COVID-19 Manual Section 3: Buildings, ventilation, air sterilisation and air cleaning
Case studies for this section
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.
Healthcare
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.
Non-healthcare
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.