There is mounting evidence of the viral transmission of coronavirus via small airborne microdroplets released when sneezing, coughing, talking, singing etc. These microdroplets become aerosolised and suspended in air. Studies have shown that aerosols and can linger for considerable lengths of time and travel considerable distances from their point of origin, thus imposing significant risks of airborne transmission, particularly within poorly ventilated buildings.

This requires us to re-think how we approach ventilation system design in high occupancy buildings frequented by the general public, such as offices, shopping centres, public transport hubs and schools. There is a need to revisit design standards for these typically low-risk sectors to ensure that ventilation systems are providing sufficient air exchange and filtration to provide appropriate indoor environments for occupants and assist in facilitating the safe return to building re-occupation.

Airborne Transmission of Coronavirus

The World Health Organization (WHO) is being challenged to modify its formal advice to recognise that transmission of the COVID-19 virus can occur via microdroplets suspended in air. In an open letter endorsed by more than 200 scientists, WHO is being called to acknowledge the overwhelming evidence regarding the danger of microdroplet transmission.

Currently, the primary precautions recommended by WHO are frequent handwashing and maintaining distance. There is little acknowledgement of airborne transmission mechanisms via microdroplets suspended within air. In a recently published article, international health authorities—including WHO and the Centers for Disease Control and Prevention (CDC)—are confronted by their lack of acknowledgement to mounting evidence regarding the airborne spread of the virus.

The difficulty with airborne transmission stems from experimentation challenges such as the requirement for regular/long sampling periods, expert understanding of local air dispersion patterns, and the inherent difficulty in dynamically testing whether a microdroplet contains the virus.

These are potential reasons why health organisations such as WHO and CDC have deflected from acknowledging airborne transmission as a major mechanism for the virus spread.

What does microdroplet transmission look like?

As water particles from breathing/coughing/sneezing/talking are released into air, they immediately begin to evaporate.

This rate of evaporation is dependent on the local relative humidity. As the droplets evaporate, they can become small enough to overcome gravity and become suspended in air. Once suspended, droplets can move. The breadth of transmission depends on external factors such as wind forces, turbulence, and diffusion.

Various peer-reviewed studies have shown how virus-laden microdroplets released from sneezing, coughing, singing and talking can travel >10m from where they originated and remain within poorly ventilated spaces for over 10 minutes, supporting growing evidence that airborne transmission of microdroplets is the only plausible explanation of many superspreading events of COVID-19.

What does this mean for the return to pre-COVID normality?

Airborne transmission unfortunately makes the return to normality more challenging, as risks of infection cannot be mitigated by simply washing hands and social distancing. Ventilation strategies such as increased air change rates, increased filtration, good HVAC hygiene, UV sterilisation technologies, and application of ionisation sterilisation techniques can all play a role to reduce the risks of airborne virus transmission. Natural ventilation remains a key recommendation.

This is underpinned by the assumption that the atmosphere provides a sufficiently large “sink” to dilute microdroplets to safe levels, and/or natural forces such as UV exposure can neutralise the virus or bacteria until it is benign.

The way forward

The application of increased standards of filtration and air processing, like those provided in hospitals, will become equally important in traditionally low-risk sectors such as offices and schools. Pressure-control regimes and containment strategies akin to laboratory systems may also become more commonplace in other sectors, providing physical air containment and separation between spaces.

Now, more than ever, the importance of appropriately designed and installed ventilation systems will become pivotal to public health and the successful reopening of economies and societies.

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