Reducing transmission of viruses in Schools and Public Buildings: Why the approach to ventilation in schools needs to change

Schools have been a common feature in headlines over the past year, given their potential role in the spread of COVID-19. As the education world reopens, there is pressure to change the previous approach of mass pupil removal during spikes and, consequently, the use of ventilation in schools has become a key theme in the industry and the press.  

The government’s SAGE issued a paper in October 2020, confirming the importance of ventilation in mitigating the transmission of SARS-CoV-2.  However, uncertainty over best methodology remains and, without the correct design and implementation of ventilation strategies, we may find their installation ineffective at reducing virus transmission rate.  At worst, running the risk of increasing it. 

In this article we discuss:

  • What is the role of ventilation in schools and public buildings?
  • Methods of ventilation
  • Is existing ventilation methodology effective in reducing the spread of viruses such as COVID-19?
  • Factors impacting ventilation choice


Why is building ventilation important?

Adequate ventilation in schools and buildings is a key requirement for a healthy interior environment, with potentially grave impacts on those using a building with insufficient levels.  In addition to ensuring suitable temperature and odour levels for user comfort, effective ventilation can help removal of harmful airborne particulates that can cause discomfort or even ill health for those inhaling them.  These particulates can be caused by damp, mould, materials & processes used and even by those working within/using the environment themselves.  


ventilation in schools

Ventilation is a serious matter, and Regulation 6 of the Workplace (Health, Safety and Welfare) Regulations 1992 (HMSO, 1992) sets out the standards to which companies must ventilate their internal environments, in particular covering supply of air conditioning and ventilation. 

In addition to this legislation, specific guidelines on thermal comfort, indoor air quality and ventilation in schools can be found in the government’s Building Bulletin 101.  

The Building Bulletin 101 Guidelines take a holistic approach within schools, to ensure overall Indoor Environmental Quality (IEQ).  This can be seen in this diagram included within the Bulletin.

Historically guidelines and legislation for building ventilation have focused primarily on user comfort and temperature, with the Regulation 6 specifically stating:

“By whatever means the appropriate ventilation is provided it must provide a comfortable, breathable atmosphere without causing uncomfortable draughts. In most instances opening windows would be sufficient.”

Recently the Chartered Institution of Building Services Engineers has added to this with specific guidelines relating to post-lockdown building ventilation. “Emerging from Lockdown” is a series of documents covering ventilation, air cleaning technologies and reoccupation of buildings.

Methods of Building Ventilation

There are three primary methods of providing ventilation in schools and buildings:

Natural Ventilation

This is where the main source of fresh air supply is through wind and/or thermal buoyancy*/stack effect.  Natural ventilation can include opening windows, dampers and roof stacks – which can be manual, automated or a hybrid approach.   (*Thermal buoyancy ventilates a building by allowing denser, cold air to lift warmer air up and out of the building.) 

Mechanical Ventilation

This is where a fan provides the main source of fresh air supply and stale air extraction.  They can be either centralised or room-based.

Hybrid ventilation

This is simply a system that combines or switches between both natural and mechanical ventilation.    These systems are useful as they give the flexibility to adapt ventilation to variations in weather and temperature to give optimal result.  A mixed mode option is available whereby the mechanical system will take over from the natural ventilation when required. 

Diagram from Building Bulletin 101 (BB 101: Ventilation, thermal comfort and indoor air quality in schools 2018)

Measuring Effectiveness of Ventilation

The performance of ventilation systems is measured in a number of ways, including specific KPI monitoring of air flow and more subjective user comfort perception.  For example, the Building regulations Part F (NBS 2013b) require a minimum ventilation rate of 10l/s per person for office applications.  

Within schools the benchmarks differ slightly, dependent also on whether it is an existing building or a new build.  For example, within existing builds, occupied areas must provide a minimum of 3l/s for each person when at maximum occupancy.   For new builds, the requirements are more complex:

  • In learning/teaching areas the average CO2 concentration can not exceed 1500 PPM (parts per million) and the maximum concentration can not exceed 5000PPM during teaching hours.


CO2 levels can also be monitored to identify concentrated areas of poor ventilation.

It remains to be answered whether current measurements will remain relevant going forward into the post-pandemic era, or whether additional units of measurement, e.g. airflow direction, should be introduced that better assess effectiveness of transmission reduction.

Why does the post-pandemic approach to ventilation need to change? An issue of extraction rather than air supply.

The historic approach to building ventilation and ventilation in schools has worked sufficiently well for a number of decades.   In today’s post-pandemic era however, the control of aerosol transmission* of diseases has understandably become a more prominent consideration in the design of ventilation systems.

* where a person inhales small particles of a virus in the air.

In July 2021 The European Centre for Disease Control cautioned that, going forward, a higher proportion of COVID-19 cases would be made up of cases among children, given their non-vaccinated status and that it was essential that the education system prepared itself, including ventilation.

There is little doubt that adequate air flow reduces the risk of aerosol transmission in public buildings such as schools; the issue is how the flow of air should be managed to ensure this is the case. 

Traditional public building ventilation design has focused not on how building air flow can reduce the risk of transmission, rather on how it can facilitate worker comfort and ensure legislative requirements for fresh air.  The management of air flow to remove contaminants is an altogether different process, and one with which traditional school and public building services/HVAC suppliers may not be so familiar. 

When dealing with contaminants, an important consideration is that the particulates (or virus droplets in the case of a pandemic) are not forced to flow through areas where they can cause more harm.  It is not a case of just moving air through,  but ensuring source capture, extraction and air flow direction are combined in a way that ensures emissions do not become fugitive and spread, thus minimising the risk it can pose on its journey.

