COVID-19 Manual Section 3: Buildings, ventilation, air sterilisation and air cleaning


The advice being given by Public Health England (PHE) is to lessen risk by,

  • avoiding inhaling/exhaling any air which might happen to contain the virus
  • wearing a mask
  • following guidelines on social distancing
  • hand washing
  • having effective ventilation.

The UK government has recently published the guidance paper produced by the SAGE Environmental and Modelling group, EMG, on the role of ventilation. It was considered at SAGE 60 on 1 October 2020 and published on 23 October 2020.

This paper makes some very important points including that assessing ventilation in many environments requires engineering expertise, and mitigation measures are ‘setting specific’ taking into account the nature of the building and its users including occupants and visitors, ventilation type, length of exposure and activity.

Professor Catherine Noakes, a Fellow of this Institution and a key government advisor on the SAGE group, provides a very clear view of the role on ventilation in dealing with the pandemic – and discusses the challenges as winter approaches and the impact of ventilation going mainstream in her article in the CIBSE Journal November 2020.

Unlike distancing and hand washing, ventilation requirements cannot easily be distilled into one simple approach that everyone can follow.

Ventilation specialists – mechanical engineers – must consider each and every building situation to determine what is the current ventilation system, whether it is operating correctly and whether it requires modification or upgrading to deal with COVID risks.  They may decide that limitations must be imposed, such as reduced occupancy numbers.

Sources of guidance

The risk of COVID-19 transmission is reduced in well-ventilated buildings. Conversely, poorly ventilated buildings pose a serious risk because any SARS-CoV-2 virus  that is present can float through the air and infect others. Viral  aerosols are generated in high numbers by an infected ‘host’ person and can spread through air flows within spaces. The paper published in the new England Journal in April 2020 entitled ‘Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1’ describes airborne and surface transmission routes.

Mechanical engineers design, install and operate ventilation systems and should apply engineered infection controls using appropriate ventilation technologies.


The Institution has published guidance which can assist engineers and others to consider options for the re-opening of buildings safely during this COVID pandemic.


The Chartered Institution of Building Services Engineers (CIBSE) has also published specific guidance which can assist by offering advice toward the re-opening of buildings safely by applying a range of modifications to existing ventilation systems and also advice which can be used for new build.


More detailed information covering a range of building sectors has been produced by the American Society of Heating Refrigeration and Air-conditioning Engineers (ASHRAE) and is publicly available on their website as an ‘infographic’, which is a dashboard approach through which anyone entering can find relevant information on a specific building sector or COVID HVAC issue.

These webpages allow questions to be raised through a ‘Frequently Asked Questions’ section. All questions go to the members of the ASHRAE Epidemic Task Force, ETF, who agree a response which gets published and available to anyone who wishes to look through them. This is an ongoing service which ASHRAE has agreed to provide through this pandemic.

Engineered infection control

Mechanical engineers must use engineered infection control by determining the most appropriate and cost-effective solution for each application, bearing in mind that there is a wide range of ventilation systems serving buildings.

Solutions range from:

  • ‘natural ventilation’ which uses outside air to provide fresh air and ‘free’ cooling to occupants, to
  • ducted mechanical systems which use fans to supply large volumes of air into occupied spaces in order to ventilate and heat or cool filtered air in order to achieve the desired indoor environmental quality (IEQ.

These systems, such as constant volume, VAV or tempered air supply, rely on the air being supplied to ventilate, heat or cool. Between these two extremes there are a variety of systems:

  • air/water
  • heat pumps
  • VRV and VRF refrigerant
  • ‘splits’
  • induction units
  • fan coils and so on.

A feature of these alternatives is they reduce the total volume of air being supplied to the minimum required to meet basic air quality needs. This means the air supply is now based only on fresh air requirements for occupancy whilst the heating and cooling is dealt with by local recirculation units which are served with water or refrigerants. It is more efficient to pump water or refrigerant than air around a building and it is possible to have individual rooms that are heated alongside rooms that are cooled, as in a hotel application where guests may desire different temperatures for their room.

COVID and increased fresh air

Since COVID-19 arrived, the advice on ventilation has been to stop recirculation of air and to increase fresh air volumes to dilute any COVID-19 virus or other pathogens in the air, and thereby reduce risk of infection.

However, whilst this is possible for ‘all air’ systems, those systems which use minimum fresh air and local recirculation heating/cooling devices cannot be used in this way to dilute any COVID virus load. Their airflow is the minimum needed and will fail to dilute any COVID virus which is generated within the room.

