From Fuel Droplets in Jet Engines to Pathogen-carrying Droplets in the Air

At around the time COVID-19 first hit the UK, I was at Heathrow Airport waiting to board a long-haul flight. I remember the uncertainty caused by the new virus in the air, manifested on passengers’ faces as we saw that those headed to Asia were all wearing masks. Little did I know, I would soon discover why wearing such a simple piece of protective equipment was of vital importance to contain the spread of the virus, as I embarked on a much more important journey: shifting part of my research from aerospace propulsion to the transmission of the SARS-CoV-2 virus.

There are fundamental principles that help us understand the behaviour of a droplet, or particle, in an airflow. George Gabriel Stokes (1819–1903), a Fellow at Pembroke College and one of the fathers of modern fluid mechanics, defined some of the physical laws that describe the motion of flows and the motion of particles as they experience a surrounding viscous flow. While large droplets fall through the air, if a particle is small and light enough, it will closely follow the surrounding fluid due to the drag it experiences. Before COVID-19, I applied such concepts to my daily research to predict, for example, how fuel droplets impact the propagation of a flame in an aeroengine combustor. When I first learnt that the SARS-CoV-2 virus was likely transmitted through droplets and that important outbreaks were taking place indoors, I immediately knew what that meant: “if these droplets are small enough, the virus can remain suspended in air for hours!” I remember telling my partner Maria as we read one of the first articles out in the New York Times, hungry for information as most people at the time.

It turns out that humans exhale droplets of different sizes during a sneeze, a cough, talking and even during breathing. These are produced in different parts of the body: large droplets in the mouth, medium to fine droplets in the vocal cords, and really fine droplets are produced deep in the lungs, in the bronchioles. Depending on where a pathogen lives, it populates droplets produced in that region, being carried by those during respiratory activity. The interaction of airflows and droplets of a range of sizes, from a thousand times smaller than bread flour (baking being another great finding of quarantine) to a millimetre, is a problem that is the bread and butter of jet engine research carried out by me and colleagues at the Hopkinson Lab, at the Department of Engineering. As University activities shifted to remote, we immediately realised the importance of airborne transmission, and that we had all the tools necessary to tackle it.

We started by addressing the ‘droplet versus aerosol transmission’ debate. At the time, the World Health Organization and governments resisted the possibility, while the media and scientists around the world started raising the likelihood of airborne transmission of the virus – that is, transmission through those really fine droplets, or aerosols, that are subject to the motion of airflows. In our first paper, published in the Proceedings of the Royal Society A, we modelled the evaporation of a respiratory droplet, a process that is critical to the motion of a droplet. As the droplet shrinks, its momentum and the drag it experiences vary until it reaches a finite size, becoming a tiny crystal. The evaporation process and the time a droplet takes to reach the floor due to gravity is controlled by many factors such as, for example, the concentration of components as proteins and salts found in the fluid, which varies depending on its origin in the respiratory tract.

Now, a year after the pandemic started, it is interesting to note that, although scientific evidence accumulates in piles suggesting airborne is the most likely route of transmission, there is still a lack of hard, experimental confirmation that there is active virus ’living’ in particles of a specific, measured size. The virus, researchers believe, is too sensitive to undergo current sampling techniques, hence the experimental confirmation of active virus in a specific particle of known micrometric size may not be possible in the near future. However, scientists have been able to successfully quantify the concentration of gene copies of virus (traces of virus) in air samples. This, along with the conditions under which known indoor outbreaks have occurred, is enough evidence to face COVID-19 as an airborne disease. In critical moments, health authorities must not wait long before acting based on worst-case scenarios. Scientists should be heard and trusted even if based only on their experience and informed judgement, without waiting for indisputable proof, subject to terrible world-scale consequences otherwise.

Surprised with our findings, we felt the need to reach out to the public. Using the framework put forward in the paper, we developed a tool available at www.airborne.cam to evaluate the risk associated with COVID-19 transmission through aerosols in indoor spaces. With the tool, the user can set the conditions of a given room, such as its size, ventilation conditions, and occupancy, which are then used to calculate the risk of infection for a healthy individual exposed to the infectious viral particles in the air over some time. This way, people can understand which practices can mitigate viral transmission in their daily activities, focusing on the reduction of risk with the use of different masks or reduced occupancy, for example. Our work is the second most read paper in Proceedings A, and the tool reached 100,000 users globally. It has been used in the design of products for aircrafts, in the reopening of concert halls, and has been a key part of the University’s risk assessment protocol, helping Colleges and departments to operate safely.

Despite all efforts, there is still a great deal to learn about the COVID-19 pandemic. For example, the reasons why there is little trust in scientists and great reluctance from governments to take decisions based on scientific evidence? Why was the West so slow in adopting a mask-wearing policy, despite seeing people’s immediate reaction in Asian countries, who very quickly adopted medical-grade masks? And how did wearing a simple piece of protective equipment become a political statement? I hope these questions will be answered by our students in the near future.

By Dr Pedro Magalhães de Oliveira (2015), College Research Associate

This article was first published in Magdalene Matters Spring/Summer 2021 Issue 51.