Comment & Analysis

Effective ventilation for air quality and infection control

Pandemic Infection Control Solutions (PICS) group

Effective ventilation for air quality and infection control
Effective ventilation for air quality and infection control

This article was original published in Science in Parliament, a journal by the Parliamentary & Scientific Committee that is given to all members of the UK Houses of Commons and House of Lords.

Authors: Frank Mills, Matt Rooney, and the IMechE Pandemic Infection Control Solutions (PICS) group.

Covid-19 has sharply highlighted the importance of indoor air-quality. In their response to the pandemic, the Government focussed attention on vaccines and had excellent success. However, during the initial months the country was slow to implement ventilation strategies that could have reduced the rate of virus transmission in enclosed indoor spaces.

Effective ventilation can reduce the level of harmful pathogens carried in respiratory particles thereby diluting the levels of virus suspended in the air. This is particularly important in buildings like hospitals and care facilities where occupants are more vulnerable to airborne infections, and schools due to the high impact on wider society of outbreaks in educational settings.

In North America, where a greater reliance on engineering systems is common for the control of indoor environments, due in part on more extreme weather conditions, ventilation improvements to control Covid-19 spread were more commonly implemented. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) formed a task force and started producing information to help building owners and operators and they ran training courses for engineers in the detailed aspects of infection control engineering focussed onto ventilation. (1)

In the UK, the importance of ventilation to mitigate transmission was highlighted in early Spring 2020 by the SAGE Environment and Modelling Group. The Institution of Mechanical Engineers (IMechE) have reacted to the Covid crisis by producing guidance on how to reduce transmission and the Chartered Institute of Building Service Engineers (CIBSE) has also published data on ventilation and air cleaning and made it freely available. (2, 3)


Ventilation and energy use

Infection control solutions, such as better ventilation, were not widely adopted or promoted by government for almost 12 months. In some instances, schools had to resort to keeping windows and doors open to promote higher ventilation rates, leading to uncomfortable classrooms in cold weather.

Advice from professional engineers included guidance on ventilation that recommended all building owners and operators to use full fresh air and to limit any recirculation as far as practically possible. In buildings that had large scale ducted systems such as VAV (variable air volume) or 100% air systems, the air handling plant had to be modified to use 100% fresh air supply and to reject all extracted air. Whilst these buildings could still be occupied and controlled for comfort, their usage of energy and associated carbon emissions increased significantly.

Although this response was appropriate as a short-term response for existing buildings, any longer-term policies or investments made in improving ventilation must consider the parallel need to reduce carbon emissions. Effective air cleaning or heat recovery strategies can be solutions that reduce emissions and deliver more sustainable and healthy environments. 

Engineering solutions for infection control

Although good ventilation should always be the number one priority, it can be difficult and expensive to implement, especially in existing buildings. Shorter term solutions such as in-room air cleaners (UVC or HEPA filter-based devices) can be deployed to remove airborne pathogens and create a safe environment in many settings. Building Regulations Part F have recently been issued and allows recirculation provided the air is cleaned using UVC or HEPA filtration.

These devices typically use fan to draw air through a standalone device and use various technologies to remove or inactivate microorganisms and other pollutants returning clean air back to the room. Devices which contain fine filters (HEPA filters) are the most common, however there is another class of devices which are as effective and use ultraviolet light in the UVC wavelength as the core technology. UVC doesn’t physically remove pathogens but works by damaging their DNA or RNA and rendering them inactive so that they are no longer infectious.

Independent and local trials at St Teresa’s school in Morden and elsewhere, using computational fluid dynamics (CFD) modelling and measurement, have shown how the installation of UVC in-room standalone air cleaners can reduce the concentration of virus in air. Direct measurement of the effect on infection rates is challenging but early indication suggests that there may be benefits in reducing sickness absence. (4)

A report published in October 2021 by the House of Commons Health and Social Care and Science and Technology Committees failed to mention ventilation in any detail and certainly not as a major strategy. Effectively applied ventilation system will achieve a relatively safe indoor environment and allow people to get back to normal, albeit alongside other strategies such as reducing occupant density and good workplace sickness policies. (5)

One of the more successful ‘immediate’ solutions being investigated uses in-room UVC air cleaners in healthcare applications, including dental theatres and Rapid Assessment Point Wards where hospitals have been able to accept patients delivered by ambulance promptly and safely into sanitized areas for triage.

