ACR Journal

April | May 2021 VENTILATION 34 Volume 7 No.3 collecting temperature and air velocity data. CO 2 concentrations picked up by this mesh of sensors gave us the exact location and intensity of the exhaled, infected air as well as the length of time it took for that air to reach other room users. Indoor environment study results Figure 5 shows that with no ventilation measures, the ‘infected’ breath fills up space immediately around the infected person and then, within about five minutes, starts to spread throughout the room and detected at the furthest desk within ten minutes, growing in concentration everywhere in the room after that. We also tested the same room using a top-down positive pressure ventilation with two diusers located above the desks, a typical system in large o ces. The ventilation rate was set at 40 l/s, matching the recommended and accepted rate of 10 l/s per person. The test highlighted several eects (figure 6): 1. There appears to be almost no dierence in the overall rate of infection spread around the room between a mechanically ventilated o ce and an unventilated o ce space. 2. The air immediately around the infected person is a lot cleaner due to fresh air being delivered from overhead. 3. The infected air is redistributed around the room almost immediately, reaching the furthest desk in less than 4 minutes. 4. Once the infected person leaves, the infection concentration drops more quickly than without ventilation. The most striking observation when looking at this ventilation scenario, which is delivered at a rate recommended by CIBSE, is that the other occupants seem to receive a higher amount of infected air than in unventilated spaces and more quickly. A remarkably similar eect was observed when using a hybrid ventilation system, a popular ventilation strategy used in schools recirculating existing air back into the room while mixing in fresh outdoor air. When set at 40 l/s of fresh air, the infected air is quickly and evenly distributed around the room, delivering the exhaled breath to every desk, whether screens or masks are in use or not. The main issue with these mechanical ventilation strategies is that they are designed to mix the air. This may be a welcome feature under “normal” conditions, but the opposite is true during a pandemic where the primary transmission route is via aerosol distribution. The results were completely dierent when the room was tested with open windows (Figure 7 ). The CO 2 concentration rose in the infected person’s immediate vicinity, but that air did not appear to spread around the room, with readings remaining relatively flat throughout the test. Even though this matches the theoretical research into airflow dynamics, it comes with a very worrying conclusion – the current o cial advice stating that any ventilation strategy is recommended as long as it provides 10 l/s per person may be wrong with potentially catastrophic consequences. Conclusion To quote Dr Rajesh Bhagat from Cambridge University: “Despite the various mechanisms generating disturbances indoors, it is clear that in many cases stratification ‘wins’. Consequently, if designed properly, displacement ventilation, which encourages vertical stratification and is designed to remove the polluted warm air near the ceiling, seems to be the most eective at reducing the exposure risk. Mixing ventilation distributes the air throughout the space and does not provide any potentially clean zones. It also has to work against the tendency of the room to stratify, while displacement ventilation takes advantage of it 2 .” Based on preliminary data, it appears not only that natural ventilation can provide safer indoor environments during airborne pandemics, it appears to be the only ventilation strategy to do so. Natural ventilation measures involve much more than just opening windows and letting the outside air in which is often too warm or too cold. Several natural ventilation systems are available - from basic roof turrets to intelligent and connected systems with heat recovery or even ventilation systems with heat pump-driven heating and cooling, such as the roof-mounted Ventive Windhive or the façade integrated Ventive Active. Successful deployment of these solutions requires careful design integration from an early stage. Still, when done correctly, the capital, running, and maintenance costs of natural ventilation systems can be significantly lower than those of mechanically driven alternatives while providing a much longer useful life due to lack of mechanical parts that invariably wear down. ventive.co.uk 1 B. Jones, P. Sharpe, C. Iddon, E. A. Hathway, C. J. Noakes, S. Fitzgerald, Modelling uncertainty in the relative risk of exposure to the SARS-Co-2 virus by airborne aerosol transmission in well mixed indoor air 2 R. K. Bhagat, M. S. D. Wykes, S. B. Dalziel, P. F. Linden, Eects of ventilation on the indoor spread of COVID-19 3 T. Lipinski, D. Ahmad, N. Serey, H. Jouhara, Review of ventilation strategies to reduce the risk of disease transmission in high occupancy buildings Figure 7. Spread of infected breath throughout Naturally Ventilated office measured using exhaled CO 2 proxy. Ventive Windhive natural ventilation installed at a Guilford school