Image: Sambeet D/pixabay.
Fluid dynamicists have long been occupied by questions about how droplets generated when humans breathe, talk or cough break down, linger and travel in the air. Today, these questions – and their answers – are even more important as the world is confronted by a virus that has evolved to spread really well among humans.
The epidemiological models that researchers use to understand how the novel coronavirus might spread depend on details at the level of a population. However, COVID-19 – and indeed every infectious disease – transmits and infects at the microscopic level. For example, the viral particles, or virions, of the novel coronavirus piggyback on tiny droplets to travel from one (human) host to another.
These droplets are first formed in the human’s respiratory tract – a network of passages through which air moves into and out of the body. These airways have a soft mucus lining, and the flow of air induces forces that partially break down this lining and eject droplets of different sizes. If a person is already infected by the novel coronavirus, the droplets are infested with virions as well.
The bigger the droplet, the more virions it has inside – but bigger droplets are also dangerous in a different way than smaller ones, thanks to the principles of fluid dynamics. Larger droplets easily overcome the drag forces exerted by the surrounding air, so they move through the air like a thrown ball or rock might – in an arc. They quickly settle down under the influence of gravity and infect nearby surfaces.
On the other hand, smaller droplets tend to evaporate quickly and become even smaller. And eventually, they become so small that they’re too light to overcome the drag, and stay suspended close to where they were breathed, coughed or sneezed out. These ‘droplet nuclei’ are almost completely dry and are called aerosols, and are presumed to be less dangerous because of their inferior viral load.
The mode of droplet transmission is based on a classification system that has a cut-off between small and large droplets — derived largely from a 1930s’ study on tuberculosis. In this mode, a virus is transmitted either by direct contact with large droplets or indirectly by touching surfaces contaminated by such droplets.
If a specific virus does spread this way, researchers invoke the system to suggest physical distancing measures, like the ‘six feet’ rule. Six feet is approximately how far large droplets can travel before falling down.
However, there’s a new, alternative theory that suggests this picture is too simple.
Droplet formation inside a respiratory airway is very sensitive to the speed and nature of air flowing through it. For example, a single sneeze produces way more droplets than a single cough, an exhalation or even five minutes of normal breathing. Droplets ejected in a sneeze also move faster.
But these events aren’t regular droplet ejectors. They’re violent processes. And in violent processes, experiments have shown that droplets don’t exit by themselves: they’re embedded in a warm cloud of turbulent air. This changes things.
The warm air cloud has a significant amount of momentum, and carries the droplets farther than they would have traveled by themselves – up to several meters in some cases. The cloud is also warm and humid, which matters because evaporation plays an important role in limiting a droplet’s lifespan. The combination of a warm, moisture-laden atmosphere provided by the ejected cloud significantly delays the evaporation of droplets inside it. So the droplets could live up to 1,000-times longer.
Aerosols have a small, but non-zero, amount of virions. We don’t yet know if each aerosol particle can carry enough virions for the particle to be potent enough to cause an infection. But if aerosols can be propelled to large distances and persist in the air for long periods, they could play a vital role in the transmission of the novel coronavirus through air.
Some scientists have already argued that airborne transmission mediated by aerosols can be a viable method of COVID-19 transmission. This idea contradicts what other scientists thought earlier – about virions relying on larger droplets ejected primarily by coughing or sneezing. If smaller droplets are a problem, then talking, exhaling and even breathing become problems.
For now, the WHO recommendation cites airborne transmission precautions only for certain “aerosol generating procedures”. It mentions neither aerosols generated by common bodily functions nor turbulent clouds of droplets.
There is another problem. Once the heavier droplets have started to settle, the cloud’s warmth still renders them lighter than the surrounding air. So the droplets are buoyed upwards and can enter indoor ventilation systems, which is a recipe for disaster. Researchers have a tough time modelling how droplets might flow through an enclosed system; the droplets might spread faster or slow down. We can’t be sure. In the great outdoors, winds and/or human activity can actively ferry small droplets that remain suspended for longer.
The prevailing wisdom is that flowing fluids interacting with suspended droplets can create problems out of seemingly harmless activities. You could, for example, accidentally walk into a cloud of infected droplets long after the infected person has left the scene. This is why physical distancing measures are so important, but also why we need to understand we can do better.
As Matthew Meselson, a biologist at Harvard University, told ScienceNews, “There’s no magic about six feet. It’s better than two feet, and 10 feet is better than six, but for aerosol, I don’t know what to say.” The fluid dynamics of virulent droplets is a difficult and confounding affair, and it’s best to not test that calculus, and exercise maximum caution.
Ronak Gupta is doing a PhD in fluid mechanics at the University of British Columbia, Vancouver.