COVID-19 | INDOOR TRANSMISSION RISK framework to estimate uncertainty of the number of SARS-CoV-2 RNA copies inhaled is employed. The reference space is a junior classroom occupied for seven hours with a ventilation rate of 5 Ls-1 per person; the form and ventilation rates for these spaces are well defined in current Department for Education (DfE) guidelines and the paper describes the reference case and assumptions fully.7,8 In 2009, Morawska et al undertook several experiments to establish the amount of aerosols produced by an individual under various respiratory activities (for example, breathing, talking and vocalisation), so it is possible to derive a volume of respiratory fluid released per m3 of exhaled breath for various respiratory activities.9 The total number of RNA copies released into the air by an infected individual is a function of the breathing rate, respiratory activity and the concentration of RNA copies per ml of respiratory fluid of the infected individual, providing the emission rate kqsus G. The analysis suggests that the volume of respiratory fluid released as aerosols is roughly 1:5:30 for breathing, speaking and vocalisation respectively. The mathematics of the model demonstrate that the RRI for aerosol transmission is most sensitive to the emission rate (G), so steps to reduce G should be considered (respiratory activities that produce more aerosols such as vocalisation should be minimised unless there are elevated levels of ventilation per person; face coverings to reduce G should be encouraged). Ventilation plays a key role in dilution poorly ventilated spaces (for example, infiltration only) increase RRI, although there is a diminishing law of returns in increasing the ventilation rate (see Figure 2); the potential benefits of increasing ventilation to a poorly ventilated space Vocalisation activities should be minimised unless there are elevated levels of ventilation per person are greater than increasing a well-ventilated space by the same amount. The model only considers the far-field risk of infection in a well-mixed room, and helps engineers reduce this risk. Social distancing and hand washing are still essential. CJ For more analysis and a spreadsheet tool see the preprint at ResearchGate via bit.ly/CJSep20CI1 or email naturalventilation@cibse.org CHRIS IDDON is chair of the CIBSE Natural Ventilation Group No gas supply? Strict NOx targets? Awkward flue run? E-Tech will solve your problems Electric boilers from ACV UK Commercial and domestic Floor standing and wall hung Single phase and three phase Heating and hot water 9kW - 259kW For more information visit www.acv.com 34 September 2020 www.cibsejournal.com CIBSE September 2020 pp32-35 Risk index.indd 34 21/08/2020 14:53