Episode 4: Airborne Transmission and Crowd Safety

Episode 4: Airborne Transmission and Crowd Safety

Recently, a group of scientists published an open letter (Morawska and Milton, 2020) urging governments and organisations to consider that airborne transmission of the COVID-19/SARS-COV-2 virus may be more impactful than surface transmission. In this letter they suggested the following low cost and effective measures to reduce the risk of airborne transmission:

  • Provide sufficient and effective ventilation (supply clean outdoor air, minimize recirculating air) particularly in public buildings, workplace environments, schools, hospitals, and aged care homes.
  • Supplement general ventilation with airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights.
  • Avoid overcrowding, particularly in public transport and public buildings.

We already know that the virus can remain on certain surfaces for periods of time and the most common government advice is to wash hands thoroughly and frequently. More recently we are being asked to wear face coverings when using public transport or visiting shops and now evidence is mounting this may be more important than we initially realised. For example, in May 2020, two hair stylists in Missouri had contracted the virus but before they were confirmed to have coronavirus, they had worked with 139 clients. Health officials contact traced all 139 clients and not one had picked up the virus. How was this so? The CDC believe vigilant mask wearing was a major factor.

Aerosol or Droplet?

Firstly, it’s important to understand the difference between ‘aerosol’ and ‘droplet’. The difference is size. Infectious aerosols are determined as particles under 100 microns (one millionth of a metre) in diameter, that are suspended in gas and can be respired. Infectious droplets are larger than 100 microns and thus fall to the ground within a short distance or shrink by evaporation into aerosols, which can travel further. Contracting the virus via infectious aerosols is also known as ‘airborne transmission’. The movement, settling and deposition of aerosols are influenced by local environment air rates (Kohanski, 2020). Understanding this means we become clear of the impact we have when we breathe, speak, cough or sneeze on the local environment. That we create this aerosol/droplet ‘mist’ in every space we stand in and, without a barrier, it spreads further than we realise. The more distance between people and the better the ventilation, the lower the risk of this ‘mist’ hanging around.

Source:: Xie, X., Y.Li, Chwang, A. T. Y., Ho, P. L. and Seto, W. H. (2007) “How far droplets can move in indoor environments – revisiting the Wells evaporation–falling curve.” Indoor Air. (Indoor Air), (17) January, pp. 211–225.


Secondly, understanding how ventilation works and how to apply it effectively in a space where crowds gather is key. Other than mechanical ventilation (opening a window or door) Nardell and Nathavitharana (2020) recommend two other practical solutions; room air cleaners and upper-room germicidal UV (GUV) fixtures. Room air filters include using filters, UV or other methods of disinfection. The CDC recommends ventilation with 6 to 12 room air changes per hour for effective disinfection (CDC, 2017).

Previously, in a study on the transmission rate of SARS in the cabin of a commercial airplane, where ventilation systems have long been in use, the results indicated that the more people moved around the plane, the higher the risk of spreading the virus via airborne transmission (Han et al, 2014). During flight, fresh air is delivered to the cabin and around 50% of air is recirculated through HEPA (High Efficiency Particulate Air) filters. Air exchange rates range from 12-15 per hour, compared to 12 in a typical office.

The airflow in a cabin is laminar, entering overhead and exiting the cabin near the floor. This separates the cabin into sections, and so disease transmission should be limited to within a few rows of an infected passenger (Mangili and Gendreau, 2005). The longitudinal movement of people through the cabin increases the risk of ‘picking up’ and spreading infectious aerosol ‘mist’ from one section of the plane and move it to another.

The less we move, the lower the risk.

Risk assessment for crowded places

Thirdly, we need to apply these principles to our crowd risk assessment. For instance, we already know that outdoor events have better ventilation than indoor events and only recently we learned that being outdoors is 20 times safer than being indoors (Jimenez, 2020). This still means we must maintain a level of physical distancing as, unless we keep our mouths and noses shut, we still create this ‘mist’. We must think about the physical space we are welcoming people into. If it’s an indoor space, does it have adequate ventilation? Is there enough room for everyone to maintain distance from each other? If not, are there suitable barriers to preventing the spread of our ‘mist’? Below are some considerations we can contemplate for our venues, events or places where crowds gather.

