Can natural ventilation provide the safe indoor environment we need?

Tom Lipinski, founder and technical director of natural ventilation specialist Ventive, shares a recent study illustrating the possible transmission of viruses with different ventilation systems.

The importance of ventilation cannot be overstated when it comes to the safe return to schools and workplaces; it is singularly the most important factor that separates safe work environments from super spreader events. This view is almost unanimously supported by the scientific community and Scientific Advisory Group for Emergencies (SAGE) advisors such as Professor Cath Noakes, an ACR Journal contributor.

There is no such thing as a safe distance indoors if the space is not adequately ventilated. The nature of buoyancy-driven air movement in enclosed and unventilated spaces is such that each breath ends up being carried to the furthest reaches of the room at some point. It will, of course, be diluted. Still, suppose we are sitting in the same room with an infected person. In that case, we are likely to receive an infectious dose of a virus at some point, regardless of masks, screens or two metre distancing - although we still do not fully understand what an effective dose is, nor how much this can vary from person to person.

Figure 1.  Air pathway of exhaled breath within an unventilated office when using screens and masks.   Driven by personal heat plume, equipment, radiators, colder surfaces (Ventive & Brunel University London).

Ventilation rates

Any ventilation method capable of the effective removal of stale and potentially infected air while providing fresh air to the occupants at a rate of about 10 litres per second per person should improve safety indoors and significantly reduce the risk of virus transmission. But – are all these methods as effective as each other at reducing the risk of infection? If we are designing an office, building, or a school, how do we make it pandemic resistant and future-proof?  Would we want to design a single ventilation measure that can handle 'normal' requirements but can adapt to suit emergency requirements such as pandemics? Or would we need a suite of measures, one for maintaining basic Indoor Air Quality and one for emergency response? An architect or a mechanical and electrical engineer would want to have answers to these questions.

As much as there is consensus that adequate ventilation is key to ensuring a safe indoor environment, there is less of a consensus, and even less data, to help us understand the impact of various ventilation measures on transmission risk levels.

Professor Ben Jones from Nottingham University states that we need a ventilation rate of 210 l/s for every infected person to mitigate the risk of infection[1]. Therefore, if we are in a meeting room of five designed to ventilate at 10 l/s/person with one positive colleague, we would all become infected. His insight provides little in the way of recommended measures to achieve this. Professor Paul Linden from Cambridge University states that displacement ventilation provides the safest indoor environment, adding: "it is clear that, as noted by Florence Nightingale, buildings with high ceilings and with large openings are optimal for natural displacement ventilation."[2]

Ventilation strategies

There has been surprisingly little research into the practical implications of different ventilation strategies and how they could influence indoor virus transmission. Some research papers have looked at spreader and super-spreader events and identified either a lack of ventilation or air conditioning as the key contributors to the spread of infection indoors. However, most commonly used air conditioning systems only recirculate stale air, so they do not provide any ventilation. There are various claims of disinfection and purification systems being effective against the virus, but the claims seem to be based on laboratory research rather than real-life scenarios. A fanned air purifier could have the opposite effect if it blows purified air straight past an infected person, picking up their breath and distributing virus-carrying aerosols around the room.

Figure 2. Schematic illustrating ventilation flows with the various flow elements such as the body plume, inlet flows, stratification and arrows indicating entrainment and mixing (Cambridge University Press) ²

Figure 3. Pre-existing CFD simulations of typical airflow in Naturally Ventilated classrooms (Ventive)³

Previous research, mainly using pre-existing computational fluid dynamics (CFD) covering various ventilation and comfort systems, shows a significant difference between natural and mechanical ventilation strategies, as the two measures handle the air very differently. One utilises buoyancy and stratification, and the other employs air-mixing as a design objective.[3] We couldn't call this research conclusive since we were missing ventilation measure specific CFD simulations in a like-for-like, matching environment and real-life test data.

Figure 4. Setup of the Indoor Infection Spread test office.

Monitoring air distribution

We teamed up with Building Services Research and Information Association (BSRIA) and Brunel University, London, to conduct a real-life indoor environment study, with financial support from Innovate UK. The study was carried out in a 40m2 office, with four desks and four 'droids' (or DIN-men - white cylindrical tubes built in accordance with BS EN 142240:2004), complete with masks, screens, various ventilation systems and a comprehensive suite of monitoring equipment.

We sourced and installed various ventilation, comfort, and 'purification' measures, including a wall mounted air conditioning unit; hybrid ventilation, popular in schools; typical top-down, ceiling void ducted HVAC, mostly used in larger office buildings and various air purifiers. We also tested different natural ventilation approaches from open windows to façade integrated systems.

We 'infected' one of the droids, using easily detectable CO₂ gas as a proxy for the infected breath, delivered at a real-life velocity of 3.5m/s. Body temperature was also mimicked to ensure we replicated a typical environment as closely as possible. The flow of that 'breath' was monitored through various scenarios, using a multitude of CO₂ sensors at multiple heights and locations. We also monitored the impact of various provisions on occupant comfort - collecting temperature and air velocity data. CO₂ 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.

Figure 5. Spread of infected breath throughout the unventilated office measured using exhaled CO₂ proxy.

Figure 6. Spread of infected breath throughout the top down mechanically ventilated office.

Figure 7. Spread of infected breath throughout naturally ventilated office measured using exhaled CO₂ proxy.

We also tested the same room using a top-down positive pressure ventilation with two diffusers located above the desks, a typical system in large offices. 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 effects (Figure 6):

1. There appears to be almost no difference in the overall rate of infection spread around the room between a mechanically ventilated office and an unventilated office 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 effect 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 different when the room was tested with open windows (Figure 7 ). The CO₂ 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 official 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 effective 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.

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