COVID-19 | INDOOR TRANSMISSION RISK CALCULATED RISK A group of UK researchers has attempted to quantify the transmission risk of airborne Covid-19 infection according to activity and type of indoor space. Dr Chris Iddon describes the methodology and how the risk of infection falls as ventilation rates are increased A lthough the near field (<2m) transmission risk is well established via droplet transmission, many scientists have warned about the possibility of indoor far field (>2m) transmission of the SARS-CoV-2 virus encapsulated in small respiratory aerosols that can remain airborne for several hours.1 Building services engineering bodies, such as CIBSE, have responded by encouraging improved ventilation strategies to reduce transmission risk. Poorly ventilated indoor spaces are at an increased risk of SARSCoV-2 transmission, and several methods have been proposed to assess the risk indoors. One challenge in developing a transmissionrisk methodology is that the required dose of SARS-CoV-2 to produce an infection is unknown, and the amount of virus per millilitre of respiratory fluid that an infected individual may shed ranges over several orders of magnitude.2,3 The methodology is described in the pre-print paper Modelling uncertainty in the relative risk of exposure to the SARS-CoV-2 virus by airborne aerosol transmission in buildings by Benjamin Jones, Patrick Sharpe, Chris Iddon, Abigail Hathway and Shaun Fitzgerald. In the HVAC filtration Ventilation Figure 1: Single-zone mass-balance model of virus transport via exhaled aerosols, showing mechanisms by which virion in aerosols can be removed4 HVAC recirculation Momentum-induced deposition and sorption Inhalation Biological decay Suspension of aerosols Emission Exfiltration Ventilation Absorption in the respiratory tract Room filtration Deposition of larger droplets Infiltration Sorption and ballistic deposition SHARING KNOWLEDGE Unprecedented times call for unprecedented measures is an oft-heard rallying call during the Covid-19 pandemic response, and this remains true in academia. Preparing research findings for academic consumption requires careful thought and rigour; ordinarily, one would ensure that results and findings are thoroughly written before submitting them for peer review and publication. A team of academics and researchers has recently made this work in progress available publicly while it is being prepared for formal publication, because of the perceived benefit of sharing the RRI assessment method. The analysis shown here is under peer review, and readers should check the DOI for the latest version: bit.ly/CJSep20CI1 paper, a dose risk for a reference space is established and the dose risk of another space can then be compared to generate a relative risk index (RRI) for aerosol transmission, such that any uncertainty in the viral load of infector or on the viral load required for infection cancel out.4 The method establishes how risky a space/activity is compared with the reference case. It can be used to establish the effectiveness of mitigation measures in reducing RRI. It is important to appreciate that this model only considers the far-field transmission of SARS-CoV-2 by aerosol in a well-mixed space, and does not consider the near field (<2m) or fomite transmission. The concentration of SARS-CoV-2-laden aerosols from an infected individual will be greater within the exhaled puff than in the well-mixed room.5 Social distancing is, therefore, still required and, if you are close to an individual and the infectious material has not become well mixed, transmission risk will be higher. In spaces where social distancing is not possible, consideration of droplet spread and appropriate PPE is required. The model imagines an infected individual emitting viral-laden aerosols (VLA) as a pollution source and assumes a well-mixed space. With no dilution mechanisms, the number of VLA in the space would increase linearly over time. However, there are several ways in which the VLA are removed from a space or rendered inactive (see Figure 1): Ventilation B allistic and momentum-induced deposition removes VLA as they become adsorbed onto surfaces B iological decay of the entrained virus, becoming inactive over time Removal by filtration I nhalation by other occupants. It is challenging to establish how many viral particles (virions) may be emitted by infected individuals. However, it is possible to measure the genetic material of the virus using the laboratory method quantitative reverse transcriptase polymerase chain reaction (RTPCR), where fragments of the viral genome in a sample are amplified via a biological 32 September 2020 www.cibsejournal.com CIBSE September 2020 pp32-35 Risk index.indd 32 21/08/2020 17:07