Air cleaners remove particles and pathogens but not gases. Therefore, areas using medical gases should be designed to ensure removal and dilution. Fresh air change rates must be sufficient to dilute gases as required. Development of these additional measures has focussed onto use of UVC with basic filtration and the air cleaning is carried out by the UVC lamps.

A range of supplementary air cleaning strategies for healthcare have been developed to supplement existing ventilation and to overcome some of the deficiencies of existing systems. Where air change rates are lower than the required standard, additional air changes can be added by installing recirculating air cleaners. Members of IMechE in conjunction with NHS Improvement have also led the development of a new standard for the use of in-room UVC air cleaners for healthcare settings which is expected to be published shortly.

4 Project Title: Opensource software simulations towards understanding, monitoring and controlling COVID-19 transmission by managing air, people distancing and adapting urban environments, UKRI grant number: 85435.
5 House of Commons Health and Social Care, and Science and Technology Committees (2021). Coronavirus: lessons learned to date, Sixth Report of the Health and Social Care Committee and Third Report of the Science and Technology Committee of Session 2021–22, House of Commons.

The importance of air quality

Engineered infection control can provide a safe and resilient environment and allow people to continue their day-to-day activity in safety. However, the UK has not required anything but the most basic standard of air cleanliness for any buildings apart from some workplaces and hospitals and even then it is only a requirement in ‘clinical areas’.  For almost all other buildings and for hospital areas that are non-clinical, such as the cafeteria, restaurant, shop, reception desk and so forth, air cleanliness is not a specific requirement.

The UK ventilation standard for hospitals is in fact a world leading code of practice and has achieved success over many years since its initial introduction in 1995. The code, now known as HTM 03-01, was revised in 2020, republished in 2021 and sets acceptable basic (minimum) requirements for hospital ventilation, air quality and control of airborne transmissions including Covid-19. (6) However, it is likely that only 45% of UK hospitals comply with this standard because 55% were constructed before 1995 and have inherited older systems. Engineers who have been involved in assisting hospitals deal with infection outbreaks report finding inadequate ventilation - or even no ventilation in some spaces.

Leading scientists have expressed concern at the poor track record in dealing with airborne infections and the somewhat optimistic viewpoint that air, and particularly outdoor air commonly referred to as ‘fresh air’, is somewhat ‘clean’, when in fact air quality varies greatly and ‘outdoor’ air may be heavily polluted or may well be ‘clean’ and relatively safe but will vary over time, depending on location and wind direction.

These concerns are summarised and expressed in a paper compiled by a group of the leading scientists and engineers (entitled A paradigm shift to combat indoor respiratory infection (7), which calls for a substantial change in society and government attitude to air quality and effectively setting out the case for taking positive steps to ensure good air quality for all.

Future work

In July 2021, engineering institutions, including the IMechE, collaborated through the National Engineering Policy Centre (NEPC) to produce Infection Resilient Environments. (8) This was a report requested by the Government Chief Scientific Adviser Sir Patrick Vallance and it lays out engineering solutions and related training and regulatory steps to achieve infection control and lower infection risk environments. This research is in the second stage of actioning clear recommendations towards a more infection-resilient infrastructure with another report to be published in the summer.

The IMechE have begun the process of purchasing or leasing air cleaners for use in our London headquarters, One Birdcage Walk. We will be trialling these systems and report the findings later in the year.

6 NHS (2021). HTM 03-01: Specialised ventilation for healthcare buildings.
7 Morawska et al, Science 372:689-691 DOI: 10.1126/science.abg2025, available open access at 
8 NEPC (2021). ‘Infection Resilient Environments: Buildings that keep us healthy and safe’, Initial Report, National Engineering Policy Centre.


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