Therefore, consideration must be given to the most effective measures to apply in each application, bearing in mind that in some circumstances it may prove impossible to re-use the existing systems and/or potentially negate the re-use of some buildings without major modifications. There could also be problems related to increased fresh air, such as lack of sufficient heating capacity, because the installed system expects to use a proportion of recirculated air.

Full fresh air systems will use much higher amounts of energy and emit much higher amounts of carbon. For example, moving from a minimum fresh air system to, say one that has six air changes of fresh air, would increase annual energy consumption by around 400% depending on building types, systems and building factors.

Before COVID arrived, HVAC systems provided ventilation for air quality, IAQ and Indoor Environmental Quality, (IEQ).

IAQ concerned the amount of fresh air needed for occupants to go about their activities. The volume of fresh air depends on several factors such as the amount of oxygen for breathing, air replacement for odours, gases etc, and limiting the build-up of dust, dirt and airborne particles including potentially harmful pathogens.

Further reading

Providing clean air

Whilst most of these systems do include filtration which removes incoming dust and dirt, they do not provide sterile or ‘clean’ air. In fact, their main objective is to reduce the amount of dust entering, to meet air quality standards for occupants, and to reduce cleaning. In naturally ventilated buildings using opening windows, there is no filtration, and outside air including dust, leaves and insects enter and consequently there is a need to do some cleaning: usually more than in sealed mechanically ventilated buildings.

As far as occupant health and well-being is concerned, the supply air is ‘clean’ if it has sufficient fresh air and meets environmental health external air standards, i.e. with oxygen content and minimal particulates.

It is expected that the human body will cope with any airborne particulates and pathogens in much the same way as it would outside a building. It should be noted that the human body does in fact have extremely robust measures of protection from these airborne threats.

However, there are two important considerations:

a. Outside Air

It is generally assumed that outside air is ‘fresh’ or perhaps even ‘clean’. This is not actually the case because outside air could have many contaminants. However, in general, the human body can cope. But there are situations where outside air is contaminated in which case all incoming air must be filtered and supplied mechanically into a sealed’ building. Note that there are standards set for outside air that local authorities are required to meet.

In the UK the 2008 Ambient Air Quality Directive sets legally binding limits for concentrations in outdoor air of major air pollutants that affect public health such as particulate matter (PM 10 and PM 2.5) and nitrogen dioxide (NO 2).

The UK also has national emission reduction commitments for overall UK emissions of five damaging air pollutants.

Similar standards exist for EU, North American and other countries worldwide and measures must be taken to prevent outside air quality falling below these standards.

In terms of pathogens, outside air enjoys some degree of sterilisation by natural UV on sunny days. However, there are times when external pathogens are drawn into the building – and although the air is partly filtered, they will get through and the bodies respiratory system must perform to protect.

b. Recirculation

Whatever the quality of outside ‘fresh’ air, there will be a proportion of indoor air being recirculated – including both mechanical and natural ventilation systems.

In the case of mechanical systems which recirculate, the return air does have some filtration as it is mixed with fresh air and passes into the supply AHU. However, in most buildings this filtration is unlikely to be sufficient to remove viral particles. Buildings which have a high proportion of recirculated air may therefore pose greater risks to the occupants. In the case of Nat Vent, the air is mixed in the room and recirculates in the airstream, but is also diluted depending on the quantity of fresh air being introduced. The effectiveness of this will depend significantly on the design of the vents, how they are used and the weather conditions.

Temperature and humidity

Although ventilation is likely to be the most important parameter for aerosol transmission, studies have shown that the SARS-CoV-2 virus is also affected by temperature and humidity.

Studies conducted under laboratory conditions have show that the virus survives better under colder and dryer conditions. Relative humidity is recognised to be an important parameter for SARS-CoV-2 and other viruses including influenza. Where possible it is recommended that buildings are maintained at 40-60% relative humidity and above 18C. Low temperature and low humidity environments, such as chilled food processing, may pose additional risks as the virus can survive for much longer periods of time under these conditions.

Air cleaning/sterilisation

Ultraviolet light - UVGI

The use of ultraviolet light for germicidal irradiation (UGVI) has been known about for a long time. In fact, the ability of sunlight to prevent microbial growth – to inactive micro-organisms – was first identified in 1877 by Downes and Blunt.