  1. Where do people move from and to in the space?
  2. What surfaces can they touch?
  3. What is the duration of time they spend in each space?
  4. Considering ventilation and room/corridor design – what direction is the airflow in the space?

Once we have this understanding of our space, consider:

  1. What is your maximum capacity ensuring everyone can maintain the set physical distance?
  2. Can you ensure your space is well ventilated either mechanically (open doors, windows, remove side walls, and airflow) or using air cleaners?
  3. Can you limit the movement of the crowd? (seated events etc.)
  4. Depending on the answers on ventilation and duration, is it important that everyone wear a face covering?

Ability to respond

If we are responsible for places where crowds gather; alongside hygiene, we must consider density, duration and ventilation as part of our risk assessment. Our plans are as strong as the weakest link in our chain. The weakest link in this case is the existence of asymptomatic carriers who may not know they have or are spreading the virus. Evidence is mounting that the wearing of a face covering can help reduce the risk of airborne transmission. Even wearing homemade cloth masks can be effective (Cheng et al, 2020) with the most important factor being that the mask is fitted to create a “seal” around the face.

In this climate, we must be responsive to the ebb and flow of an ever changing environment. We are learning more each day on how the virus transmits and which activities (being outdoors instead of indoors) are safer than others. This evidence can guide us to us to rebuild our industry, by designing events that are low risk regarding virus transmission. This in turn helps to rebuild confidence in our audience to return to events. We can do this; by being responsible, adaptable and knowing we are all in this together.


Cheng, V. C. C., Wong, S.-C., Chuang, V. W. M., So, S. Y. C., Chen, J. H. K., Sridhar, S., To, K. K. W., Chan, J. F. W., Hung, I. F. N., Ho, P.-L. and Yuen, K.-Y. (2020) “The role of community-wide wearing of face mask for control of coronavirus disease 2019 (COVID-19) epidemic due to SARS-CoV-2.” Journal of Infection, 81(1) pp. 107–114.

CDC (2017) Guidelines for Environmental Infection Control in Health-Care Facilities. [Online] [Accessed on July 9th, 2020] https://www.cdc.gov/infectioncontrol/guidelines/environmental/index.html.

Han, Z., To, G. N. S., Fu, S. C., Chao, C. Y.-H., Weng, W. and Huang, Q. (2014) “Effect of human movement on airborne disease transmission in an airplane cabin: study using numerical modeling and quantitative risk analysis.” BMC Infectious Diseases, 14(1) p. 434.

Jimenez, J. L. (2020) “COVID-19 Is Transmitted Through Aerosols. We Have Enough Evidence, Now It Is Time to Act.” Time. [Online] 25th August. https://time.com/5883081/covid-19-transmitted-aerosols.

Kohanski, M. A., Palmer, J. N. and Cohen, N. A. (2020) “Aerosol or droplet: critical definitions in the COVID‐19 era.” International Forum of Allergy & Rhinology.

Nardell, E. A. and Nathavitharana, R. R. (2020) “Airborne Spread of SARS-CoV-2 and a Potential Role for Air Disinfection.” JAMA, 324(2).

Mangili, A. and Gendreau, M. A. (2005) “Transmission of infectious diseases during commercial air travel.” The Lancet, 365(9463) pp. 989–996.

Morawska, L. and Milton, D. K. (2020) “It is Time to Address Airborne Transmission of COVID-19.”

Xie, X., Y.Li, Chwang, A. T. Y., Ho, P. L. and Seto, W. H. (2007) “How far droplets can move in indoor environments – revisiting the Wells evaporation–falling curve.” Indoor Air. (Indoor Air), (17) January, pp. 211–225.