Take these examples of traditional solutions:

Natural Ventilation Example

Windows and mechanical fans have historically been designed to simply pull air in, with little regard for the direction of air flow that it causes. 

Mechanical Ventilation Example

Grilles are traditionally placed overhead in easy-to-access voids, thus pushing air down onto the lower areas. 

In these diagrams, we can see that the ventilation solutions have actually exacerbated the potential for transmission, and consequently put the building users at greater risk.   

Removal of contaminated air is not an issue of air supply (or blowing air in), but of extraction (taking air out).  By pulling contaminated air away from populated areas, you are more able to effectively reduce the risk of transmission.    Conversely, consider this solution.

An Extraction-first Approach

Here we can see that the contaminated air is being pulled away from where it poses greater risk, with the opportunity for fresh air supply to replace it.  This is a system for ventilation in schools that has been built under the premise of reducing transmission first, with an extraction-led approach, rather than a supply-led one.


Put simply, any ventilation system designed to reduce the risk of transmission should first consider how it is going to remove the contaminated air to create a space in which fresh air can then flow.  This may mean that schools and public buildings need to look beyond traditional HVAC and Building Services suppliers and instead to those who are familiar with designing systems for the removal of contaminated air.  HSE advice specifically says:


“To help you decide which actions to take, you need to carry out an appropriate COVID-19 risk assessment, just as you would for other health and safety related hazards. This risk assessment must be done in consultation with unions or workers. Undertaking this risk assessment may require advice from competent persons, such as professionally registered engineers who are chartered or incorporated engineers registered with the Engineering Council.”

Use of Filters and Recirculated Air

Common guidance is quite clear that these solutions should be used as last resorts.   Recirculated air essentially just pushes the air (and most of what is in it) around the building.  HSE guidance states that recirculation should only be considered where the supply of outside air would cause too great a level of discomfort or demand for energy.  Today there are excellent heat exchanger and heat wheel options that enable easier reclamation of heat without recirculating air and thus not incur high energy usage. 

Air cleaning and filtration units run the risk of being poorly managed, with filters not being changed sufficiently often.  This results in the multiplying of the bugs harboured within the filters.  Combined with a recirculated air system, this can cause a dangerous scenario where more contaminated particulates are actually being placed into the air than before filtration.  

The Role of CO2 Monitors

CO2 monitors certainly have a role to play in identifying poorly ventilated areas.  When we breathe out, we emit CO2, and these levels in populated areas can be measured to see if fresh air supply is meeting required levels. 

They are not a solution in their own right however, they do give a good indication if there are sufficient levels of fresh air and the right composition of air to cover respiratory needs.   

It is also important to ensure that measurements are taken over time and in varying circumstances to give an average, rather than just snapshots.   Changes in factors such as building user behaviour and external temperatures will all impact the levels of CO2. 

There are instances where CO2 monitors become less reliable/unsuitable.  For example, the use of air cleaning units may mean that levels of CO2 are disproportionately higher than levels of contaminants in the same air.  Large, vacuous spaces such as factories and warehouses and with ventilation in schools mean that air flow is not always mixed and in less densely populated areas the CO2 monitors be of little use at all. 

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

Characteristics of space


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

Suitability of CO2 monitoring in different types of space.  Table taken from

GUV Devices

Preliminary studies have been positive on the use of UV devices in destroying viruses such as SARS-CoV-2 and potentially plasma too.  However, there are still a number of unknowns to be tested and considered before their use finds far-reaching recommendation.   For example, residence time is difficult to achieve in most ventilation systems.  There are also construction material and safety considerations in their operation and maintenance.  One limitation is that their use within a fresh air supply or extraction system discharging into the atmosphere would offer little benefit, although their use in recirculation air systems would prove more beneficial.  Similarly, their use with a stand-alone, air-cleaning device is of little use, as only those virus droplets passing through the device would be killed, leaving those droplets elsewhere in the air flow to flourish and contaminate surfaces.  


Approaching Ventilation System Design for Reduction of Transmission Risk

When considering the redesign of ventilation systems post-pandemic, wherever possible, we would recommend engaging the services of a professional experienced in the extraction of contaminated air.  At a minimum, any building owner should be asking existing suppliers their experience and approach to the extraction of contaminated air, and how this could be applied.

The process should always incorporate the following key questions at planning stage:

  1. What is to be removed from the air and what is the volume?
  2. Where should it be removed from and to where?
  3. What are the constraints on possible solutions?
  4. What are the edge cases and varying circumstances in which the system must operated?
  5. What are the desired outcomes and how can they be measured?


As with any ventilation system, the specific circumstances should be taken into account.  Factors impacting specification and installation of the system include:

  1. Purpose of the building
  2. Energy consumption considerations
  3. Cost
  4. Flexibility required for varying environments of building
  5. Ability of team to understand and use the system appropriately
  6. Make-up of the building (e.g. constraints on what can/can’t be designed)
  7. Movement and activities of users
  8. Perception of user comfort
  9. Volume of users


Air flow volume and direction is the key additional consideration to historic ventilation system design – and what role extraction should play in addition to air supply.  The air flow through the building should always be examined under multiple variations of building usage and volume of occupants.  This will not only help in identifying poorly ventilated areas, but also in the potential flow of transmission. 

We're experts
Talk to us about ventilation

Our team is highly experienced in Air Pollution Control across a number of building environments.  If you require support or assistance with your building ventilation needs, please do not hesitate to contact us.