rooms frequently improves ventilation. Opening doors and windows fully — preferably when a room is not occupied — is a good idea.

Improving mechanical ventilations — including air conditioning

Two priorities here are to:

  • maximise fresh air

  • avoid recirculating air.

Making sure that mechanical ventilation systems do bring fresh air into buildings often means speaking to the people who manage them to ensure that flow levels are adequate and units are maintained in line with manufacturers’ instructions.

Lowering mechanical ventilation rates just because the number of people in a room happens to be reduced temporarily is not good practice. Rather, base ventilation rates should be calculated on the maximum normal number of people using a specific area. Extending ventilation operational times before and after people occupy any particular room or space is also recommended.

As already mentioned, recirculation units, including air conditioning systems, can actively pose risks. Systems should, therefore, be set on maximum fresh air and minimum recirculation. Fan convection heaters should only be used in well-ventilated areas.

Balancing ventilation with keeping workplace temperatures comfortable

Adequate ventilation does not mean people working in an uncomfortably in chilly or cold workplace. Simple steps to take include:

  • having windows and doors partially open — higher-level windows may create fewer draughts

  • relaxing dress codes in colder areas so people can wear extra layers and warmer clothing

  • only using fan convector heaters in well ventilated areas.

Air cleaning and filtration units

These can be used to reduce airborne transmission where maintaining adequate ventilation is otherwise not possible.

But they are not substitutes for ventilation. The most suitable types are high-efficiency filters and ultraviolet-based units. The size must be appropriate for the area in which they are to be used.

Ventilation in vehicles

A few simple rules are useful here. Vehicle systems should be switched on whenever vehicles are in use; they should be set to draw in fresh air and not on recirculation.

Windows should preferably be kept open at all times; leaving the heater on can make this more comfortable. Whenever safe and possible, doors should be left open between use by different groups of passengers to change air quickly.

Department for Transport guidance is available here.


Ventilation is important. Reducing airborne transmissions of the Covid-19 virus very much depends on good natural and mechanical ventilation of school work spaces, or a combination of both.

HSE issued in March 2021 detailed guidance notes — “Ventilation and air conditioning during the coronavirus (Covid-19) pandemic” (available here). This was reviewed on 31 August and will be updated again on 30 September 2021.

The guidance shows: why ventilation is important; simple steps that can be taken; how to identify poorly ventilated areas (with CO2 monitors); assessing fresh air flows; the importance of room size; how activities can increase risk; why desk and ceiling fans should not be used; how to manage complex ventilation systems; why talking to staff members is important; improving natural ventilation, purging and airing rooms; improving mechanical ventilation — including air conditioning systems; balancing ventilation with keeping workplace temperatures comfortable; air cleaning and filtration units; plus ventilation in vehicles.