Sunlight contains high levels of UVA and UVB light, and experiments with artificial sunlight have shown that the SARS-CoV-2 virus is highly susceptible to strong sunlight. A calculation tool provided by the US Department of Homeland Security showing that a high UV index reduces the half-life of the virus from hours to minutes.

While sunlight is effective, it is impractical for building applications, and hence researchers and practitioners have focused instead on the use of ultraviolet germicidal irradiation (UVGI) which uses UV-C wavelength irradiation.

This was shown to be biocidal in the 1940’s  and there have been many further research applications since then. UVGI has been of particular interest whenever an airborne infectious disease becomes active in a community.

For example, in modern times, the use of UVGI to inactivate pathogens in air became popular as a way of limiting TB spread in the 1980’s  and has since become an accepted method of ‘air cleaning’ in many countries, notably in North America where a range of products are available.

Interest in the application of UVGI has grown and technical articles have been published such as ‘Limit the spread of contagious diseases and bacteria with germicidal UV’ published by HPAC journal.

Further reading

Further background information on the application of UVGI to reduce COVID risks can be found in a webinar presentation made by consulting engineers, Arups, entitled 'How lighting can help fight COVID-19' presenting some of our internal research, tools, and opportunities of harnessing UV light to effectively mitigate virus exposure in the built environment’ and are available via their website.

The four presentations, which have been made available, cover the basics and not-so-basics of Ultraviolet Germicidal Irradiation (UVGI).

Arup also presented at the LuxLive digital kicker on the topic of 'How fluid dynamics can make UV-C More effective against the coronavirus’ which described the current Arup research and design tools developed to combine the application of UVGI and CFD analysis to better understand the effectiveness of the technique.


UVGI has not been widely adopted in the UK. In fact, there appears to have been some resistance to it from microbiologists who prefer filtration. However, filters have difficulty preventing viruses due to their very small size.  In practice it would be sensible to use a combination of UV to ‘kill’ viruses and use filters to remove particles from the air.

UV has been applied to overcome serious infection outbreaks in some NHS hospitals.

Manufacturers have developed and marketed high performance portable UVGI unit. This type of model could be simply wheeled in to position and plugged in to a standard 240volt single phase electrical supply.

UVGI is a technology that can reduce viral and bacterial loads and lead to healthy buildings. The article published the CIBSE Journal in November 2020 provides an updated view and refers to the research work being carried out to support its application.

Researchers from industry and academia have found evidence that suggests SARS-CoV-2, when suspended in air, is reasonably easy to inactivate using UV light at 254nm.

Clive Beggs, Emeritus Professor at Leeds Beckett University, has been researching UV air disinfection to reduce COVID-19 transmission in buildings. His research paper has recently been published by "PeerJ" the biological, medical and environmental sciences journal.

Air sanitation

Air sanitation has a role to play in the fight against COVID-19 and indeed other airborne pathogens such as influenza, spores, yeasts and moulds. These pathogens continue to cause harm whilst little notice is taken about them, in " normal times". The adoption of passive UV-C air sanitiser units in places where vulnerable people need protecting / shielding will go a long way towards future proofing these spaces as well as reducing COVID risk.

UV-C can prevent COVID spreading by neutralising its ability to replicate. UV is best used for air sanitisation but can also be used on surface sanitisation too. It is best to refer to sanitisation rather than sterilisation because it cannot be considered as being 100% effective unlike, for instance, steam or hydrogen peroxide because ‘ shadowing’ prevents it reaching any hidden virus.

Signify (formerly Philips Lighting) are a leader in UV-C lamps and equipment, commissioned Boston University to carry out research into how effective UV-C light was in disinfecting the COVID-19 virus. The results demonstrated the parameters that need to be adhered to, to be sure of a good result. Whilst this study is currently out under peer review there are other, older, studies that show UV-C is effective in destroying viruses and other pathogens.

Further reading

The design parameters used to make an air sanitiser all revolve around the size of UV-C dose is required to kill the particular pathogen in question. Test results have shown that a 90% "lethal dose”, LD90, for COVID-19 is 3.3mJ/cm2.  To achieve LD99 would require 6.6 mJ/cm2. This is very similar to a typical Influenza virus. To achieve such a dose using UV-C light, it is a product of the UV-C intensity and time spent within that light. In a typical air sanitiser each time the air is recirculated through it, it receives the same dose. All doses received are cumulative so the amount of disinfecting increases logarithmically.

