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Outdated Air Conditioning Systems Help The Spread Of Covid

Most ventilation systems in schools and offices are just not up to the job when it comes to helping curb the spread of COVID‑19. That’s the message from a study of how infectious diseases spread in tightly packed buildings using different ventilation systems.

Sick building syndrome, which blames factors like heating and air conditioning for sickness, days off and poor performance has been a worry since the 80’s. Now, with COVID-19, a fresh focus on intelligent ventilation systems could be key to public health.

“The most commonly used ventilation systems are inadequate at lowering airborne transmission risks,” said Professor Hussam Jouhara at Brunel University London. “High occupancy buildings must be better adapted as a matter of urgency to reduce disease transmission resulting from inappropriate or inadequate ventilation,” said Prof Jouhara.

“Covid-19 has exposed areas requiring urgent development to protect our health, well-being and the economy by providing safe indoor environments for employees or students,” he said in a study published by the International Journal of Thermofluids [1].

While social distancing and wearing a mask can lower COVID-19 transmission, indoor airflows in buildings holding lots of people must work better to remove airborne pathogens, the study finds.

Depending on the outside weather, most ventilation systems suck in outdoor air, heat it or cool it, and then circulate it round the building. Later, the ‘used’ air either gets pumped back outside or is recirculated in the system. Most current systems use centralised air distribution and ceiling level air supply or recirculation, which create the best conditions for disease to spread, the study reports.

And with claims that the virus can remain in the air for up to three hours, the more people come and go from the building, the more the virus spreading pathogens people are exposed to.

Figure 1. Air stream simulation of a typical comfort ceiling fan in an office [2].

Recirculation and air mixing from a ceiling fan in an office environment causes a turbulent airflow and mixes aerosol particles people produce when they talk, cough or sneeze and recirculates them inside the room. This drastically raises the risk of airborne disease transmission. Bringing fresh air into the room by opening a window or door for instance would help lessen transmission risk here and be a good way to make public buildings safe again to use, the study recommends.

Poor ventilation systems maintenance and alterations made to cut energy use or noise usually mean a significant discrepancy between the level of ventilation laid down by building standards and reality. In classrooms, ventilation rates often fail to reach required minimum standard, several studies show. The peak CO₂ concentration often exceeded the recommended levels. “Since the vast majority of air conditioners and hybrid ventilation systems in public buildings mix the indoor air like a blender and use air recirculation, it raises the question over their safety and indicates the need for further research, so safety can be improved, especially in the light of the COVID-19 pandemic,” said Prof Jouhara.

Figure 2. Velocity profile of air in a classroom where air conditioning systems are the main source of indoor air quality control [3].

Conventional air conditioning systems pose a very high risk for airborne disease transmission. Most are based on mixing indoor air together which recirculates back into the system. In a classroom (see above) this could potentially cause never-ending disease spread among school children, students or staff in an office. “If designed and implemented appropriately, natural ventilation measures, or a combination of localised mechanical exhaust and large cross section natural inlets, can provide an adequate displacement ventilation solution, significantly reducing the risk of infection.” Prof Jouhara explained.

To show the visual differences between mixed and displacement ventilation mechanisms, Figure 3 shows the air bubble that surrounds a person in a room. The displacement system on the right draws air in from the outside and the warm air is taken out through the exhaust along with any airborne particles people produce without mixing with the surrounding air. Compare this with the mixed air system on the left. This illustrates how displacement mechanism systems are better at preventing disease transmission than other systems.

Figure 3. Differences between mixed and displacement ventilation systems [4].

Natural ventilation systems available on the market produced by Ventive Ltd [5], show how efficiently displacement mechanism ventilation systems work in classrooms in ventilating classrooms (Figure 4).

Figure 4. Airflow velocity profiles simulations for natural ventilation mechanisms in classrooms [5].

Forward thinking facilities managers are now well-advised to re-evaluate their current ventilation systems and look at how they might better manage and reduce the risk of airborne virus transmission. Reviewing current ventilation systems in light of COVID-19 and developing new lower risk solutions is no small task. A quick and simple but effective first measure would be to add a high efficiency particulate air filter (HEPA) into their current system. HEPA filters remove particles that are 0.01 microns, smaller than the 0.0125 microns diameter of the coronavirus particle. However, this doesn’t eliminate the fact that the mechanism of ventilation is now dated and the risk of pathogen particles escaping from the filters will require strict scheduled maintenance.

Displacement ventilation using continuous extraction ventilation and natural fresh air supply is the best way to combat this risk of virus transmission. Some of the advantages are a laminar airflow and no recirculation of air. Conventionally, displacement ventilation strategies use the natural forces of buoyancy that drive this mechanism.

Natural ventilation mechanisms may not be always easily applicable in scenarios such as high rise buildings where various engineering challenges will arise. In such environments, intelligent ventilation mechanisms need to be studied further.

[1] Review of ventilation strategies to reduce the risk of disease transmission in high occupancy buildings published in November’s International Journal of Thermofluids https://www.sciencedirect.com/science/journal/26662027

https://doi.org/10.1016/j.ijft.2020.100045

[2] Cfd Modeling Approach For Turbomachinery Using Mrf Model | LearnCAx, (n.d.). https://www.learncax.com/knowledge-base/blog/by-author/ganesh-visavale/cfd-modeling-approach-for-turbomachinery-using-mrf-model

[3] S. Lin, B.T. Tee, C.F. Tan, Indoor airflow simulation inside lecture room: A CFD approach, IOP Conf. Ser. Mater. Sci. Eng. 88 (2015).

[4] Anastasia Churazova, Displacement Ventilation vs. Mixing Ventilation I SimScale, March 10th, 2020. (n.d.).

[5] Ventive Ltd www.ventive.co.uk