BioShift UV-C chamber

The BioShift UV-C chamber was originally designed for use in the animal agriculture industry, until recently when it started to gather interest from police stations, retailers, and more due to the outbreak of COVID-19. This eco-friendly tool provides an effective and immediate way of deactivating bacteria and viruses found on the exterior of everyday objects, like cell phones and retail scanners. With two size options available, the BioShift chambers offer an all-encompassing solution to inactivate COVID-19 in a recommended 1-minute disinfection cycle time.

Figure 1. Bioshift chamber

Figure 1. BioShift UV-C Chamber

Further information

COVID transmission: mitigating measures

An update paper prepared by the Environmental and Modelling Group (EMG) of SAGE on transmission of SARS-CoV-2 and mitigating measures, was considered at SAGE 40 on 4 June 2020.

It should be viewed in context: the paper was the best assessment of the evidence at that time. The picture has been developing rapidly and as new evidence or date emerges SAGE updates its advice accordingly.  They published an update on 23 October, which has been referred to at the start of Section 3 as it has raised the status of ventilation as a mitigating measure.


In healthcare there are many reasons why supply air must be much cleaner than for other buildings, the principal one being that hospital patients are likely to be immune compromised due to their illness and/or their course of treatment. Therefore, in all areas where patients are being treated and are likely to suffer due to contaminated air, the HVAC system must supply ‘clean’ air to the standard set out in the UK NHS Code of Practice HTM 03 and the ASHRAE Standard 170.


It is evident that pre-COVID-19, it had been accepted practice to supply air which can be a mix of recirculated air with fresh air in order to provide acceptable air quality to occupants of any buildings which are not designated as ‘healthcare’ in that the occupants require ‘clean’ air.

This is based on the premise that provided a basic level of fresh air supplied, the human body can protect itself from any airborne threats.

However,  all buildings are at risk of a COVID-19 outbreak which places all occupants at risk. COVID-19 literally walks into a building as the infected person enters. If such an event occurs there is no time to alter or add to existing ventilation systems, COVID-19 is immediately present and posing a threat.

Living with COVID-19

Until COVID-19 has been eradicated, or until a protective vaccine has been developed, it will be necessary to consider how to adapt building ventilation systems to provide suitably safe spaces for occupants.

This will require an analysis of the existing ventilation system, its component parts, its control system and the identification of all areas of risk of COVID-19 transmission. This should be followed by the application of all necessary corrective measures needed to reduce COVID-19 risk. These measures may be simple to apply, such as increasing fresh air rates, or may be complex and expensive requiring major changes to the HVAC installation.

In order to undertake work, it is necessary to consider the modes of transmission for COVID-19 and to understand how these relate to ventilation.

Since COVID-19 arrived there has been a significant change in the basic assumptions as described above, and an understanding that COVID-19 is an airborne virus which can infect people through airborne routes and also through surface routes as and when airborne virus settles onto surfaces.

Coronavirus is carried into a building by an infected person who will transmit it into the airstreams inside a building by breathing, coughing and/or sneezing. The air inside the building is therefore carrying droplets and aerosols which contain COVID-19 and will infect others if they breath the virus in.

SAGE guidance

The paper published by SAGE reports although the relative importance of airborne transmission of the SARS-CoV-2 virus is controversial, increasing evidence suggests that understanding airflows is important for estimation of the risk of contracting COVID-19. The data available so far indicates that indoor transmission of the virus far outstrips outdoor transmission, possibly due to longer exposure times and the decreased turbulence levels (and therefore dispersion) found indoors. This paper discusses the role of building ventilation on the possible pathways of airborne particles and examine the fluid mechanics of the processes involved.

Update papers prepared by the Environmental and Modelling Group (EMG) of SAGE, and considered at SAGE 40 on 4 June 2020, on transmission of SARS-CoV-2 and mitigating measures Update paper prepared by Environmental and Modelling Group (EMG) on transmission of SARS-CoV-2 and mitigating measures, can be read on their website.

It should be viewed in context: the paper was the best assessment of the evidence at that time. The picture is developing rapidly and, as new evidence or data emerges, SAGE updates its advice accordingly. Therefore, some of the information in this paper may have been superseded and the author’s opinion or conclusion may since have developed.

These documents are released as pre-print publications that have provided the government with rapid evidence during an emergency. These documents have not been peer-reviewed and there is no restriction on authors submitting and publishing this evidence in peer-reviewed journals.

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